I recently had side pipes and headers installed on my 74 Corvette. After driving it around I have noticed when I let off the gas at speeds above 50mph I get quite a bit of back firing. I was wondering if anyone knows what could be the cause of this? I am very much a novice when it comes to mechanical knowledge and will answer questions to the best of my abilities. Thanks in advance for any help or advice.

 

My money is on an exhaust leak at the header gasket or a lean misfire.

 

If you opened up the exhaust, installed headers, and reduced backpressure without re-jetting the stock carb, you are now running lean, and the popping you are getting (which is called "afterfire" if it happens in the exhaust) is caused by a lean misfire when you close the throttle. Richen it up.

Lars

 

I didn't adjust the carburetor after installing the headers. How would I go about making it richer?

 

What Lars said, but I'll add one more.

I had this exact situation with my 80 when I put on sidepipe headers: Afterfire when letting off the throttle at speed (4-speed, so I could put it in neutral to reduce the noise). I tried adjusting the idle mixture screws, and that helped a bit. But what really did it was moving the distributor vacuum advance back to the manifold vacuum port of my (stock) Q-Jet. I had previously done the Lars-style ignition tune, so I don't know what happened to that vacuum line, but the car ran much better, and the afterfire went away.

 

I think that might be a bit advanced of a solution for my mechanical knowledge. I think I am going to try adjusting the carburetor. I'll be honest I don't really know anything about the vacuum system in the car.

 

I sent you a PM. I only posted my anectdote because I had exactly the same symptoms, and 90% of the solution turned out to be correct timing. To do both the carb adjustment, and the ignition timing, I followed the instructions that @lars wrote!

The vacuum line to the distributor vacuum advance should be a single hose running from the (correct) manifold vacuum port to the vacuum can on the distributor. It couldn't be easier! But please read up before you adjust timing or the carb.

 

I had the same issue, lean misfire when decelerating

 

You have to disassemble the carb, re-jet it for the headers (usually 2 jet sizes larger than stock), re-set the APT height of the power piston, verify correct float level, and then re-set idle mixture once it's re-assembled and running. That's part of the job involved when switching out stock manifolds with headers. Making sure the timing curve is correct, with vacuum advance on manifold vacuum as outlined in my papers, is also of significant benefit.

Lars

 

Lean decel, afterfire or whatever you want to call it is popular with the younger crowd. My vette only burbles alot on decel with the side mount exhaust but my CTS-V with a catback exhaust likes to pop on decel quite often. On newer cars, lean decel can be tuned out by richening the fuel mixture on deceleration or it can also be tuned in to make it pop. Someone can correct if I'm wrong but I read that Chev actually tuned in decel pop in the C7 performance exhaust mode.....because alot of people think its just plain cool. Imo, unless it bothers you, don't worry about it.

 

Exactly - Re-jetting the carb is good for power because of the headers.

You bring up a good point - Fixing the lean misfire is cured by the idle mixture enrichment and added ignition advance that can more easily fire the (hard to fire) lean mixture. Carb butterflies snap shut at high RPM while the engine keeps pulling harder than normal for air on the butterflies. Vacuum goes VERY high pulling in way more air around the butterflies than it would normally see at idle (where you have set the mix for a normal everyday "idle sized" amount of vacuum). The idle circuit "as adjusted" is unable to meter/feed enough gas to ensure good mixture for proper combustion at very high vacuum levels. Mix does not burn fully and gets exhausted into the manifolds where it lights off making a "bang" or burbling sound. Extra manifold vacuum advance (vs ported vacuum advance) will help insure the lean mix gets burned better during the combustion event rather than an afterfire in the exhaust. As you noted, putting the car in neutral provides a normal amount of vacuum signal to the carb and then no afterfiring.

Makes me want to try a bigger spark gap here too. That is why the HEI used bigger spark gaps were used in the first place. The mid 70's lean factory emissions calibrations REQUIRED a bigger plug gap and that required more spark energy to light off those harder to fire "extra lean" emissions calibrations.