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Backfires on deceleration
Posted:
Wed Sep 23, 2015 9:18 pm
by 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!
Re: Backfires on deceleration
Posted:
Thu Sep 24, 2015 11:12 pm
by 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.
Re: Backfires on deceleration
Posted:
Fri Sep 25, 2015 4:57 pm
by 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.
Re: Backfires on deceleration
Posted:
Fri Sep 25, 2015 6:40 pm
by Steve
Re: Backfires on deceleration
Posted:
Wed Sep 30, 2015 11:24 am
by 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
Re: Backfires on deceleration
Posted:
Wed Sep 30, 2015 10:47 pm
by 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.