Hi all, new member here. Looking forward to interacting with you all.

I have a Series 1 240z that is mostly original CA car that was mechanically restored by Rod Schmitt a few years ago. Recently I’ve been having problems with back firing upon deceleration. The only thing I can think of that I have done which may have contributed to this was to install a new MSA exhaust (2.5 inch).

Anyway, I contacted Rod who suggested I remove the belt from the air pump to see if that helps. It did a little but she’s still popping off some. I have since rebuilt the original carbs and removed the air pump altogether and verified it is still working, which it does appear to be. Valve clearance and timing set at factory specs.

Could anyone please help suggest what could be causing this problem? Could it be a faulty anti-backfire valve? If so, how can I check it?

Thanks in advance.

Mark

If there are sharp hot parts in the exhaust. They can set off unburnt fuel.

You could experiment with a different muffler or try to eliminate the richness on decel

Thanks. Ive tried leaning out the mixture which doesn’t appear to help much

Leaning out might be be going the wrong way.

The anti-backfire screen in the PCV hose isn't for exhaust backfires.

When you say "backfire" do you mean one big pop or a string of small pops? Seems like you're also suggesting that there were no backfires before installing the MSA exhaust system.

More a series of small pops. I had not noticed it before installing the exhaust. Could that be the culprit? Or is it more a matter of tuning as a result of the less restrictive exhaust? Removing the belt from the air pump definitely helped.

Thanks for your help!

I was thinking of the wrong thing when I mentioned the PCV system. That is the flame arrester.

You might be on to something with the anti-backfire valve. I don't know how to check it, but it appears to open the large passage when vacuum is applied to the small tube. You might check the hoses and see if the valve opens correctly.

Thanks. I’ll take it off and inspect it.

EC section of the FSM. I would not be surprised if your car lacks the parts listed on EC-7. In operation they prevent the butterflies from slamming shut, thus limiting the unburned hydrocarbons in your exhaust.

See if easing off the gas instead of just lifting your foot off the throttle reduces/eliminates the backfire.