My sons car is setup with a 355 small block chevy. The timing on the car is set at an overall of 35 degrees. When he is on the high way crusing and he takes his foot off of the gas, there is some detonation in the headers and exhaust untill he puts his foot back on the gas. It only doesnt while driving, not when the engine is reved in park. Is the problem his overall timing. I am not sure what size cam is in the engine, but it is big.(very little vaccume of idle). It has a 750 carb and high rise team G intake. Any information as to what may be his problem will be greatly appreciated. Thanks Kevin

What do you mean by "timing is set at an overall of 35 degrees"? Are you saying that WITHOUT the vacuum advance, he has 35 degrees of advance at or below 3000 rpm? Then the vacuum advance adds some additional? If so, timing is not the issue.

First thought I have is that you have an air pump connected to the exhaust, and the diverter valve is shot.

first easy thing to check is for an exhaust leak. When there is an exhaust leak when at higher rpm's and you let off the gas it will run a little ruff and backfire some times, just an fyi. I had it happen to my 350.

Originally posted by malibuman400:
first easy thing to check is for an exhaust leak. When there is an exhaust leak when at higher rpm's and you let off the gas it will run a little ruff and backfire some times, just an fyi. I had it happen to my 350. Thats what I would suggest too. I had the same thing happen with my 72, it turned out one of the bolts in the headers had come loose and when I would get up around 65-70mph and let off the gas I would get a pop....pop pop. I would check all the bolts on your headers and see if that doesn't fix it.

I believe MalibuMan and Brandon are right on this one. Last summer I was smashing the gas on my 1966 Malibu, which has a 355, and when I let off the gas, POP,POP,POP. I got back to my garage only to find a header bolt about to fall out. Tightened it up and never had a problem again.

later, graemlins/thumbsup.gif

I had the same problem and it turned out to be a crack in one of the headers. It pulled in just enough air to make the exhaust gases ignite.

Check for leaks.

Tom