Hey fellas, I bought my first rx7 a few weeks ago. It's a 1979 that is completely stock. The motor is eventually gonna need a rebuild but since it is still relatively new to me I want to put a few miles on it first.

Question is: The thing backfires when I let off the gas. So basically everytime I shift I get a loud pop. The problem seems to get better once the car is warm. A lot of time after the backfire I get a trail of smoke out the exhaust for a couple seconds after. I did change plugs, checked the wires, made sure there was no arcing, etc. I have searched the forums and went through the troubleshooting in the FSM but I cannot find where to go next trying to troubleshoot this. Maybe I just need to deal with it until I get the engine rebuilt? Any advice/input is greatly appreciated.

 

congrats have fun with it, you have now learned don't run it in the garage nor in front of your BMW with the plastic bumper lol

 

Maybe start by getting a wideband installed to see what your air/fuel is like.

 

lol.

The number one cause of backfiring is holes in the exhaust system. When you let off the gas, fresh air is sucked into the exhaust through those leaks, then detonates whatever is remaining of the fuel. Patch the holes, and the problem will go away.

Congrats on your new toy. Enjoy.




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Sort of like Kentetsu said, it is most likely a little bit of unburnt fuel in the exhaust that is getting burnt off when you let off the gas. That is probably what the brief smoke is from.

 

It is more than likely your emissions control system acting up. If you live in an exempt area you could remove it and see a decrease in your Backfires.

 

A bad air control valve will do the same thing.

 

And the thermal reactor exhaust will always backfire a little anyway. It's just part of the charm. Fix any exhaust leaks and it will be minimal, however.