Popping when throttle is let off
Thread Starter
Instructor
Joined: May 2004
Posts: 127
Likes: 0
From: Aurora CO
Popping when throttle is let off
We just got done doing the top end on our 68 327. We had the valves seats done and new springs and the heads were ported. We put a Edelbrock 2701 intake on. The engine runs well but when you let off the gas you get popping noise like backfiring through the exhaust.. Any idea what can be causing this? Again its only when you let off the throttle
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Race Director
Joined: Nov 2000
Posts: 11,872
Likes: 12
From: Waterloo ontario Canada
Originally Posted by FeedVaal
I would think the opposite. Unburnt hydrocarbons igniting in the exhaust due to a too rich condition.
Melting Slicks
Joined: Feb 2003
Posts: 3,053
Likes: 4
St. Jude Donor '03,'11
There may be some confusion here.
There is a big difference between the pop pop pop when you let off the gas with a very low backpressure exhaust and the very loud bang of a real explosion in the exhaust created by raw gas mixture igniting in the exhaust system itself.
I have been experiencing the latter and i believe as Norval that the carb is putting fuel into the system with both throttle plates closed.
There is a big difference between the pop pop pop when you let off the gas with a very low backpressure exhaust and the very loud bang of a real explosion in the exhaust created by raw gas mixture igniting in the exhaust system itself.
I have been experiencing the latter and i believe as Norval that the carb is putting fuel into the system with both throttle plates closed.
Thread Starter
Instructor
Joined: May 2004
Posts: 127
Likes: 0
From: Aurora CO
Any advice on how to check or adjust the holley carb for this condition?
We were very carfull adjusting valve lash we went 5/8 past 0 lash on the valves. Could the valve lash be causing such a problem? Also I notice that if I let the throttle off slowly back to Idle it does not do it only when you run the engine up and then take your foot off. Seams to be more noticable from higher RPMs say above 2000 then it is below 2000.
The only other thing I notice is the vacuum seams to be higher now then before the rebuild. It will go to about 23 when i raise the rpm to 2500 to 3000 range and hold it there in the garage and at idle seams to be hanging around 15. These numbers appear to be sevel point higher then befor the Head work and new intake. Not sure if this higher vacuum would require the power valve to be changed. The other change we did was removed the headers and underbelly exhaust and put the original manifolds and original 69 side pipes back on.
As always thanks for the input
We were very carfull adjusting valve lash we went 5/8 past 0 lash on the valves. Could the valve lash be causing such a problem? Also I notice that if I let the throttle off slowly back to Idle it does not do it only when you run the engine up and then take your foot off. Seams to be more noticable from higher RPMs say above 2000 then it is below 2000.
The only other thing I notice is the vacuum seams to be higher now then before the rebuild. It will go to about 23 when i raise the rpm to 2500 to 3000 range and hold it there in the garage and at idle seams to be hanging around 15. These numbers appear to be sevel point higher then befor the Head work and new intake. Not sure if this higher vacuum would require the power valve to be changed. The other change we did was removed the headers and underbelly exhaust and put the original manifolds and original 69 side pipes back on.
As always thanks for the input
Last edited by MPSJM; May 21, 2005 at 01:28 PM.
Senior Member since 1492
Joined: Aug 2000
Posts: 87,942
Likes: 156
From: Just because I'm paranoid doesn't mean people aren't out to get me...
St. Jude Donor '09
I got it too. 
I don't think mine is due to a lean condition. It was popping when I was running a little rich and leaving black carbon deposits on my driveway where the side exhaust dumps. I lean the carb out a little and that seemed to help. I think I'm still running a little rich. Timing is around 20deg.
I think my side ehaust must be leaking some air into the side pipes. It's primarily on the passenger side. Same side that I was getting more carbon deposits.
Other than that, it runs like a raped ape.
I don't think mine is due to a lean condition. It was popping when I was running a little rich and leaving black carbon deposits on my driveway where the side exhaust dumps. I lean the carb out a little and that seemed to help. I think I'm still running a little rich. Timing is around 20deg.I think my side ehaust must be leaking some air into the side pipes. It's primarily on the passenger side. Same side that I was getting more carbon deposits.
Other than that, it runs like a raped ape.
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
Vacuum advance?
Originally Posted by MPSJM
Any advice on how to check or adjust the holley carb for this condition?
We were very carfull adjusting valve lash we went 5/8 past 0 lash on the valves. Could the valve lash be causing such a problem? Also I notice that if I let the throttle off slowly back to Idle it does not do it only when you run the engine up and then take your foot off. Seams to be more noticable from higher RPMs say above 2000 then it is below 2000.
The only other thing I notice is the vacuum seams to be higher now then before the rebuild. It will go to about 23 when i raise the rpm to 2500 to 3000 range and hold it there in the garage and at idle seams to be hanging around 15. These numbers appear to be sevel point higher then befor the Head work and new intake. Not sure if this higher vacuum would require the power valve to be changed. The other change we did was removed the headers and underbelly exhaust and put the original manifolds and original 69 side pipes back on.
As always thanks for the input
We were very carfull adjusting valve lash we went 5/8 past 0 lash on the valves. Could the valve lash be causing such a problem? Also I notice that if I let the throttle off slowly back to Idle it does not do it only when you run the engine up and then take your foot off. Seams to be more noticable from higher RPMs say above 2000 then it is below 2000.
The only other thing I notice is the vacuum seams to be higher now then before the rebuild. It will go to about 23 when i raise the rpm to 2500 to 3000 range and hold it there in the garage and at idle seams to be hanging around 15. These numbers appear to be sevel point higher then befor the Head work and new intake. Not sure if this higher vacuum would require the power valve to be changed. The other change we did was removed the headers and underbelly exhaust and put the original manifolds and original 69 side pipes back on.
As always thanks for the input
If u have higher vacuum at idle then maybe u could use more initial advance. Since the vac adv should already be full in from the previous lower vac adjustment with the same vac canister. U can change cans to match ur new idle vac but to get more adv u will have to add more initial adv.
And thats what vac adv is for - low speed/low load cruise or throttle back-down.
I check my vac can with a Mighty Vac brake hand pump that has a vac gauge. I pop the dist cap and watch the timing plate move as i increase vac. Mine is set for full vac adv at 15"Hg and none at 4&1/2-5"Hg.
Hope this helps.
cardo0
