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Discussion Starter · #1 ·
Hello everyone:

I'm back again with more issues! Just scratching my head over this whole delema and I'm about out of ideas.

I'm having a hard time trying to find the root of this problem.
Basically the title says it all... it has a small miss every so often at part throttle cruise. Changed plugs thinking they were gunked up from when the engine was first fired up and the rings weren't seated yet.... that didn't fix the problem. Then I metered my plug wires IAW Taylor Cable's specs for ohms per foot and it checked good... then i changed my ignition module.. and then I recurved my distributor from 18 initial 36 total all in by 2200 because I found shiny specs on the porcelain :0. Now I'm at 14 initial, 34 total all in by 3200... still missing. So I tried backing off vacuum advance a few turns at a time until eventually I just completely disconnected it. Still missing.... not AS bad, but it still misses. I don't think it's anything mechanical per se, because I just finished a rebuild on it and it only has about 500 miles on it. Jets are at 78/82, and when I read the plugs, they don't look to be carbon fouled due to a rich condition... so I'm kinda lost here. I even adjusted my transfer slots to the .040" Holley recommendation. Any ideas?
Should I try backing spark plug gap back to about .040"?
Could it be a gas quality issue?

`355 SBC
`10.25:1 SCR
`156 cranking PSI according to the wallace DCR calculator
`Flat top pistons @ .040" quench height (.025" deck heigh with .015" fel-pro shim style head gaskets)
`Autolite 3924 plugs gapped to .050" (High volt coil)
`Comp Cams 282s cam
`Performer RPM heads and intake
`Holley 670 carb running 93 octane pump gas
`Coolant temp runs between 170 and 190 depending on the OAT and whether I'm in traffic or not.
 

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Close plug gap to .035"...all you are doing with a big plug gap is forcing the electricity to try to find an easier way out, not creating a bigger spark like you may think.

Carb may be too lean on low speed and transition circuit also, but close the plug gap first.
 

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Hello everyone:

I'm back again with more issues! Just scratching my head over this whole delema and I'm about out of ideas.

I'm having a hard time trying to find the root of this problem.
Basically the title says it all... it has a small miss every so often at part throttle cruise. Changed plugs thinking they were gunked up from when the engine was first fired up and the rings weren't seated yet.... that didn't fix the problem. Then I metered my plug wires IAW Taylor Cable's specs for ohms per foot and it checked good... then i changed my ignition module.. and then I recurved my distributor from 18 initial 36 total all in by 2200 because I found shiny specs on the porcelain :0. Now I'm at 14 initial, 34 total all in by 3200... still missing. So I tried backing off vacuum advance a few turns at a time until eventually I just completely disconnected it. Still missing.... not AS bad, but it still misses. I don't think it's anything mechanical per se, because I just finished a rebuild on it and it only has about 500 miles on it. Jets are at 78/82, and when I read the plugs, they don't look to be carbon fouled due to a rich condition... so I'm kinda lost here. I even adjusted my transfer slots to the .040" Holley recommendation. Any ideas?
Should I try backing spark plug gap back to about .040"?
Could it be a gas quality issue?

`355 SBC
`10.25:1 SCR
`156 cranking PSI according to the wallace DCR calculator
`Flat top pistons @ .040" quench height (.025" deck heigh with .015" fel-pro shim style head gaskets)
`Autolite 3924 plugs gapped to .050" (High volt coil)
`Comp Cams 282s cam
`Performer RPM heads and intake
`Holley 670 carb running 93 octane pump gas
`Coolant temp runs between 170 and 190 depending on the OAT and whether I'm in traffic or not.
Shiny specks on the porcelain are a strong indication of preignition. This is often the result of a lean mixture. The hesitation is also another indication of that.

Since this is intermittent it might be the fuel pump doesn't always keep up which may be a filtration issue.

I wouldn't increase the plug gap maybe even pull it back to .035.

I'd recommend trying to richen the mixture a little. This could be raising the float level to larger primary main jets or a power valve that comes on sooner, but given the occasional nature of this that would be my first response.

Coolant temp of 170 seems low especially if this is running an air gap. They have a habit if too cold and RPMs are low of puddling fuel that fouls out a cylinder when it breaks loose. This fuel separation can actually make the engine appear to be running lean most of the time.

Bogie
 

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Discussion Starter · #5 ·
Shiney specks on the porcelain are a strong indication of preignition. This is often the result of a lean mixture. The hesitation is also another indication of that.

Since this is intermittent it might be the fuel pump doesn't always keep up which may be a filtration issue.

I wouldn't increase the plug gap maybe even pull it back to .035.

I'd recommend trying to richen the mixture a little. This could be raising the float level to larger primary main jets or a power valve that comes on sooner, but given the occasional nature of this that would be my first response.

Coolant temp of 170 seems low especially if this is running an air gap. They have a habit if too cold and RPMs are low of puddleing fuel that fouls out a cylinder when it breaks loose. This fuel seperation can actually make the engine appear to be running lean most of the time.

Bogie
So does this mean I should try richening mixture or leaning the mixture? The reason I ask is because you recommended richening it a bit, but then your later statement implied I am already running rich?? Or that coolant temperature is too low? I agree with you, 170 is low, especially with the 180 thermostat I'm using.
 

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Discussion Starter · #6 ·
Later tonight I'm going to close the plug gaps to .035", and when I do so I'm going to try and get a decent picture of the spark plug I just put in 50 miles ago to check for preignition specs. I was skeptical of preignition, that's why I backed off timing and gave a more modest timing curve.
 

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So does this mean I should try richening mixture or leaning the mixture? The reason I ask is because you recommended richening it a bit, but then your later statement implied I am already running rich?? Or that coolant temperature is too low? I agree with you, 170 is low, especially with the 180 thermostat I'm using.
Richen the mixture.

The latter input is a situation where the mixture can be spot on (or a bit lean) but because of a cold intake fuel droplets that are thrown to the outside when the flow must turn can form puddles. The mixture actually does read lean but now and then a puddle is sucked off to one cylinder or another causing a miss fire. Then things go back to a little lean and the evidence on the plug insulator burns off till next time.

Bogie
 

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How would I go about richening the transition circuit??? Accelerator pump cams and nozzles?
Idle passage feed restrictor needs to be enlarged, or low speed (idle) air bleed made smaller.

Accel pump cam and nozzles are only adjusted when there is a stumble or bog when slamming throttle wide open.

Carb typically runs on idle and transition circuit at cruise, maybe on the main jet if rpm is high enough (generally above 2500-2600 rpm to run on main jet at cruise)
 

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Discussion Starter · #10 ·
[/ATTACH]

These are the plugs. From the looks of it, I need to go up one heat range due to the build up on the threaded base (idle circuit is only 1.25 turns out), and it would appear it's running slightly lean.
 

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Discussion Starter · #11 ·
I checked float levels and they were good, maybe a LITTLE low and only needed the float levels backed out one flat on the adjuster nut. Fuel pressure was a tiny bit high at 7 psi, so i backed it off to 6.5, and I closed plug gaps to .035" and jumped up to 80 jets on the primary. Tomorrow I'll see how it reacts.
 

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Discussion Starter · #13 ·
Hey everyone, just here to update. As it turns out I must of just been running rich. The 80 jets on the primary dropped operating temperature about 5 degrees, and when I got on it in 2nd gear for about 5 seconds, I noticed temperature drop a significant amount.. making me think the sudden *** load of cold fuel dropped operating temperature. I checked the plugs and they were already showing color on the porcelain after about 15 miles.. So I dropped jetting back to 76 primary 80 secondary, and now it's closer to operating temperature, and the weird hiccup seems to be almost completely gone.
 

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If your running a holley 670 street avenger then your jetting is way off as it comes out of box with 65/68 jetting and that carb is the same as the old holley 1850-80457 600cfm vacuum secondary carbs but just with center hung bowls and a rear metering block and vacuum quick change secondary as I have had both carbs and they are identical and use the same primary metering block and tower and base plate. Those particular carbs are way leaner compared to a double pumper or performance calibrated carbs and you might have to modify the idle fuel curve for it to run with that big of a cam.

I am running a way smaller hyd roller lunati voodoo cam270/278 219/227 @50 515/530 lift with a 112 lobe separation angle so I have way less overlap and duration then what you do and that holley would never run out of the box even with jetting upped some. The idle circuit was too lean and I had backfire city and I had to open up the idle feed restrictors .001 at a time till I got it right. I had like you a constant misfire at light cruise but 2000 rpm and up it ran fine. I ended up .031 on the front and the rear's are already .031 but my cam is smaller then yours and I also have 17 inches of vacuum in park. I would rejet it to 65 to 67 front and 72 to 74 on the rears as that would be more in the range that carb is calibrated for.

Don't go just drilling things out as once you change it too big then you will either have to get new metering blocks are have them tapped for screw in restrictors. If I was you I would get a book on tunning holley carbs and understand how each circuit works with each other before even trying to change anything. That cam is going to need plenty of timing at initial as well. My hydraulic roller does not like anything less then 16 initial and I have about 18 or so but I also only have 15 degrees mechanical so I don't get into pinging issues.

The holley 670 carb is calibrated for very mild engines with high vacuum and small cams and I would say they work out of box best for cams with 220 @50 or less as I had to always modify them to work correctly on my small blocks that have had bigger cams then that.
 

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Discussion Starter · #15 ·
Yeah trust me, I had the same problem with it too when I first got it out of the box... misfiring and popping all over the place. I did run 68/74 jetting on it for a while, but that was back before the rebuild and I had lower compression. Now I'm at 76/80, and I'll probably run it for a while and see how the plugs look, and chances are I'll have to drop it back some. I still haven't had a chance to check my secondary jetting.. I usually just run secondary 4 sizes larger than primary for whatever reason haha.

What are you talking about drilling out the idle bleed restrictions? How/what does this do to help the performance of the carburetor? That might be what it needs to fix the idle on this thing. Idle is rich even with the idle bleeds adjusted where I need them. It has a rough surgey idle... not too bad, only fluctuates about 50 or 70 rpm or so (bounces between 950 and 1000 in park, and bounces between 700 and about 770 in gear with my foot on the brake)

Yeah, I need to screw with my timing curve agian. For a while I had it at 18 initial, 36 total, all in by 2000, but I was getting light preignition at low RPM due to the radical advance curve. It really liked the 18 initial though. Now I have it set at 14 initial 34 total all in by 3200. Preignition is gone and it drives just fine, plenty of power, just maybe not quite enough initial for it when it's in gear with my foot on the brake.
 

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On the primary and secondary metering blocks there are called idle feed restrictor's and they control the amount of fuel that flows for you idle mixture and they work hand in hand with the idle air bleeds to give your mixture at idle and off idle before the main jets kick in. Below is a article for you to read and get familiar with some basics of things.

The only thing adjusting the idle mixture screws does is allow more or less fuel flow but does not affect the fuel curve of the carburetor idle and transition circuit itself for the most part but there is a science on how things work on all the different circuits on the carb and how they all have to be able to work together.

I would not go drilling anything yet till you get your timing in good order first and also do your homework before doing anything as that really effects everything on how a carb will tune.

I sold my holley street avenger and 600cfm carb for a quick fuel which has everything adjustable for screw in bleeds etc. The stock holley idle feed restrictors on the 670 street avenger are .028 to .031 on the primary metering block and the secondary block is .031. As for example for comparing the two carbs when I got my quick fuel 680 VS carb it came with .034 primary and secondary idle feed restrictors and .070 idle air bleeds and that would make a richer fuel curve compared to the holley 670 which has .078 primary idle air bleeds and .048 secondary idle air bleeds. Since the holley is a 2 corner idle it has richer idle air bleeds compared to the primary ones. Mine is also 4 corner idle as well so the secondary idle air bleeds is the same as the front.

Messing with them two areas is very tricky and just a slight change can make a big difference for better or worse wise for a tune. I had to lean out my quick fuel carb cause it was so rich when I installed it and would puff black smoke out my exhaust. I went down smaller on the idle feed restrictors till I got a nice clean idle but yet not to lean to make it stumble or surge while cruising.

I have read countless posts and have 5 holley tuning books and learned from many on here so I know more then just a basic tuner but I am not an expert either. I just have learned to familiar myself how everything works on them. Below is two good links to help you out with understanding things more.

Engine Tuning Guide - Carburetor Track Tuning For Speed - Car Craft Magazine

Carburetor Tuning the Scientific Way
 

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Also one last thing is to make sure your timing is dead on. I recently marked my distributor before I loosened it up to remove my cap and take out my control module to switch over to a msd box and I bolted everything back up and put it right where I marked my distributor and thought my timing should be right on and after chasing my tail so I thought to recheck it even though I marked it and even though it was about right were it was before I was still off on my timing about 10 degrees and after giving it just about a 1/8 of a turn advance to what my engine likes and a lot of my tuning issues went away pretty quickly and I got better adjustment out of things and got rid of a lot of my cruising surge issues. I still had to lean out my carb on my idle circuit as well and also after getting the idle mixture screws adjusted correctly she runs really awesome now. Good luck on getting things working good.
 
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