Lean AFR on deceleration / engine braking - Pontiac GTO Forum
User Tag List

 4Likes
Reply
 
LinkBack Thread Tools Display Modes
post #1 of 12 (permalink) Old 05-07-2019, 07:58 AM Thread Starter
 
Join Date: Oct 2017
Posts: 668
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 162 Post(s)
Lean AFR on deceleration / engine braking

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

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie
cij911 is offline  
Sponsored Links
Advertisement
 
post #2 of 12 (permalink) Old 05-07-2019, 09:19 AM
 
GTO44's Avatar
 
Join Date: Apr 2016
Posts: 350
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 45 Post(s)
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.

2006 GTO M6

Motor - Stock with Vararam
Bunch of suspension
Bunch of brakes
Stock tune

365/368 std
12.81 @ 110.6 2.03 60'
GTO44 is online now  
post #3 of 12 (permalink) Old 05-07-2019, 05:45 PM
64-67 Expert
 
geeteeohguy's Avatar
 
Join Date: Feb 2008
Location: Fresno, California
Posts: 8,561
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 119 Post(s)
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.....
cij911 likes this.
geeteeohguy is offline  
 
post #4 of 12 (permalink) Old 05-07-2019, 07:00 PM Thread Starter
 
Join Date: Oct 2017
Posts: 668
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 162 Post(s)
Quote:
Originally Posted by geeteeohguy View Post
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?

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie
cij911 is offline  
post #5 of 12 (permalink) Old 05-07-2019, 07:57 PM
 
PontiacJim's Avatar
 
Join Date: Dec 2012
Location: Gastonia, NC - Born & raised in Connecticut - 31 years
Posts: 3,739
Mentioned: 1 Post(s)
Tagged: 1 Thread(s)
Quoted: 404 Post(s)
Garage
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.
cij911 likes this.
PontiacJim is offline  
post #6 of 12 (permalink) Old 05-07-2019, 08:18 PM
 
Lemans guy's Avatar
 
Join Date: Oct 2014
Location: Nashville, Tennessee
Posts: 766
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 79 Post(s)
Back your base timing off 4 degrees and try it and see if it still backfires....
cij911 likes this.
Lemans guy is online now  
post #7 of 12 (permalink) Old 05-07-2019, 08:24 PM
 
Lemans guy's Avatar
 
Join Date: Oct 2014
Location: Nashville, Tennessee
Posts: 766
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 79 Post(s)
Cj what is your idle AFR?
cij911 likes this.
Lemans guy is online now  
post #8 of 12 (permalink) Old 05-08-2019, 08:58 AM Thread Starter
 
Join Date: Oct 2017
Posts: 668
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 162 Post(s)
Quote:
Originally Posted by PontiacJim View Post
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


Quote:
Originally Posted by Lemans guy View Post
Back your base timing off 4 degrees and try it and see if it still backfires....
Quote:
Originally Posted by Lemans guy View Post
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

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie
cij911 is offline  
post #9 of 12 (permalink) Old 05-08-2019, 09:19 AM
 
GTO44's Avatar
 
Join Date: Apr 2016
Posts: 350
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 45 Post(s)
Soooo forgot to ask... is the popping/backfire thru the exhaust or thru the carb? 2 totally different situations there...

2006 GTO M6

Motor - Stock with Vararam
Bunch of suspension
Bunch of brakes
Stock tune

365/368 std
12.81 @ 110.6 2.03 60'
GTO44 is online now  
post #10 of 12 (permalink) Old 05-08-2019, 09:29 AM
 
Lemans guy's Avatar
 
Join Date: Oct 2014
Location: Nashville, Tennessee
Posts: 766
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 79 Post(s)
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...
Lemans guy is online now  
Sponsored Links
Advertisement
 
Reply

  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, LeMans & GTO Engine Tuning and High Performance

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Pontiac GTO Forum forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in











Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Similar Threads
Thread Thread Starter Forum Replies Last Post
Afr problem Nz66gto 1964-1974 Tempest, Lemans & GTO General Discussion 9 05-30-2016 03:15 PM
Tires rub on deceleration? Poolshark1321 Wheels and tires 4 07-22-2009 12:04 PM
05 w/AFR 225 heads and cam wildchildm39 Engine Discussions 14 09-18-2006 09:54 PM
AFR 205s/EDC Cam = 112 rwhp gain GTO1_OHIO Engine Discussions 12 04-20-2006 08:28 PM
Better Braking with a Pad change only! Xman Service, Maintenance and Technical Discussion 7 12-05-2004 02:07 AM

Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome