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Holley off idle backfire problem

42K views 41 replies 12 participants last post by  cobalt327 
#1 ·
Hello guys sorry to post here again but I need some pointers on what to do next.
I have checked everything and there are no vacuum leaks and ignition system is all new and all has been checked along with timing. I am running plenty of initial timing with this cam so its not the issue as was checked before even messing with the carb.


First thing engine 350 sbc high rise dual plane intake dart shp 180cc aluminum heads 9 to 1 cr lunati voodoo hydraulic roller 219/227@50 515/530 lift.

The carb I am using is a holley 600 vacuum secondary with a metering block conversion on the rear. So its jetted 65 front and 72 rear. Float levels checked etc.

First problem I had was the transfer slot being opened up to much but got that solved and is now correctly at about .20 to .030. with the secondary's opened as well and are almost equal with the primary's. Next problem has was getting it to idle smooth but always quit on me so it seemed very lean.

From reading on setting idle mixture screws and idle circuit tuning I got the symptoms mine was way lean and these holley 600 and street avenger carbs are known to have a very lean idle circuit.

I had to also open my primary metering block idle feed restriction from a .027 to .029 cause the idle mixture was lean and having idle mixture screws out almost 1 3/4 turns and turning them in more then a 1/4 of a turn killed the engine and making them richer made no difference so I needed to get more adjustability so after opening the ifr on the primary metering block it worked better and I was able to get it to adjust more so using a vacuum gauge they both sit at 1 1/4 turn out.


Problem I am having is if you are at a dead stop and gently ease into the throttle its fine and if your cruising and your easy getting into the throttle its fine but if you give it a quick shot it will sometimes stall or it will back fire and from reading holley's tech stuff it indicates its too lean.

So I bought a shooter kit and I went from the stock .031 to a .035 and it helped some but still get the back fire if giving it a big shot. So changed the orange pump cam to a blue pump cam to give a bigger longer duration shot and it still did not help either on the number 1 or 2 position.

I called holley tech and they about suck as they said they don't know why its doing this and asked if I went to a smaller shooter and I said how would that help as its a lean backfire? He said to run ported vacuum advance and not full but that makes no difference here as it still does it.

Vacuum advance it limited to 12-14 degrees and from the whole debate of ported vs full manifold setup it does it on either one but from what I get with bigger cams they like full manifold more then ported.


Normally going to a bigger shooter solves this problem and my pump arm is set correctly and has instant shot. I don't have funds at the moment for getting a 02 sensor installed etc so please don't be mean and say get one cause if I could I would but trying to just do one thing at a time so in order to not mess things up.

Any ideas on what to try next? Please be kind guys I am trying to learn.
Thanks all
Eric
 
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#2 ·
What is your timing actually at? What is the curve on the distributor. I would try locking the timing first (since you indicate you have a 'big' cam) at 34-36 advance and see what that does. What are the cam specs? Is it possible you are getting too much advance? What do your plugs look like?
 
#3 ·
hello the cam is not that big as I put the at 50 spec but numbers all around the cam is 270/278 219/227 @ 50 515/530 lift with a 112 LSA and the timing is set at approximately 18 initial and 36-38 all in by 3000 rpm. The mechanical advance does not start till 1200 rpm and gets to total timing at 3000-3500 rpm.

Using less timing the thing does not like it and adding more then where its at does not make it perform better. I have another chevy 350 with a much bigger cam then this with a 650 vacuum secondary carb and it idles and runs excellent with the same timing so I know that is not a factor here. I don't need locked out timing for something like this.

Being easy on the throttle its fine its just when you punch it fast from a dead stop to half throttle or if your cruising down highway and you open it to 3/4 throttle or so and if your easy its fine but hit it fast and it just sputter's or backfires. The carb works excellent all the way around except that one problem and plugs are good and tan color so jet size is pretty close but this has to do with the transition circuit somehow.
 
#5 ·
I am going to put my orange pump cam back on and back to the 31 squirter and start from scratch again. I am going to jet up as it seems to have a faint surge at 55 mph and I have read that many of the holley vacuum secondary carbs whether its the 600 or the 750 or the street avenger series all have these problems when used with anything bigger then a stock motor and small cam. If I had the bucks I would get a wideband sensor and litter bigger quick fuel carb which would be a little bit better for my combo but I have what I have and need to make it work. I read up on tuning with a cheaper 02 shortband sensor but I get mixed results from others who use them to tune a carb. It runs fine all the way around just that backfire if you nail it to quick. Once and if I can get rid of it I am finally done with this thing as everything else is good on it.
 
#7 · (Edited)
It's prollably the accelerator pump but be sure the secondaries aren't trying to open too soon.

A smaller shooter will extend the pump shot duration. That could help if the engine were to hit hard then lay down shortly after. Changing the pump cam might help some but if the pump shot really is lean (and this isn't a rich bog- and I believe you know the diff), going up in shooter size will show an improvement regardless of the pump cam.

If you're depending on a lot of vacuum advance at idle (manifold vacuum sourced), losing that when the vacuum drops at the hit of the throttle can cause a pop through the carb if the combo is off.
 
#8 ·
Personally, I don't think that changing main jets will make any difference as a lean backfire is a TRANSITION problem.

Accelerator pump and discharge nozzles are the first stage of "off-idle" transition. The power valve is the second stage ... providing additional fuel until the venturi boosters start drawing fuel through the main jets.

In your situation, I'd be looking at the power valve. You might want the High Flow single-stage "picture window" style

versus the dual-stage one with the small round hole.

Read this

These are available in different vacuum ratings.

As far as smaller squirters as recommended by Holley tech ... they may be trying to extend the duration of the shot, rather than the volume?

It sounds like you have a "Street Avenger" carb?
List #80570 or #80670 ?

Check the carb numerical listing to see which power valve your carb came equipped with. I believe that each power valve is also stamped with it's "rating" if you want to confirm what you actually have.

Also, being that this is a vacuum-secondary carb with a "quick-change" secondary throttle spring ... is it possible that the current spring might be too light, and allowing the secondaries to open too soon?

That would also cause a lean condition, as airflow through the secondary booster venturis may not be sufficient to draw up any fuel. I recall having friends that used to actually put a screw into the secondary throttle cam ... forcing the secondaries to open manually. This is NOT a good idea, for the reasons stated above.
 
#9 ·
Hello guys my carb is the 80457 600 vacuum secondary but it has the exact same metering blocks and parts the holley 670 street avenger has. I have a 670 on the side and they are basically the same carb and the only difference is the 670 has slightly different booster size that's all but metering blocks along with the tower and base plate are the same part number.

Anyways I already have a 6.5 high flow power valve and the secondary's I have a quick fuel vacuum secondary pod on the rear with a purple spring and you can adjust its rate by just turning a screw and mine comes in around 2000 rpm and if your on the highway and you smash it you can tell when they start to open and it causes no bog.

I know the off idle or pedal smashing bog is lean cause it back fires and if your at a dead stop and if you punch it with the smaller shooter on it sometimes just quits. I am going to put my orange pump cam back on and then put a .037 shooter on and see how it does.

If that does not fix it them I am going to have to save some coin and see if I can get a carb shop to make me a custom set of metering blocks to better work with my application as the holley vacuum secondary carbs have a reputation of having this problem.

Just do a google search and type in holley street avenger off idle stumble and you will see what I mean. Thanks for tips guys will try just a few minor changes and see how it goes.
 
#11 ·
I run a 600 dp so it is not exactly the same. The carb's PO could not get it to work right. It had a stumble and would backfire, much like you allude to.

The PO had changed the stock shooter (which is a 25) to a 31 chasing the problem thinking, as most people do, that it was a "lean bog."

Dyno testing showed the opposite and it was a rich bog cause by too large of a pump shot. I believe this lets fuel puddle in the manifold that eventually backfires. I changed the squirters to 21 and that cured most of the problem. I then moved the pink cam from position 1 to 2 on the secondaries to get a smaller pump shot and that seems to have done the trick.

BTW CarCraft had a similar problem with their 600 dps and filled the 25s with epoxy and drilled them out to 22s before realizing that Holley sold 21s. It fixed their bog too.

 
#24 ·
I run a 600 dp so it is not exactly the same. The carb's PO could not get it to work right. It had a stumble and would backfire, much like you allude to.

The PO had changed the stock shooter (which is a 25) to a 31 chasing the problem thinking, as most people do, that it was a "lean bog."

Dyno testing showed the opposite and it was a rich bog cause by too large of a pump shot. I believe this lets fuel puddle in the manifold that eventually backfires. I changed the squirters to 21 and that cured most of the problem. I then moved the pink cam from position 1 to 2 on the secondaries to get a smaller pump shot and that seems to have done the trick.

BTW CarCraft had a similar problem with their 600 dps and filled the 25s with epoxy and drilled them out to 22s before realizing that Holley sold 21s. It fixed their bog too.

not a holley guru, but didn't holley tech line basicly say the same thing?
and you brushed it off..
have you even tried a smaller shooter than the first one??
 
#12 ·
Hello thanks for the tip. In all manner it seems like its lean as when it was really bad it would completely stall at times. But I will put a smaller shooter on and see how it goes. I put the orange pump cam back on and in the number 2 spot where it was I have opened up my primary idle feed restrictor to .030 ( stock was about .028) and that got rid of all hesitation as the idle mixture was very lean and the mixture screws were 1 and 3/4 turns out and with my very slight adjustment they are now at 1 1/4 turns out and man what a difference. I have no more surging on idle transition cruise. I also went up two jet sizes to 67 and wow what a difference these few changes made. I still have the backfire on the occasion but not like before and when you get on it the throttle response is instant and is almost like fuel injection. I have to just get rid of that one thing then this tune is good to go. Thanks guys
Eric
 
#14 ·
Well besides jetting up I opened up my idle feed restriction's on the primary metering block and that alone got rid of my low speed transition circuit surge as jets don't affect that. I had slight power at 40 plus mph but did not go as good as it should and going up to 67 cured that and seems pretty spot on. I can now give it a quick shot and it will not backfire all the time. I am going to try just a few minor things like putting a smaller .028 (stock is .031) shooter on just to see and if its worse then I know it will have to go even bigger then .035. Had a suggestions above to try that so will try it. If none of these other changes work then I don't know what left to do. This does not need a 50cc pump shot and way big shooter as that is mostly for all out racing motors and this is nothing compared to those engines.
 
#15 ·
Check the metering plates for warpage, this being an old and somewhat odd Holley problem. It runs pretty well after reinstalling the plates especially with a new gasket then slowing falls lean with time.

I suggest that your cam is wilder than you're giving it credit for. This would be happier with more advance sooner. Camshaft, timing lead and Dynamic Compression Ratio and Static Compression Ratio by association with the SCR all walk hand in hand. An aggressive cam (define that by lift, this has quite a bit, and where in crankshaft degrees the intake closes) mixed with insufficient compression needs a lot of timing advance. As the compression ratio comes up the timing can be reduced and/or slowed down. One also needs to take into consideration the amount of overlap, that period where the intake and exhaust valves are open can result in strange coupling between the exhaust and the intake at different RPMs; this can be a real difficult tuning issue when the exhaust system is particularly efficient. The wave interaction into the intake can drag the carb a lot leaner than one would expect.

You can get away with a reduction of initial advance if you run the vacuum canister off manifold vacuum. This makes the engine easier to start and provides the benefit of more idle advance as soon as it fires up. You also have to watch foe excessive advance, what goes into the base setting and what comes from the mechanisms should not exceed an amount somewhere between 34 and 40 degrees. This total amount depends a lot on the compression ratio and the type of combustion chamber as well as cam timing and lift. Modern tight chambers burn faster thus need less timing advance than older design open chambers or even older closed chambers where the spark plug still tends to be poorly placed relative to the center of the bore.

The intake can also be a contributor, unheated and or large open plenums can cause the engine to appear to run much leaner than the jetting would cause you to suppose and at the same time may appear rich in the exhaust. This is because the mixture is not sufficiently atomizing leaving large globules of fuel that don't mix with the air and don't burn. Such a condition leaves the engine running like it's lean while tossing unburnt fuel out the exhaust. Again this is also related to compression ratio, ignition timing, intake closing point, intake and exhaust manifolding configurations, and certainly combustion chamber design and the amount of squish/quench generated between the piston crown and the step of the combustion chamber.

Bogie
 
#16 ·
Hello bogie I don't have a metering plate on the rear because I got the secondary metering block to replace it. The carb is brand new and the metering blocks are nice and good as I checked them with a straight edge. I am also using vacuum advance timing on full manifold ( about 30 at idle with vacuum advance). Holley said that was my problem and I tried it on ported and it did not make any difference. I like full manifold as it give better throttle response and nicer idle quality. I am running 16-18 initial and advancing it more makes no difference in how it runs. Cruising timing is around 50 degrees total and total timing is around 36 all in by 3000 rpm give or take a few. Using a lighter spring makes it come in too quick and one light and medium is still a little to quick and does not make much of a difference. I am using two medium springs and starts to advance at 1200 rpm. As of right now it has instant throttle response and just blipping the throttle will throw you back in the seat. If it was not for the backfire at times it's good to go.
 
#17 ·
Dont make the mistake of thinking you only have 1 issue. You could have a couple things hurting you. It seem you got the idle and cruise surge cleared up and these are both usually mixture related issues. The backfire on hard acceleration is almost always pump shot related and there is not 1 correct answer. I have fixed a lot of these type issues and it is usually a combination of the correct cam and shooter size.
When you changed the pump cam did you check the linkage to make sure there was no play in it. Most people miss this and it is important. Also when you go over a certain size shooter, the exact size escapes me at this time, you need to also change the shooter screw to the hollow one to allow more fuel.
The proper adjustment of the pump arm is critical. There should be no play between the arm and cam at idle and you need to be sure that at wot there is still some extra room for movement (.015)so you dont bend anything.

I have found the key to carb tuning is to change 1 thing at a time and on things like the pump arm adjustment. Mark it at base line and count turns when adjusting, this will allow you to go back to scratch if need be
 
#18 ·
Hello Tbucket I am only working on the accelerator pump circuit at this time as that's the last thing to get right. I have made notes of everything I have done with carb from stock setting. Trust me I know my way around a holley more then just a beginner. I have done hours and hours of research before I even do anything and one way is not always the right way as every engine is different. I have use the .035 shooter and it did not really help so I changed over the pump cam to a bigger shot with shorter duration and it still did not make a difference. I bought a holley shooter kit which has all the pump cams and shooter sizes so I have everything I need to tune.

Seems these holley's are known for having this issue on there vacuum secondary carbs mostly the 600 and street avenger series. And yes I always reset the pump arm when changing cams out. So far it seems to make no different if the orange cam is in the 1 or 2 position and the blue cam in the no 2 position. With the orange cam and the .035 shooter man it gets plenty of shot and its not a rich bog as it backfires.

I have had rich bogs before with other holley's and they never backfired just got a little sluggish and picked up. The backfire is not as bad as it once was and does not happen all the time just at certain times when you punch it quick.

From what I read when you go over the .040 size you have to use the hollow screw and the 50cc pump but man that is overkill and I should never have to use that.
 
#19 ·
just my 2 cents on this one, iv had this problem before. you will need a GOOD timing light, and a vacuum gauge. if its a holley double pumper put 72 primary jets in it and 73 in the rear for a higher rated cam. set your squirter cam back to the original position. the vacuum gauge needs to be hooked up to the intake manifold. fire up the engine let it idle till warm. start with your idle setting, typicall you want an idle of 500-800 rpms. than after you set the idle you want to set the idle fuel mixture. on either the left or right side of the metering blocks front and rear is a small recessed screw, start on the front metering block turn the screw in clockwise (right) until you hear the engine "stumble" then back out the screw until the idles smoothes out, repeat for the rear metering block. now your carb is set...for now.. with these setting you should be able to run the car and get rid of the popping and stalling if it persists you have to considerimproper valve adjustment, improper timing etc there are many many factors that will affect the way an engine runs
 
#20 ·
Hello this is not a double pumper but a vacuum secondary carb. I know its carb related cause everything else has been checked and set before any carb tuning was done.

I had a 600 quick fuel street slayer carb on this last year and it ran fine out of the box but the idle fuel curve was to rich and could only go 1/2 a turn on the idle mixture screws or it would be pig rich but unfortunately I had an accident with the primary metering block but quick fuel does not sell separate cast metering blocks so its on the side for now but you could smash the throttle or do what ever you wanted to and it ran fine with no backfire off idle just with this holley.

The quick fuel carb had also smaller air bleeds so it had a richer fuel curve and the idle feed restrictions were a massive .035 front and .040 in rear as to where my holley setup was .027 front and .031 rear and that makes a big difference on idle transition af ratio. I opened the holley ifr's on the front to .030 and left the rear block alone and it now has a way better idle and no more surging during low rpm driving but I don't want to go any bigger and mess up my block.

I have done hours and hours on the holley 600 vacuum secondary carb which shares the same metering blocks as the holley 670 street avenger and do a google search and you will see how many people have this problem with all sorts of engine combos and cam sizes. This is what I am stuck with so need to make it work.
 
#22 ·
I got a new set of metering blocks coming in the mail for my quick fuel carb. Its not the original blocks as they are not for sale by quick fuel but they are for vacuum secondary carbs 600-750 so I am going to try a few more things and if it does not work then I am putting it on and see how it goes. I have compared both carbs and the quick fuel carb tower has smaller air bleeds by a small number so it had not only a richer fuel curve but richer air bleed size as well. The idle was way richer then my holley but I never had any backfire problems or stumbles as it ran like a charm. Metering block got stuck and no matter how much I tapped it with a rubber mallet I could not get it off. So from user error I tried a small screwdriver to pry it off and well I ended up breaking the timed port tube off and gouged the block across the sealing line:nono: I learned that's not a good way to do this. Lesson learned. I can't make my holley air bleeds any difference as they are not screw in type but the quick fuel carb has all that stuff.
Eric
 
#26 ·
I am going to try a smaller shooter but I have not had a chance yet to do so. The metering block is fine as it was from my old carb and it did not have all these problems like I am having now. All I did was swap out the towers and they were both holley 600 vacuum secondary's I just kept the 80457 tower cause it has the electric choke and a new base plate. I sold my old one on ebay but for some reason it did not have all these problems like this one is. I will post back what I find out by going to a smaller shooter.
 
#28 ·
I am going to try a smaller shooter but I have not had a chance yet to do so.
Are you now thinking the hesitation is a too-rich condition? I think you know a smaller shooter will extend the shot duration, but won't help a too-lean at the hit of the throttle accelerator pump condition.

Too rich of an accelerator pump shot can sometimes be confirmed by having someone follow your vehicle to watch the exhaust at the hit of the throttle. Black smoke = too rich.
 
#29 ·
gearsheadlife said he had something like this before and to try a smaller shooter. It won't hurt anything as I can always go back to where I was. I am going to test it tomorrow when I have some time as I had stuff to do today. I went cruising up a small hill and the funniest thing happened I gave it a small easy shot of gas to go up the hill from a stand still and it backfired twice really fast and not the normal once like it usually does. Yes the gaskets are the correct ones as the only thing I changed out on the carb was the front and rear metering blocks and they have the correct gaskets. Base plates is whatever holley has on it and I have not had the bottom apart as it was all new. I have no vacuum leaks as the idle is nice and steady on the vacuum gauge and goes back and forth between 15-16 inches. I am about ready to take this thing and throw it over a fence. The previous carb was the exact same thing and it did not do this at all and I took the metering block from it and put it on this holley which is the same thing except it came with electric choke. This is driving me nuts as it seems its getting plenty of a pump shot and changing things really does not make any difference. Something is going on with the carb as previous one it did not do this backfire at all just when I put this one on:smash: I will post tomorrow of what it does changing the shooter does.
 
#30 ·
All right guys here is the final results. I went down to a .028 shooter and the backfire still happened so in order to rule things out as it no being enough I put a .037 shooter on and that still did not get rid of the backfire and it hurt throttle response as at that point it was getting too much shot.

Best results for throttle response is with the stock .031 shooter so that is what I put back on and it give a nice clean crisp throttle response. Now the backfire problem does not happen all the time just at certain times. I reved the throttle once and just barely touching the throttle it backfired in park so I don't know what else to check on this damn thing.

Shooter size is not the issue as nothing gets rid of it and I opened up my idle feed restriction on the primary block to .033 from .031. This is a older block and checking the new one I just got its .031 already so I did not make too much of a change to it and now hooking up my vacuum gauge my idle is a lot more stable since its not so lean and the idle mixture screws are at 1 1/8 turn out and I also checked my float level and they are at the half way point on the glass window so they are good.

Everything had checked out ok and I don't know what else to do with the dang thing. It runs very strong on 2000 and up and also cruises very good and idles good just the occasional backfire if you hit it really fast and hard. Even unhooked the pcv valve to hook up my vacuum gauge and did not make any difference so I know it is not an issue.

One interesting thing I tried is I put a towel over top the primary and the engine died so could that mean anything? Doing it by hand does not do anything.

Its had so many backfires it looks like a cannon that was shot. I checked the power valve and its still good.

Holley was no help and looking through all my books I did all the checking and tricks and don't know what else to do. Might take it totally apart and pull the base plate off and check for any casting flaws as I read someone had a off idle stumble but no backfire problem and he finally took his carb apart and he had casting flash in the throttle plate keeping the mixture almost blocked off but still run. O well back to the drawing board. Thanks folks for input.
Eric
 
#32 ·
Its not ignition related the distributor has its power straight from the battery through a toggle switch and I have a volt meter hooked up and its 13.5 to 14 volts and I also put all new plug wires and spark plugs in just a month ago and did not have any backfiring till I put this new carb on. Previous one did not have this problems like this. Distributor is all new as well with everything top to bottom so there is nothing else to check. I have already looked over everything and I also checked each wire with a ohm meter before I even hooked them up and they all were spot on and very close to one another. Timing has been checked and verified and tested with different settings and no difference in how this just happens. This thing is going to make me pull my hair out soon. High speed rpms are nice and good with no missing or anything. If I had money to buy a better carb and chuck this one I would but can't right now. I just paid over 1500 bucks for my heads just last month so can't afford stuff right now but I just don't know why this tiny but problem issue keeps rising its ugly head and no amount of carb tuning is changing it. Something is bad somewhere but its not constant only at certain times does it do it. I will have to see if I dig in some more but for now I don't know what else to check as have been through everything.
 
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