Decel "popping"/backfire?

TeeJayHoward · 07-06-2022, 09:59 AM

'81 3.0L SC w/ PMO carb conversion. AFR @ idle set to 12.5.

I've noticed that when I'm decelerating and in-gear, the car has what sounds like a steady backfire. It happens about once a second, and appears to be independent of engine RPM - It'll happen at the same frequency regardless of if I'm slowing down at 5000RPM or at 3000RPM. If I put the car in neutral or use the clutch, the sound goes away. If I'm accelerating, the problem doesn't occur. I've noticed the same problem, at the same intervals, at both 35MPH and 55MPH, so it's not speed dependent, either.

What the devil is going on? It's not like it's preventing me from driving the car, but it is altering the way I'm doing it.

pete3799 · pete3799's Garage

Checked timing?

VFRPeter · VFRPeter's Garage

An air leak that only occurs under deeper vacuum (decelerating) but not at tickover (coasting in neutral). Could be in the inlet or exhaust system.

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proporsche · proporsche's Garage

backfire to what carbs or exhaust?

1979-930 · 1979-930's Garage

Excess fuel getting sucked through the engine due to higher RPM and not getting burned from lack of oxygen (throttle closed). The fuel is hot in the exhaust and once some air mixes with it (Bang).
So yea a small exhaust leak or just the air in the muffler. It's not an issue if it's a minor pop/burble.

If it's a loud bang. The butterflies on the carbs could be set too far open at idle and then the idle mixture is probably set rich to compensate. I had a carb way out like that one time. Some times people struggle with getting them to idle and set the linkage too far open to compensate for too rich on the settings. Best to go back to zero with the butterflies and start over on adjustments.

MrBonus · 07-06-2022, 01:30 PM

My 2.7 has Webers. When I purchased the car, I switched from a tired old Dansk muffler to a much sleeker M&K unit. However, upon changing, I kept getting a BAP BAP BAP at around 3000 RPMs under deceleration that occasionally would become a backfire. I checked the plugs and timing, nope.

Welp, sure enough, I left a few of the new bolts on the exhaust slightly loose and after tightening them up, it's a very sexy overrun/burble rather than an embarrassing series of .22 shots.