Popping noise on decel!

[vette70cdg]

I am not sure when it started, but each time I use the 4 speed trans to decelerate my 1970 454/390, especially in first gear, there is this continous popping sound out the tail pipe. I have tried re-jetting the carb towards more lean, no change. I thought it was back-firing, but it does it when the engine is hot or cold. I thought maybe unspent fuel. It has done it with two different Quadrajet carbs. Engine timing has been checked, don't remember what the numbers are. Any ideas?

 

[pws69]

I am not sure when it started, but each time I use the 4 speed trans to decelerate my 1970 454/390, especially in first gear, there is this continous popping sound out the tail pipe. I have tried re-jetting the carb towards more lean, no change. I thought it was back-firing, but it does it when the engine is hot or cold. I thought maybe unspent fuel. It has done it with two different Quadrajet carbs. Engine timing has been checked, don't remember what the numbers are. Any ideas?

If you have a leak in the system up close to the headers, that can cause this.

 

[Remo]

Chris,

That popping noise is generally associated with uneven exhaust pressure between the left and right sides of the system. Most manufacturers use an H-pipe (located just past the headers) to help mix the exhaust in an effort to balance both sides, smooth out the flow and reduce the popping sounds. Recently many owners have started replacing the H-pipes with X-pipes. The X-pipe design does a better job of mixing the exhaust pressures between the left and right sides. One of the results is the reduction of that irratating popping sound.

Remo:cool

 

[Vettehead Mikey]

If your smog system is still intact, the diaphragm in the diverter valve may have ruptured. This will force air to be pumped to the manifolds almost continously and will give you the popping sound you mention.

 

[vette70cdg]

I have heard of these exhaust presesures before. I don't understand why they are occurring within the last 2 years. I finished restoring the car 8 years ago and don't recall the noise back then! 1970 big blocks do not have any emmisions hardware at all. No smog pump! I have checked for exhaust leaks. Car was restored to original stock condition.