Exhaust Backfire

[Turpin]

Hello guys,
I've recently updated my exhaust system to a slightly larger size and now I am experiencing a exhaust back-fire after about 5seconds when I shut the engine off.

FYI...The engine uses carburetors

I've heard several different causes...Too lean, Too rich, exhaust manifold leak...

What do you think?

Thanks,
Russell

[NY65912]

Jetting or exhaust leak IMO

[Turpin]

Mike2,
By jetting do you mean too rich or too lean?

Thanks,
Russell

[petrolblue83911]

when this has happened to me with a carb'd car it was generally too rich

essentially raw gas is blowing by and into the exhaust where it's hot and likes to explode

[Turpin]

Good deal. I will lean it out a little and see what happens.

Thanks,
Russell

[cabmandone]

Do you have a wide band or color tune setup?

[Turpin]

Originally Posted by cabmando

Do you have a wide band or color tune setup?

Very interesting question. I've never been asked that before. What does "wide band and color tune" mean?

Thanks,
Russell

[cabmandone]

Originally Posted by Turpin

Very interesting question. I've never been asked that before. What does "wide band and color tune" mean?

Thanks,
Russell

Wide band 02 gives you realtime AFR which tells you if the engine is running lean or rich. I have a 14Point7 setup in my 85. My brother used a handheld unit in his 83 with PMO carbs to tune the carbs. He also has a colortune that will change color based on whether the cylinder is lean or rich. The wide band 02 is nice for tuning if you put a sensor in each bank. The colortune fine tunes each cylinder.

[Turpin]

Cabmando,
I do not have either of those tuning devices. However, they do sound interesting. I will research them...

Thanks,
Russell

[cabmandone]

Originally Posted by Turpin

Cabmando,
I do not have either of those tuning devices. However, they do sound interesting. I will research them...

Thanks,
Russell

If you're going to play around with the mixture you should probably have a wideband o2 setup. I think my setup cost around $500 and it's a mounted system with a display and data logging. You can buy handheld setups that will log data so you can adjust your carbs, do a run and then tweak the settings on the carbs to get the best mixture.
The color tune is interesting and I wish my brother and I could have played around with it while I was there a few weeks ago. It seems like a good way to get all 6 cylinders firing at optimal levels.

[antares]

… So do anyone of you have a link to purchase either this user friendly wide band o2 setup or the color tune unit as I will be getting onto my CIS tuning soon and want to buy a good sensor … Also if you jump over from the porsche to a V8 with dual exhausts would you have to have yet another additional input sensor for the other exhaust bank when using this wide band o2 unit … ? dave

[cabmandone]

Originally Posted by antares

… So do anyone of you have a link to purchase either this user friendly wide band o2 setup or the color tune unit as I will be getting onto my CIS tuning soon and want to buy a good sensor … Also if you jump over from the porsche to a V8 with dual exhausts would you have to have yet another additional input sensor for the other exhaust bank when using this wide band o2 unit … ? dave

If you have true duals one sensor in each bank is the recommended setup. I'll have to ask my brother where he bought his colortune and post back. As for the Wb02 I believe our host offers one and as mentioned I have a setup made by 14point7. Most recommend buying one that has the LSU4.9 sensor which does not require free air calibration. My brother had the older 4.2 (I think) which did require free air calibration.
Here's a thread on different setups people are using.
AFR Logging

[T77911S]

you mean when you turn the key OFF and the engine stops, 5 seconds later you get one backfire?

what did you change? muffler, header?
what did you have on there and what did you goto?


sounds like fuel is getting dumped in at idle or after you turn the ignition off that is not burning, then it ignites from heat

[Turpin]

Originally Posted by T77911S

you mean when you turn the key OFF and the engine stops, 5 seconds later you get one backfire?

what did you change? muffler, header?
what did you have on there and what did you goto?


sounds like fuel is getting dumped in at idle or after you turn the ignition off that is not burning, then it ignites from heat

Hello,
I changed the header system to larger diameter. The engine did not backfire at shutdown before. Maybe I didn't get the header studs tight enough?? It is possible that the throttle plates are not closing and allowing fuel to be dumped??

I'll put the car on the lift this weekend and check it out.

Thanks,
Russell

[T77911S]

probably and exhaust leak, but you should hear it.
look for exhaust residue around the ports.

even with throttle plates closed if you have an injection nozzle leak. long shot but worth a look.

[cabmandone]

Btw, if the backfire wasn't there before the exhaust swap, I'd check all of your gaskets. When my brother and I were messing with his car we went between an oversized B&B 2 in 2 out header system and a 2 in one out SSI with Dansk sport muffler setup and I don't recall either one having much effect on the mixture and certainly not in the idle circuit.

[Turpin]

Yep. Replaced the exhaust manifold gaskets and tightened it down and "whola" no back-fire.

Thanks for your comments and help,
Russell

[cabmandone]

Originally Posted by Turpin

Yep. Replaced the exhaust manifold gaskets and tightened it down and "whola" no back-fire.

Thanks for your comments and help,
Russell

Sweeeeeeeeeeeeeeeeeeeeeet! Glad you found it.