# Lean AFR on deceleration / engine braking



## cij911 (Oct 25, 2017)

I have been playing with timing for a while now and seem to have one problem that is troubling me - car goes lean on engine braking. AFRs are fine everywhere else - idle, part throttle, wide open throttle. Car is a tri power.

Example 1: I run the car hard through first, second , and third gear (up to 5K rpm) and then take my foot off the gas and just let the engine brake and/or use the brakes at the same time (basically butterflies closed), the AFRs spike to ~18:1 and I get backfires / pops. This can be immediately remedied by pushing in the clutch (rpms drop to idle and AFRs are perfect).

Example 2: Going down a hill in second or third gear and using engine braking to maintain speed, the car will backfire / pop. AFRs are again lean.

Is this an example of too small primary jets in the center carb? Thanks in advance


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## GTO44 (Apr 11, 2016)

The back fire and pops are one of my favorite things about decel in my late model gto lol.

It’s normal to see the gauge go lean right after letting off. Butterflies close, back to strong vacuum signal, no additional fuel, temporary lean condition.

If you’re worried or concerned about the popping, up the primaries a couple sizes. The primaries are the only thing supplying fuel when you let off after hammering down.


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## geeteeohguy (Feb 2, 2008)

I agree. Or exhaust leaks can cause popping on deceleration. Your condition of full lean at high vacuum and closed throttle is totally normal. Physics in action. No cause for concern. I would be checking for exhaust manifold leaks myself.....


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## cij911 (Oct 25, 2017)

geeteeohguy said:


> I agree. Or exhaust leaks can cause popping on deceleration. Your condition of full lean at high vacuum and closed throttle is totally normal. Physics in action. No cause for concern. I would be checking for exhaust manifold leaks myself.....


Hmmm...So why would the AFR be fine at idle? Wouldn't the exhaust leak cause a backfire at idle and lean conditions?


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## PontiacJim (Dec 29, 2012)

I can't recall if you have vacuum advance or not. At 5,000 RPM, your mechanical advance is fully in. IF you do have vacuum advance, it is possible that when you are letting off the gas quickly and snapping shut the throttle blades, the engine vacuum is building up instantly and adding the vacuum advance to full mechanical and the timing is over advanced at that moment. Maybe pull and plug the vacuum advance/line and try it without it and see if it still does this.

Second thought is that one of your valves is adjusted too tight. You may not have a problem at lower RPM's, but at the upper RPM's with those valves really working and the lifters pumped up to max, it could be that a valve is being held slightly open. As I recall, you have all new valve springs(?). A weak valve spring could also be the problem - valve bounce.

Next, don't let off the throttle completely and let the carb throttle blades snap closed, rather, ease off the gas, but not completely off. Does it still do it? Could be a carb or carb adjustment issue even though you have good AFR readings.


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## Lemans guy (Oct 14, 2014)

Back your base timing off 4 degrees and try it and see if it still backfires....


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## Lemans guy (Oct 14, 2014)

Cj what is your idle AFR?


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## cij911 (Oct 25, 2017)

PontiacJim said:


> I can't recall if you have vacuum advance or not. At 5,000 RPM, your mechanical advance is fully in. IF you do have vacuum advance, it is possible that when you are letting off the gas quickly and snapping shut the throttle blades, the engine vacuum is building up instantly and adding the vacuum advance to full mechanical and the timing is over advanced at that moment. Maybe pull and plug the vacuum advance/line and try it without it and see if it still does this.
> 
> Second thought is that one of your valves is adjusted too tight. You may not have a problem at lower RPM's, but at the upper RPM's with those valves really working and the lifters pumped up to max, it could be that a valve is being held slightly open. As I recall, you have all new valve springs(?). A weak valve spring could also be the problem - valve bounce.
> 
> Next, don't let off the throttle completely and let the carb throttle blades snap closed, rather, ease off the gas, but not completely off. Does it still do it? Could be a carb or carb adjustment issue even though you have good AFR readings.


Heads are old...no idea on valve adjustment or spring condition...I'll try the ease of the deal trick and see what that does. Thanks




Lemans guy said:


> Back your base timing off 4 degrees and try it and see if it still backfires....





Lemans guy said:


> Cj what is your idle AFR?


Base timing is ~8-10* & total advance is ~38* (vacuum advance is 10*). AFR at idle, when warm, is ~13-14:1. Thanks


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## GTO44 (Apr 11, 2016)

Soooo forgot to ask... is the popping/backfire thru the exhaust or thru the carb? 2 totally different situations there...


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## Lemans guy (Oct 14, 2014)

That AFR is good at idle, 38 degrees advance some cars can take others not..

Back the base off 4 degrees just for the test, run it up and decelerations see about the backfire, if gone or better or no change then you can go from there. All carbs go lean when the throttle slams shut, but lean relative to the timing advance could be the culprit.

Of course as the guys said, exhaust leaks, carb jets and full throttle mixture. When you are going 5,000 Rpm all carb circuits are engaged,....idle and transfer slots a little they never shut off, primaries and secondary jets and PVCR for the power valve, like mini jets...

So your mixture should be like 12.2 neighborhood....you could be real lean there as well.

One final thought is that idle dashpots were designed to keep throttle from slamming shut....they worked off vac and electric and there were various kinds, it just eases throttle down in such a condition....some cars did not have others did, and they have aftermarket ones also...


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## cij911 (Oct 25, 2017)

GTO44 said:


> Soooo forgot to ask... is the popping/backfire thru the exhaust or thru the carb? 2 totally different situations there...


Exhaust.



Lemans guy said:


> That AFR is good at idle, 38 degrees advance some cars can take others not..
> 
> Back the base off 4 degrees just for the test, run it up and decelerations see about the backfire, if gone or better or no change then you can go from there. All carbs go lean when the throttle slams shut, but lean relative to the timing advance could be the culprit.
> 
> ...


Sorry I don't completely understand. Would disconnecting the vacuum can (pulling 10 degrees) be even a better test? I do believe I have a small exhaust leak (Doug's headers) at the head to flange. I've tried to eliminate the exhaust leak, but suspect it is still present (very small).

Thanks


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## Lemans guy (Oct 14, 2014)

I think last time you had a vac can pulling more than 10 degrees, I don’t recall the results if you disconnected it....did the backfire stop

Recommended a 10 degree can which you now have...that is right so if timing now too much it is from total timing not vac advance..

Try 34 total....

When you throttle down vac advance snaps up and adds 10 and RPM’s drop and reduce centrifugal advance...they are crossing one drops one increases....

Most guys have too much vac there...you fixed that....now tweak the total

Fix the exhaust leak as well...and you may need a litttle riches up


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