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Holley Carb Tuning- Misfire/Lean surge below 2500RPM

48K views 81 replies 10 participants last post by  HeavyChevyDeluxe 
#1 ·
Hey everyone! Back again!

I'm here again with another tuning issue. I come here because y'all never steer me in the wrong direction and tend to get me right back on track. I try and do everything I can to my knowledge or extent before I come here through researching forums and trying to figure things out on my own.. but I'm lost on this one.

Alright, so heres the deal:

Slight misfire below 2500 RPM. At first I thought it was too low of an octane rating or possible the chance of getting bad gas (due to high ethanol content and getting 93 from a station that doesn't sell it too often causing the gas to absorb water).. so I tried mixing 110 with the 93 and that seemed to cure the problem until recently it started it's weird little misfire again. This could be due to weather changes such as humidity or temperature. So I've kind of ruled that whole octane thing out. So I checked my plugs the other day and didn't see any signs of detonation (at least that I could see with my untrained eyes) and the plugs were showing decent color from probably running too rich of a jet (75/80).. but I did notice the electrode tips on some plugs were beginning to slightly melt! Which leads me to believe I'm running way lean at low rpm/throttle causing cylinder temperatures to sky rocket?? I'm only running 185* max temp and usually closer to 180*. So my main question is: What is causing this carb to go so lean at low rpm like this? Is there something I can do in terms of carburetor tuning to eliminate this low rpm miss and quit melting electrodes? To my knowledge primaries don't even kick in until about 2500 RPM which is what leads me to believe this is due to something in some circuit that I am completely clueless on tuning such as the power valve or idle air bleeds or PCVRs or some junk. As I've said this carb has no issues up above 2500 RPM or when I seem to get above about 15% throttle other than a flat spot in hard acceleration from lower RPM from what I assume is the incorrect secondary diaphragm spring. I do feel a surge in the throttle when I'm in first gear and cruising through a parking lot at around 2000 to 2500 rpm (like it's lean surging). I took the carburetor off and checked both primary and secondary transfer slots and they were both at the recommended .040" square and both idle air mixture screws are 1.5 turns out. What do I do to fix this? Powervalve? PCVRs? Mixture screws? Air bleeds???

The carb is a Holley 670 street avenger with the 6.5 PV, 75/80 jets, .035" squirter and blue cam on 2.. and fuel pressure is set to just shy of 7 psi with an electric pump.

Timing is 14 initial and 34 total all in by 2800 no vacuum advance.

355 Chevy 10.2:1
Flat top pistons
Edelbrock Performer RPM heads and intake (air gap)
236/236 @ .050" solid flat cam with .521/.521" lift
.040" squish (.025" deck .015" head gasket)

I'm hoping this is a carb tuning issue and I can go back to running straight 93 and maybe even hook my vacuum advance back up to try and save SOME gas.
 
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#6 ·
Damn, I don't remember if they were screw in bleeds... IIRC they're just the plain old brass insert looking deals.. I'll have to check on that next time I head out to the garage. Would these have any effect on my erratic idle speed? It likes to bounce about 300 to 400 RPM randomly. I used to just think it was the larger cam, but now I'm beginning to thing it has something to do with the idle circuit.
 
#8 ·
how much vacuum is it pulling???? get it hot, normal temp, hook up a vacuum gauge to a full manifold vacumm port, put it in gear(if auto) block wheels and see how much vacuum its pullin with that 236* cam. you might have to drop down to a 5.5, 4.5 or even a 3.5 pv. I also don't care for the avenger carbs. I like the 8007, 1850 with external float adjustments, and 3310 carbs. they just seem so much easier for me to tune. another thing, try tuning it with the black (heaviest) secondary spring installed. get your primary side tuned right, then go to the secondaries. you might end up liking the black spring. when you start changing the secondary spring, ONLY go up to the NEXT lighter spring and put miles and hours on it, THEN TRY THE NEXT LIGHTER SPRING. I see way too many guys make a mile test then come back and change springs. get that cotton picker HOT, and put it in different situations, stop and goes, highway, up hills, decel, burn outs, hard launches, hell I even test in reverse, lol. I just can't understand tuning a cold motor, run it a few blocks and tune some more. keep tuning, you're getting closer:thumbup:. rick
 
#13 ·
I was planning on doing that here once I got the idle restriction drilled out and my pressure regulator fixed. I need to see what vacuum I pull in park at idle. It pulls about 14.5 in park 1000 RPM, but there's no telling what it drops to with my 2500 stall and 700 rpm in drive with my foot on the brake. The method I read was to record idle vaccum in gear and go down 1" from that for your PV size.

Also I totally agree with everyone so far hahaha. All I've had with this 670 is a hell. It wont run smooth ever. Idles like ****. PITA to find the proper tune.. and all I've read online is nothing but people having problems getting these things to tune! I bought it sheerly for CFM. I wanted to compromise between 650 and 750 for a street driven mild 355.

I always wait til the engine is warm to do ANYTHING. I don't even leave the driveway until it's up to at least 160*. Especially tuning the carb, taking vacuum readings, and setting timing.

And if I'm not mistaken I have the plain spring in there now. I think I need to go one heavier which is purple? I'll have to look it up again but I'm pretty sure that's the next heavy spring from plain.
 
#9 ·
I had a Street Avenger 670 that came on a 350 crate engine. Wouldn't idle worth a damn to the point of stalling and never ran smooth. Took it apart and cleaned it twice, re-jetted up and down, piece of crap.

I put an Edlebrock 600 on and needed to turn the idle screws about 1/4 turn. DONE.

The 670 went on ebay. Sorry if anyone here got it.
 
#10 ·
Here is my experience with my holley 670 street avenger (same carb tower and base plate and primary metering block as holley 600 but with side hung fuel bowls and metering plate on rear)I used to have. I had a 350 with a 222/230 @50 hydraulic roller crane cam.

My holley stock came with 65 jets front and 68 rear and was way to lean. The primary idle feed restrictor was .028 and the rear idle feed restrictor was .031 and the idle air bleeds were .078 and the rear was .048 and the high speed bleeds are .025.

Be aware the rear transfer slot sits up .010 higher then the front transfer slots. Those holley street avenger 670 carbs are calibrated very lean from holley and need to be recurved for any motor with 220@50 or more for camshaft size as these carbs are based for mild builds but can be made to work well. I had to always open up my butterflies way to much causing my primary transfer slots being open up to much and causing idle issues and adjusting the rear to be close to the front helped but this thing always had a nasty backfire by giving it a quick throttle shot and no amount of changing shooters or pump cams would get rid of it and had the famous off idles stumbling and hesitation like you mentioned. Timing is very critical for carb tuning and you might need more initial and I did not require more then 16 degrees on mine and it did fine and adding more did not make any difference on how the carb reacted to tuning.

So I started to read more and more on how each circuit works in relation with the others. Now please be very careful when doing these type of mods as once you drill to big then your metering block will either need to be tapped for screw in bleeds or you will have to toss it so make very small adjustments at a time if you do decide to do this. I rejetted mine to 67 front and the rear to 73 as most of the time there is a 6 to 10 split difference between front and rear since there is no power valve in the rear.

First thing is to make sure you have the correct power valve and its not opening up at idle and making for a rich mixture and other issues. I put the vehicle in gear and divide by half. Mine had plenty of vacuum so I kept the 6.5 but if you have less then say 9 in gear you would want a 4.5 or lower depending on your reading. The first thing I did was to tackle the idle part of things and I never had to do anything with the air bleeds and just the idle feed restrictors.

I started off with my primary metering block and drilled the idle feed restriction just .001 at a time and ended up with .034 primary and left the secondary at .031 but my engine had a smaller cam then yours and yours might need more or around the same but its a trial and error thing and you want to make a little step at a time and its time consuming but once you done it will be all worth it.

Just so you can see the difference in the fuel curves from a 600/670 cfm holley vacuum secondary compared to a 3310 750 holley vacuum secondary. The 750 comes with .034 primary and the rear metering plate comes with a .039 idle feed restrictor. In comparison my holley 650 double pumper came with .034 primary and .040 secondary for the idle feed restrictors so that along with the idle air bleeds work hand in hand for how things work on the idle circuit.

Your street avenger might have a .031 for the primary as I have bought a few extra metering blocks that are matched to that carb and the older ones had a .028 and more recent ones where they moved the idle feed restrictor to the top area of the metering blocks have .031 so it will vary depending on what year it was made. If you need any more help I am sure ericnova72 will posting a lot of help as he knows his stuff pretty good.
 
#12 ·
Buying used carbs off of ebay or similar is a risk but I have only bought from holley directly or ones that have not been modded or from a reputable carb carb builders like quick fuel etc. I have never hacked up a carb to where it was junk or unusable and always put it back to factory specs before selling it and full details are given before I ever sold one if any slight modification was done.

I know my way around holley style 4150/4160 carbs pretty well but don't claim to be an expert by any means but thanks to kind folks on here and my library of holley tuning books and doing a lot of research on other peoples experience I have gained a lot of knowledge about tuning these things and still have more to gain.

Nothing wrong with the street avenger carbs and its no different trying to put a carb tuned for a racing motor on a mild street motor as no carb is ready out of the box and they all have to be tuned to run on what ever engine its going on from mild to wild if its not to far out from proper cfm size wise for what its going on.
 
#15 ·
Yes that is the idle feed restrictor. That is the only thing you would want to drill at first. Be carful though as just a .001 change is a certain percentage difference.

I had the quick fuel vacuum secondary pod on my old holley and comes standard on all quick fuel vacuum secondary carbs which allows you to change how quick the secondary's open on the carb. They come with a purple spring.

The silver spring really never lets the secondary's open all the way till clear past the engines rpm. On the quick fuel pod you turn a screw to adjust how quickly it will allow the secondary's to open and I started out by adjusting it till it bogged then backed it off a 1/4 of a turn and it runs just as good as my double pumper on my other engine and I love how it works.

I tried the yellow spring in the quick fuel pod and it would bog sometimes regardless where I set the screw and from my experience the purple spring or yellow depending how your engine reacts are the best springs to get your secondary's open all the way. My builds are in s10's so it does not take much to get them moving as soon as you hit wide open throttle.

With my old holley quick change secondary I could change springs but I could never fine adjust when and how quickly the secondary's could respond and the quick fuel pod is worth the money to upgrade to for the best tune on the secondary side of things.

http://www.summitracing.com/parts/qft-63-1
 
#17 ·
Oh wow, that QFT diaphragm is bad ***! And that's a doable price. I may look into that once I figure out this lean cruise/stumble/miss issue. I need to get this all taken care of then find my primary jet size.. then eventually move my way up to secondaries and that sweet vac secondary pod.

And yeah I was gonna do one little bitty drill bit at a time until the engine runs crisp and smooth at cruise.. since the misfire isn't constant and it's not a terrible miss (more of an annoying misfire and surge in low gear) I am assuming it shouldn't take much to get the IFR drilled to the size I need... maybe just one drill bit larger than the current diameter.
 
#16 ·
Purple is the next lighter spring than the stock plain spring, and it is the one I most often end up with on almost every street/strip vacuum secondary.

The List # 3916 950cfm Three Barrel on my 406 SBC has the long yellow in it, with a 50cc pump on the front of the carb and a 37 shooter. 3600 stall and 4.56 gear. 275/60-15 MT Drag Radial.

Next one heavier than plain silver is brown, then black is the heaviest....I have never found either of these to be good for anything, maybe a motorhome or trailer towing 3/4 ton truck??

They won't fully open on 400 or smaller engines, and won't even fully open on 427/454 big block size engines until way up in the rpm range, like 6500 rpm or more.

Plain silver is not fully open on a 750 vacuum until something like 5600 rpm.
 
#18 · (Edited)
Purple is the next lighter spring than the stock plain spring, and it is the one I most often end up with on almost every street/strip vacuum secondary.

The List # 3916 950cfm Three Barrel on my 406 SBC has the long yellow in it, with a 50cc pump on the front of the carb and a 37 shooter. 3600 stall and 4.56 gear. 275/60-15 MT Drag Radial.

Next one heavier than plain silver is brown, then black is the heaviest....I have never found either of these to be good for anything, maybe a motorhome or trailer towing 3/4 ton truck??

They won't fully open on 400 or smaller engines, and won't even fully open on 427/454 big block size engines until way up in the rpm range, l rpm or more.

Plain silver is not fully open on a 750 vacuum until something like 5600 rpm.
DAMN! That's a lot of carb and a very very soft spring. That beast must get down the track alright.. especially with that stall and gear. You must be running a pretty decent cam in there for that stall and gear.

Yeah if I'm not mistaken my current spring fully opens around 6900 or so.. and I shift no later than 6200 when I hammer down.. but even with that spring I still feel a flat spot when I'm below 3000 and stomp it. Maybe incorrect PV or just the simple fact that the cam doesn't have a lot of balls down low haha.
 
#19 · (Edited)
Below is a copy of the drill bit set I got from ebay and I did not have to pay $30 bucks for it. I think I paid around $15 shipped give or take a few. They jump a few sizes up from .028 to .029 then to .031. So on some you might have to jump a few sizes but in my experience when I did mine I did .02 to .03 but I also kind of knew what range to not go over as well.

I used comparisons of all the different brands from quick fuel to demon and holley to see a base line pattern on all the 650ish range carbs on double pumpers and vacuum secondary's in order to know my limit and I got mine really nice and good with just a few changes on mine.
JEGS Performance Products 150111, JEGS Precision Drill Bit Sets | JEGS Performance Products
 
#22 · (Edited)
I would expect your idle feed restrictor to end up around .031" to .035", that's what I've found it ends up being on the 750/780/800 vacuums I do, got a couple on drag machines that are .037" but they weren't interested in the best possible mileage either, not using a vacuum advance at all.

Cam in the 406 in my Nova was solid lifter 260°/266°@.050" with .575"/.590" lift, 108 lobe separation, installed at 102° intake centerline. Ran 11.50's on Pump 93 @3550 lbs w/driver and was capable of a bit more...I had no roll bar, so I was short shifting it at 4800 rpm from low to 2nd to keep it slower than 11.49.

I think your flat spot you feel is too early secondary spring is basically you are just too lean on the idle transition circuit, not that the secondary's are opening too fast....they are going too lean as the secondary starts to open which will feel like secondary opening too soon. You may need to increase the pump shot a bit more and go to a lighter spring like the purple once you have the surge tuned away....maybe even go .001-.002" over on the idle bleed than what removes the surge, so it will be rich enough to transition quicker and allow use of the lighter secondary spring....but get the surge out of primary and then check to see how it does on the purple spring before just drilling more on the bleed, because it will impact mileage a lot if you go too big on the bleed.

If the rear has it's own idle feed restriction you could make the secondary low speed enrichment/transition a bit richer to allow the earlier opening spring instead of doing it all with the primary bleed. If you have adjustable air bleeds you could decrease the idle air bleed on the secondary to help richen the earlier opening spring quicker also, and may not need to touch the idle feed restrictor in that case.

Just work at one change at a time, take care of all the idle and cruise issues first, then secondary spring rate, then work the jetting for WOT power. Once you find the best jetting, you could step back smaller on primary jetting and increase the Power Valve Channel Restrictions to make up for going back to a smaller jet, and that may help mileage also.
 
#25 ·
I would expect your idle feed restrictor to end up around .031" to .035", that's what I've found it ends up being on the 750/780/800 vacuums I do, got a couple on drag machines that are .037" but they weren't interested in the best possible mileage either, not using a vacuum advance at all.

Cam in the 406 in my Nova was solid lifter 260°/266°@.050" with .575"/.590" lift, 108 lobe separation, installed at 102° intake centerline. Ran 11.50's on Pump 93 @3550 lbs w/driver and was capable of a bit more...I had no roll bar, so I was short shifting it at 4800 rpm from low to 2nd to keep it slower than 11.49.

I think your flat spot you feel is too early secondary spring is basically you are just too lean on the idle transition circuit, not that the secondary's are opening too fast....they are going too lean as the secondary starts to open which will feel like secondary opening too soon. You may need to increase the pump shot a bit more and go to a lighter spring like the purple once you have the surge tuned away....maybe even go .001-.002" over on the idle bleed than what removes the surge, so it will be rich enough to transition quicker and allow use of the lighter secondary spring....but get the surge out of primary and then check to see how it does on the purple spring before just drilling more on the bleed, because it will impact mileage a lot if you go too big on the bleed.

If the rear has it's own idle feed restriction you could make the secondary low speed enrichment/transition a bit richer to allow the earlier opening spring instead of doing it all with the primary bleed. If you have adjustable air bleeds you could decrease the idle air bleed on the secondary to help richen the earlier opening spring quicker also, and may not need to touch the idle feed restrictor in that case.

Just work at one change at a time, take care of all the idle and cruise issues first, then secondary spring rate, then work the jetting for WOT power. Once you find the best jetting, you could step back smaller on primary jetting and increase the Power Valve Channel Restrictions to make up for going back to a smaller jet, and that may help mileage also.
As always you've got it all figured out it would seem. I'm about to copy and paste this and print it out to put into my tune/reference binder. This'll definitely help when I do get down to getting it all dialed in perfect.
 
#26 ·
On my small block chevy with the 231/239 @ 50 hydraulic roller cam with 9.5 to 1 compression with my old proform 650 vacuum secondary I had .034 primary and .040 secondary which was out of box that way but that was with a idle air bleed of .070 and my current holley 650 double pumper I ended up about .037 (.003 more then out of box) primary and .040 secondary but that is with a idle air bleed of .078 which is leaner then what my proform had so that is why there is a difference between the two for the ifr size on the primary block and it works in relation with the air bleed for metering things on the idle circuit and transition circuit.

On my quick fuel 680 vacuum secondary carb its a 4 corner idle so it is not set up the same as the 2 corner carbs but I do have a quick fuel slayer 600 vacuum secondary carb sitting at the side for a spare and on my milder 350 with 9:1 compression and a 219/227 @ 50 with it set up as a 2 corner idle out of box I had .031 primary and .040 secondary with a .070 idle air bleed and it was a little rich but did not keep it on for too long to tune the idle circuit leaner as I wanted a bigger carb and its a long story but probably would have ended up with a idle air bleed of around .078 and it would have been good.

That is what the average base line for carbs recommended for cams in the 230 and over range till you get into the 240 plus then I can't honestly say what the base line would be but would not be too much more for a street motor. This is not etched in stone as every engine even if similar will need more or less then this and camshaft specs and timing and cylinder head flow and compression all play a part in a tune.

Specs below is out of box setting so you can get a idea of comparison between brands.

Holley 650 double pumper stock specs
.077 to .078 idle air bleed primary .050 secondary idle air bleed
.030 to .031 high speed air bleed
67/73 jetting
.034 primary idle feed restrictor .040 secondary idle feed restrictor

Proform 650 vacuum secondary
.070 idle air bleed primary .039 secondary idle air bleed
.034 primary idle feed restriction and .040 secondary idle feed restriction
70/76 jetting

Holley 670 vacuum secondary street avenger
.078 primary idle air bleed and .048 secondary idle air bleed
.025 high speed air bleeds
.028 to .031 primary idle feed restriction ( depends on metering block) .031 secondary idle feed restriction.
65/68 Jetting
 
#27 ·
On my small block chevy with the 231/239 @ 50 hydraulic roller cam with 9.5 to 1 compression with my old proform 650 vacuum secondary I had .034 primary and .040 secondary which was out of box that way but that was with a idle air bleed of .070 and my current holley 650 double pumper I ended up about .037 (.003 more then out of box) primary and .040 secondary but that is with a idle air bleed of .078 which is leaner then what my proform had so that is why there is a difference between the two for the ifr size on the primary block and it works in relation with the air bleed for metering things on the idle circuit and transition circuit.

On my quick fuel 680 vacuum secondary carb its a 4 corner idle so it is not set up the same as the 2 corner carbs but I do have a quick fuel slayer 600 vacuum secondary carb sitting at the side for a spare and on my milder 350 with 9:1 compression and a 219/227 @ 50 with it set up as a 2 corner idle out of box I had .031 primary and .040 secondary with a .070 idle air bleed and it was a little rich but did not keep it on for too long to tune the idle circuit leaner as I wanted a bigger carb and its a long story but probably would have ended up with a idle air bleed of around .078 and it would have been good.

That is what the average base line for carbs recommended for cams in the 230 and over range till you get into the 240 plus then I can't honestly say what the base line would be but would not be too much more for a street motor. This is not etched in stone as every engine even if similar will need more or less then this and camshaft specs and timing and cylinder head flow and compression all play a part in a tune.

Specs below is out of box setting so you can get a idea of comparison between brands.

Holley 650 double pumper stock specs
.077 to .078 idle air bleed primary .050 secondary idle air bleed
.030 to .031 high speed air bleed
67/73 jetting
.034 primary idle feed restrictor .040 secondary idle feed restrictor

Proform 650 vacuum secondary
.070 idle air bleed primary .039 secondary idle air bleed
.034 primary idle feed restriction and .040 secondary idle feed restriction
70/76 jetting

Holley 670 vacuum secondary street avenger
.078 primary idle air bleed and .048 secondary idle air bleed
.025 high speed air bleeds
.028 to .031 primary idle feed restriction ( depends on metering block) .031 secondary idle feed restriction.
65/68 Jetting
So basically all performance oriented carbs have around a .034" Idle restrictor on the primary side? Does the Street Avenger Have a secondary IFR, and if so will it require modification as well? Sorry, I didn't bust down the secondary bowl and I'm too lazy to take it off and look for myself at this moment lol.

Wait.. nevermind. It's a two corner idle carb.
 
#28 ·
http://www.summitracing.com/parts/hly-0-82750/overview/

^I'll probably end up slapping one of these on one day if this damn 670 keeps up with its shenanigans. Anyone on here have any good luck with this carb? Is it a good choice for my given application? I've kinda been weary of the 750CFM for my 355 due to the general rule of thumb (C.I. x 2 = CFM).. hence why I chose the 670 for my application.. keeps me on the lower side of the two (both 40 CFM low or high of my C.I. X 2 formula {lol formula}) to keep the lower range crisp due to my street driving usage.
 
#29 · (Edited)
I have never found anyone really into hot rods and performance who is also pretty savvy with Holley carbs who feels that the "formula" is right....most agree the "formula" figure plus 50 to 100cfm is the place to be.

This is the closest one I've found, most all of the calculators at this site are good ones. You may want to bookmark or favorites list this site
Wallace Racing - Intake CFM Requirement

2 corner idle carbs still have an idle feed restrictor, it feeds the constant idle discharge holes in the baseplate.....Holley's have this to keep the fuel fresh in the rear bowl even if the secondaries don't get used for a while.
 
#30 ·
One of the main reasons why I sold my holley is I wonder if it really flows what they state on that particular carb. I am by no means an expert when it comes to this sort of thing but I know the size of the booster in relation to the throttle bore size and the design of the tower all play in hand how much cfm it will truly flow.

The holley 600/670 vacuum secondary carbs models1850,80457,80670 share the same tower boosters and same base plate and primary metering block. The booster size is 1.250 primary and 1.312 secondary and has 1 9/16 throttle bores.

Then you look at the holley 650 double pumper I have it has 1.250 primary boosters and 1.312 secondary boosters but it has a 1 11/16 throttle bore which will allow more flow. I figured my milder 350 engine just needed a little bit better of carb and I bought a quick fuel hotrod series carb rated at 680cfm with vacuum secondarys for my build and it has 1.312 primary and secondary boosters along with the bigger 1 11/16 throttle bore and when I put it on wow what a difference it made over my old 600/670 holley carb.

The 3310 holley 750 vacuum secondary has the 1 11/16 throttle bores as well but even bigger boosters then the other carbs and that is how it gets its 750 flow rate.

In my old Demon carburetor book Demon even has a article on flow rating a carburetor and they used what is called a wet flow rate and that holley uses a older dry flow rate but I have not found much on it as far as that stuff goes.

The holley 670 street avenger and 600 carbs I had still ran well after spending a lot of time tuning and buying better upgrades for parts wise for them but in the end I would have been better off buying my current quick fuel 680 carb as it is the best bang for the buck for a street carb as it has 4 corner idle along with screw in air bleeds and screw in idle feed restrictors and screw in power valve channel restrictions and they are made quite well and don't really cost more then the holley street avenger.

I like you don't want to over carb my engine and honestly both of my builds could use a 750 cfm rated carb just fine but I like my slightly better throttle action and strong signal I get with my 650/680 rated carbs for street use. I once ran a 750 and it did not have as much on the bottom end for signal and crispness but was just a hair more on the top end but that was after 5500 rpm and not really that much and we are talking less then 10 horsepower at best.

I get 19mpg with mine and I don't take them past 6000 rpm anyways and never race or take them to the track so I don't need every last ounce my engines can give me and I get excellent with what I already have so why change it.

With some time and changes you will be able to bring that carb to life its just finding its sweet spot and allowing your engine to get what it wants from this with what it can offer. Took me some time but was well worth the effort once I was done.
 
#39 ·
One of the main reasons why I sold my holley is I wonder if it really flows what they state on that particular carb. I am by no means an expert when it comes to this sort of thing but I know the size of the booster in relation to the throttle bore size and the design of the tower all play in hand how much cfm it will truly flow.

The holley 600/670 vacuum secondary carbs models1850,80457,80670 share the same tower boosters and same base plate and primary metering block. The booster size is 1.250 primary and 1.312 secondary and has 1 9/16 throttle bores.

Then you look at the holley 650 double pumper I have it has 1.250 primary boosters and 1.312 secondary boosters but it has a 1 11/16 throttle bore which will allow more flow. I figured my milder 350 engine just needed a little bit better of carb and I bought a quick fuel hotrod series carb rated at 680cfm with vacuum secondarys for my build and it has 1.312 primary and secondary boosters along with the bigger 1 11/16 throttle bore and when I put it on wow what a difference it made over my old 600/670 holley carb.

The 3310 holley 750 vacuum secondary has the 1 11/16 throttle bores as well but even bigger boosters then the other carbs and that is how it gets its 750 flow rate.

In my old Demon carburetor book Demon even has a article on flow rating a carburetor and they used what is called a wet flow rate and that holley uses a older dry flow rate but I have not found much on it as far as that stuff goes.

The holley 670 street avenger and 600 carbs I had still ran well after spending a lot of time tuning and buying better upgrades for parts wise for them but in the end I would have been better off buying my current quick fuel 680 carb as it is the best bang for the buck for a street carb as it has 4 corner idle along with screw in air bleeds and screw in idle feed restrictors and screw in power valve channel restrictions and they are made quite well and don't really cost more then the holley street avenger.

I like you don't want to over carb my engine and honestly both of my builds could use a 750 cfm rated carb just fine but I like my slightly better throttle action and strong signal I get with my 650/680 rated carbs for street use. I once ran a 750 and it did not have as much on the bottom end for signal and crispness but was just a hair more on the top end but that was after 5500 rpm and not really that much and we are talking less then 10 horsepower at best.

I get 19mpg with mine and I don't take them past 6000 rpm anyways and never race or take them to the track so I don't need every last ounce my engines can give me and I get excellent with what I already have so why change it.

With some time and changes you will be able to bring that carb to life its just finding its sweet spot and allowing your engine to get what it wants from this with what it can offer. Took me some time but was well worth the effort once I was done.
You have your engine(s) built for the same exact usage as I have. Fun street usage up to 6000 RPM. So realistically the 670 is pretty much just a 600? lol looks like one day I'll get that quick fuel 680.. here in a bit I'm gonna hop on summit and check it out.

Oh and FYI, drill bits got in. I measured the IFRs out to be .029".. went ahead and drilled up to .031". Haven't gotten a chance to test it out yet as I'm still waiting on my new pressure regulator to get in. My summit brand one just died on me, and I even tried changing the spring and diaphragm but that didn't help. So ordered a quick fuel regulator and I'll get to slapping that on tonight.
 
#31 ·
Have you tried the brown or white or pink accelerator pump cam?

I've always found the brown works the best with any performance engine, stock seems to like pink or white.

I've never drilled a Holley in my life, everytime I get one thats been tampered with it will never work properly again.

Holley used to have a tuning guide on their website thats now gone, basically it said.

Get the mains sizes right, get the secondary sizes right, get the pump cam right and reset the idle mixture after as is said and done.

Sounds too simple to be true but every Holley I have ever tuned I did this way and ran perfect, you need to read plugs to do it but no drilling required.

Of course you need the performance carb for the performance engine, trying to get a stock vac sec to work on a cammed engine doesn't work...calibration is wrong.

I would think twice before drilling a Holley for anything, they are usually junk after. Thats why I would never buy a used one, someone got lost and took a set of drill bits to it and she's done.

Think twice before you do it, I've yet to see it work out the way it was envisioned.
 
#33 ·
I agree this is all good basic advice but all for a few things...

#1 - A brown cam will not work with the stock 30cc accelerator pump, as it is a 37cc cam and needs to be used with the 50cc "REO" pump.

#2 - Holley isn't God, they don't magically make a carb for every application, and I believe their tech line is actually staffed by the Devil, because they don't know crap about their own carbs....that's why there are so many custom carb companies out there.

#3 - Once you get the basic jetting, pump cam/nozzle changes, secondary spring tuning and it still feels like something is not performing the way you like, it is time to get out of the manual and into the custom calibration mindset.

I can understand your point about a used Holley and hacked up, but there is really nothing that can't be fixed if you are a more veteran tuner. Drilled bleeds are not a big problem....you just replace them back to stock. Harder to do with old carbs and press-in bleeds, you either have to send it back to Holley and pay them outrageous money, or convert it yourself to screw-in bleeds. For that reason most all but the very basic performance Holleys now come with screw-in bleeds and orifice restrictors, makes swapping an easy thing to do.

If you're a guy who likes/wants to run vacuum secondaries on their street/strip cars, then you need to be prepared to modify them. Educate yourself as much as possible and confer with others of the same mindset, buy a couple of the better performance tuning books available...or buy from a custom carb tuner and not straight from Holley.

I've drilled on Holleys a lot, only one of about 85+ I couldn't get to do what I wanted was a reverse idle circuit #7001-2 Spread bore vacuum 4175 model....but that's an emissions idle circuit carb and I was very early in my learning curve, it was the first Holley I ever owned. I wouldn't even attempt to misapply it's application today, I'd just get a different carb. I eventually used up most of the parts except the metering blocks off it to repair swap meet low-buck parts missing carbs.

I've got one friend (10-second street/strip car) with a 780cfm Holley VS I did for him with a replacement HP main body and all my "tricks"(now 810cfm), he got crazy and replaced it with an $900 775/875/995 RaceDemon with all the fancy removable sleeves and adjustable everything....car is no faster with it than the upgrade swap meet $250 invested modified I did.

This is Hotrodders.com, I thought hotrodders are guys who do modify stuff to fit their needs. :mwink:
 
#32 ·
I agree 4 jaw chuck on some things but I have never had to hack any of my holley's up to where they were junk by any means. All I had to do on my old holley 600 and 670 street avenger carbs was recalibrate the idle circuit a little and it was nothing major to make it work really well. They just have a leaner idle circuit and I don't think there is nothing wrong with recalibrating things if you know what your doing as that is part of fine tuning.

I only had to drill mine out just a hair to get my idle circuit and idle mixture screws and it running good below 2000 rpm and a minor jet change and it ran really strong with no issues. That's why quick fuel and other brand style holley carbs come with screw in idle feed restrictors, power valve channel restrictors and air bleeds so you can really fine tune the carb for your application.

Jet changes won't do anything for a idle circuit and pump cams and shooters is a whole different tune as well and all have to work together to make them run good. I always tune my idle circuit first then the pump shot and shooter combo to get throttle response nice clean and crisp then once that is done I go to jetting and reading plugs and how the engine performs.
 
#36 ·
Chuck, I agree with you, never, ever drill jets because you can't control both the length of the lead-in and exit chamfers when you drill the jet bigger so you have no idea what the jet actually flows when it is modified....just buy those.

Bleeds on the other hand are just a simple drilled hole. Accelerator pump nozzles are the same way,

I would never try to make a true performance carb out of a 600 Vacuum either, any motor mild enough to only need a 600 vacuum doesn't need more carb or custom work.

The 670 Avengers everybody seems to be unhappy with can be tuned up if your stuck with one, I have no problem trying to help/guide someone on the ins and outs of the various circuits and how to modify/fix the lean problems everyone reports, because it is near the same work the 735/750/780/855 Vacuum Secondaries need for more serious performance use. Seems like info around the web points to all the Avenger line being overly lean, and maybe/likely folks are purchasing them for the wrong applications....but Holley Advertising and Tech are doing them no favors by pushing these or telling you to spend another $2-300 on the HP or Ultra HP lines in order to get something usable....it's all a profit game for them.

I respect you're line of thinking, you're a straight-up guy, and your advice is what the majority of carb buyers should follow. Hope there is no animosity here. I'm not attacking you, even if the verbiage might read that way sometimes.

I just try to help the guys asking for it, on things I have figured out. I'm the guy who has never bought a new Holley because it pisses me off that they charge roughly $35-50 more for each 50 cfm increase in the size of their carbs, when an 850 or 950 costs them no more to make than a 600 does..... they are just de facto punishing the guys who need the larger carbs, I find that unfair and rather rotten.

Buying a used or swap meet carb isn't for everyone, and I never one buy blind(like Ebay). I want to at least look over closely what I'm dealing with beforehand, and weed out the real trashy looking stuff. It's just all in what you have experience with, I'm the go-to guy in my crowd on carbs, intakes and heads, and rear ends. Others I hang with have their areas of knowledge and interest, and it works out for us in the end.

I'm sure you're the same way, you've got your special interest also. :) It's not the tool, but the artist, who creates the work. :cool:

I feel like I've hi-jacked this thread a bit too much now :spank:
 
#37 ·
I'm the guy who has never bought a new Holley because it pisses me off that they charge roughly $35-50 more for each 50 cfm increase in the size of their carbs, when an 850 or 950 costs them no more to make than a 600 does..... they are just de facto punishing the guys who need the larger carbs, I find that unfair and rather rotten.
. LOL! Eric, same for tires... those rubber band sized performance tires cost 4 - 10 times what a real tire costs for a pickup truck...!!!

. Yeah, if I was building an engine like the OP's/HeavyCHevy's, a 7,000 RPM solid cam 400-450 HP engine (prolly limited by the Eddy RPM heads), I'd just slap a Holley 750 on it and be done with it... maybe some slight re-jetting needed... get one for $150 from a local swap... BTW, where's that mountain of $50 Holleys... I'll take a truck load of them...

. Harbor Freight has number size drill bits usually for $9.99/set... may not be good enough for drilling steel, but should drill brass or aluminum... and, yeah, drilling jets leaves swirls inside them that can slow fuel flow...

. That size cam is going to lope good at idle, so don't try to jet it smooth/pig bog rich... just let it lope...

. Surging at idle can be vacuum advance... advancing and dropping back as vacuum changes with lope... may need a different vacuum advance can that stays above idle vacuum... or use ported vacuum...

. May also need a different ignition mechanical advance curve...

. WHAT heat range sparkplugs are you using? That may be part of melting electrodes...

. Also, if vacuum secondaries opening to soon, that creates a mid-RPMs LEAN condition... that's what all the popping is about... and could also be damaging the plugs...
 
#38 ·
All is good Eric, your advice is good...I just want to the voice of reason that says sell it and get a 750 LOL!

Carburators man, thats what life is all about.:D
 
#43 ·
According to Holley the 670 street avenger has different boosters then the 600 but I had both carbs right beside each other and even in there specs book they are identical except the street avenger has a rear metering block and and center hung fuel bowls and a quick change secondary.

Both towers I measured with a digital caliber and everything measured exactly the same and the boosters are straight and look and measure identical. When I got the metering block conversion kit for my 600 to replace the metering plate it is the same exact rear metering block on the 670 street avenger. Part numbers all match.

I think with holley its a marketing thing as holley has 570,670,770 and quick fuel has 580,680,780 lol but I honestly do believe the quick fuel carbs do flow closer to what they rate theirs at. They each though might use slightly different methods on measuring flow as well so it could just be a difference how holley measures air flow on the street avengers compared to the other 600 and 750 rated vacuum secondary carbs which has been around longer then there street avenger lines.

In all points and purposes I believe the 770 is the same as the 750 holley vacuum secondary just with a rear metering block and the vacuum quick change as it has the same specs as the 770.

Honestly its not really that big of a deal as we are talking maybe 2 to 4 hp if that between a 600 and 650ish range for carb cfm.

All I had to do after I got my timing set right after an error on my part on my quick fuel was lean the idle circuit out some as it was way richer out of box then what I had my old holley tuned at and its a 4 corner idle and a minor jet change and it was done but with its bigger 1 11/16 throttle bores and bigger primary boosters it I know would flow more cfm then the old holley and I know I did not really pick up anything horsepower wise but my throttle response picked up quite a bit.

It has down leg boosters so what I have read they are less restrictive then straight leg boosters. I basically wanted a true 650ish rated carb and even holley's 650 80783 vacuum secondary has the 1 11/16 throttle bores compared to the 670 street avenger's 1 9/16 throttle bores and that 1/8 inch bigger size in each bore I know would equate to way more flow.

Even the quick fuel 600 vacuum secondary slayer carb only has 1 9/16 bores. O well I am just rambling on here. Outside of a little better throttle response and a little better acceleration on my engine if the holley street avenger had all the adjustability like the quick fuel I would have kept it but I like to be able to really fine tune my carbs and like to be able to change each circuit as needed instead of saying well its adjusted as best as it can be.

If they would have had the hotrod series quick fuel carbs out years ago when I got my holley's then I would have gotten them instead as they are pretty close in price. Good luck on your tuning and keep me posted as I am hoping it all works out good.
 
#44 ·
Haha that's funny because that's the whole reason I got this damn 670. I wanted to upgrade from the 600 cfm 4160 I had.. but didn't wanna shoot for the moon and go overboard with a 750. Oh well. S**t happens.. I'll just tweak the hell out of this 670 and run it! After all, it is just a street motor anyway... not like that's stopped me before lol.
 
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