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eric32 05-14-2013 02:25 PM

Holley off idle backfire problem
 
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

33Willys77 05-14-2013 02:47 PM

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?

eric32 05-14-2013 04:22 PM

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.

TommyK 05-14-2013 05:14 PM

Go up 3 jet sizes on the primary and see if it gets any better.

eric32 05-14-2013 06:12 PM

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.

Richiehd 05-14-2013 06:13 PM

Quote:

Originally Posted by TommyK (Post 1675573)
Go up 3 jet sizes on the primary and see if it gets any better.

When I read the initial post I was going to say 2 sizes.

cobalt327 05-14-2013 07:18 PM

Quote:

Originally Posted by eric32 (Post 1675579)
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.

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.

66GMC 05-14-2013 07:38 PM

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
http://holley.com/data/products/pict...um125-65v2.jpg
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.

eric32 05-15-2013 08:36 AM

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.

cobalt327 05-15-2013 10:27 AM

This fix ain't gonna take any custom one-off parts, I can assure you. Add squirter size. Fine tune w/the cams.

toddalin 05-15-2013 01:22 PM

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.

http://www.largescaleonline.com/eima.../Dyno_Pull.jpg

eric32 05-17-2013 12:18 PM

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

toddalin 05-17-2013 01:57 PM

I run 69 primaries and 74 on the secondaries. I had 67s in the primaries and would get a lean surge at light cruising. The 69s cured that.

eric32 05-17-2013 04:51 PM

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.

oldbogie 05-17-2013 06:24 PM

Quote:

Originally Posted by eric32 (Post 1676400)
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.

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


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