Exaust backfiring - Pontiac GTO Forum
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post #1 of 65 (permalink) Old 04-04-2018, 04:03 PM Thread Starter
 
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Exaust backfiring

Hello,

I can’t seem to find the reason as to why my car back fires seem like a lean backfire. I checked my air fuel at higher rpms and its goes to about 12.5-12.7. When im at idle and reving it wont backfire but when im driving and i slam the throttle after about 3,000 RPM it starts backfiring like crazy and when i let go off the throttle i hear lots of pops from the exaust.

My timing is at 14 initial and 35 total.

I have a dual feed carb.

What do you think it may be?
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post #2 of 65 (permalink) Old 04-04-2018, 04:09 PM
 
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Hello,

I canít seem to find the reason as to why my car back fires seem like a lean backfire. I checked my air fuel at higher rpms and its goes to about 12.5-12.7. When im at idle and reving it wont backfire but when im driving and i slam the throttle after about 3,000 RPM it starts backfiring like crazy and when i let go off the throttle i hear lots of pops from the exaust.

My timing is at 14 initial and 35 total.

I have a dual feed carb.

What do you think it may be?
Are you logging AFRs when you slam the throttle ? My guess is you are running rich and then the unburnt fuel is exploding in the exhaust....

I am using a wideband to log AFRs right now and it has been interesting....

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie
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post #3 of 65 (permalink) Old 04-05-2018, 01:14 AM Thread Starter
 
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Hi ,

I was using an air/fuel reader by innovate. On my tesrlt run i was getting readings between 13-15 but as the rpms pass 3000rpm im at about 12.6-13.5, on my second test run , the clamp said goodbye lol, but for that run thats the reading i got. Im going to show you my spark plug , looks a bit lean.
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post #4 of 65 (permalink) Old 04-05-2018, 08:25 AM
 
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Originally Posted by TheGoatFather1965 View Post
Hi ,

I was using an air/fuel reader by innovate. On my tesrlt run i was getting readings between 13-15 but as the rpms pass 3000rpm im at about 12.6-13.5, on my second test run , the clamp said goodbye lol, but for that run thats the reading i got. Im going to show you my spark plug , looks a bit lean.
Well I am using the innovate wideband as well (LM2). Under throttle I am more like 12.5 - 13 range at the leanest. If you are seeing an AFR of 15 when WOT then you are tuned too lean IMHO. Optimal AFR is 14.7 (as I am sure you know), but there are a number of other factors to consider and with a carb I doubt you will ever be able to tune the car as precisely as a newer computer controlled fuel injected motor. I generally am happy if the car is at 12 - 13 AFR under WOT.

Obviously when you let off the throttle, your AFR will be much greater than 14.7. I believe I see greater than 20 IIRC while coasting.

Your plugs don't look "bad" to me, but you are definitely not running rich by the looks of them (syncs to your wideband readings). Please confirm when you are hearing the backfire.

Generally a backfire indicates you are running rich (as there is unburnt fuel in the exhaust that ignites) or you have a timing issue. (That said, it can happen if you are too lean as well.)

What is your vacuum at idle? Have you verified you don't have a vacuum leak? What is your AFR at idle?

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie

Last edited by cij911; 04-05-2018 at 08:44 AM.
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post #5 of 65 (permalink) Old 04-05-2018, 11:04 AM Thread Starter
 
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Originally Posted by TheGoatFather1965 View Post
Hi ,

I was using an air/fuel reader by innovate. On my tesrlt run i was getting readings between 13-15 but as the rpms pass 3000rpm im at about 12.6-13.5, on my second test run , the clamp said goodbye lol, but for that run thats the reading i got. Im going to show you my spark plug , looks a bit lean.
Well I am using the innovate wideband as well (LM2). Under throttle I am more like 12.5 - 13 range at the leanest. If you are seeing an AFR of 15 when WOT then you are tuned too lean IMHO. Optimal AFR is 14.7 (as I am sure you know), but there are a number of other factors to consider and with a carb I doubt you will ever be able to tune the car as precisely as a newer computer controlled fuel injected motor. I generally am happy if the car is at 12 - 13 AFR under WOT.

Obviously when you let off the throttle, your AFR will be much greater than 14.7. I believe I see greater than 20 IIRC while coasting.

Your plugs don't look "bad" to me, but you are definitely not running rich by the looks of them (syncs to your wideband readings). Please confirm when you are hearing the backfire.

Generally a backfire indicates you are running rich (as there is unburnt fuel in the exhaust that ignites) or you have a timing issue. (That said, it can happen if you are too lean as well.)

What is your vacuum at idle? Have you verified you don't have a vacuum leak? What is your AFR at idle?
Hello,

As i said , my AFR at Wot goes between 12.5 to 13.7 (+\-) and when i let go of the throttle like you said it goes high but that’s completely normal (Coasting). I start hearing the backfiring at around 3000RPM + when im on WOT.

I double check my timing and it still the same as what i set it at. My initial is 14 and my total @ 35.

I shouldnt have any vacuum leaks i check a while ago but i used to have the same backfiring issue as well. The device im using isnt accurate at idle shows me 25+ but as soon as i raise the rpm to about 1500 starts reading better
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post #6 of 65 (permalink) Old 04-05-2018, 12:35 PM
 
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Plug wires might be getting old. What kind of ignition are you running?
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post #7 of 65 (permalink) Old 04-05-2018, 12:52 PM
 
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Hello,

As i said , my AFR at Wot goes between 12.5 to 13.7 (+\-) and when i let go of the throttle like you said it goes high but thatís completely normal (Coasting). I start hearing the backfiring at around 3000RPM + when im on WOT.
Well I think you are running a bit lean and may not be getting a great combustion. You could try retarding the timing 2* and see if that helps.

25" of vacuum at idle is great (congrats) - that would tell me you don't have a vacuum leak.

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie
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post #8 of 65 (permalink) Old 04-05-2018, 02:59 PM Thread Starter
 
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Plug wires might be getting old. What kind of ignition are you running?
Hii,

Im running the stock distributor, the spark plug wires are new , taylor thundervolt and new spark plugs installed together about 3 monthes ago.
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post #9 of 65 (permalink) Old 04-05-2018, 03:02 PM Thread Starter
 
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Originally Posted by TheGoatFather1965 View Post
Hello,

As i said , my AFR at Wot goes between 12.5 to 13.7 (+\-) and when i let go of the throttle like you said it goes high but that’s completely normal (Coasting). I start hearing the backfiring at around 3000RPM + when im on WOT.
Well I think you are running a bit lean and may not be getting a great combustion. You could try retarding the timing 2* and see if that helps.

25" of vacuum at idle is great (congrats) - that would tell me you don't have a vacuum leak.
I tried changing the timing but still got the same backfiring.

Im going to check the vacuum and let you know what happens, but isnt 25” of vacuum a bit too high?
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post #10 of 65 (permalink) Old 04-05-2018, 04:38 PM
 
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I tried changing the timing but still got the same backfiring.

Im going to check the vacuum and let you know what happens, but isnt 25” of vacuum a bit too high?
If you retarded timing and still had the backfiring then I'm guessing it is just too lean a mixture and you will need to richen it up a bit.

25" of vacuum would be nice, but I believe is atypical for Pontiacs....

When does your full timing come into play (RPM?) ?

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie
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