Backfires on deceleration

[rw11]

I mostly notice this at about 5500 rpm, in 3rd, coming down from redline or near it. This is with the throttle all the way closed (and the throttle plates look all the way closed with the engine stopped).

This is a twin-plugged modifies 3.2L with PMO carbs & a not too hot cam. The sound is sometimes sharp & loud, and at other times somewhat muffled.

I'm not sure if it is running rich or not - or what else might be causing this problem. Any ideas?

Thx much!

[Steve]

Hi,

Four things can cause this:

1) Too rich

2) Too lean

3) Late ignition timing

4) Exhaust leaks


You will need to check AFR's to determine #1 or #2. Without knowing what your jetting is, I am unable to offer anything further.

[rw11]

So it is definitely not normal with carbs?

Can you recommend an AFR Meter? I have a place on the SSIs to screw in an O2 sensor.

[Steve]

[rw11]

Thanks. Is the Basic LM-2 suitable? (the one that retails for $305).

I also found some documentation for the PMO carbs on it:

40 mm PMO carbs

“inserts” are marked 36

size?? vents

main jets 145

idle jets 55

air correction 200

F-11 emulsion tubes

[Steve]

The LM-2 is fine.

Without knowing precisely what your cams are and your CR, I cannot offer much about jetting.

I would recommend taking your car (and a handful of different main jets and A/C ones) to the dyno to measure and set the WOT AFR's. This helps prevent holes in pistons.