OK, I've got a complex fuel injected engine that needs tuning, but I'd like your high level theories on what causes backfire thru mufflers. Specifically:

Lean or Rich fuel?

Advanced or retarded timing?

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Jameel Qazi
#'s 67 SS
65 TPI / 700R4 El Camino

I'd guess rich and/or retarded timing. Just my theory though http://www.chevelles.com/forum/wink.gif

If exhaust is burning in the pipes, it has too much fuel. If the timing is too late, it might push still burning fuel into the exhaust.

What do you guys think?

chris

I agree because I've done it more than once.

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My Web Page (http://www.landry-family.com) (updated 06-21-02)

"Long Live Freedom!"

Chad Landry
TC Member #643
ACES Member #4556
'68 El Camino, 357, L31 Vortec heads, 700R4,
8.2 10-bolt, 3.55 gears, Auburn Pro Posi.

good. That's what I thought. Now I have to figure out which one. I've been leaning out the fuel. It seems to help a little. Now I am thinking I just need to start advancing the timing.

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Jameel Qazi
#'s 67 SS
65 TPI / 700R4 El Camino

Don't lean the fuel too much. With lean fuel/air mixture and retarded timing you'll get overheating. Too lean even with proper timing will make it run hotter.

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My Web Page (http://www.landry-family.com) (updated 06-21-02)

"Long Live Freedom!"

Chad Landry
TC Member #643
ACES Member #4556
'68 El Camino, 357, L31 Vortec heads, 700R4,
8.2 10-bolt, 3.55 gears, Auburn Pro Posi.

Good point. I am watching the air / fuel ratio carefully with an oxygen sensor. My Oxygen sensor is still trying to lean me out 5 to 10% in light throttle situations.

The problem is that the oxygen sensor only tells me there is unburnt fuel entering the exhaust. It does not tell me if that is because I am pumping to much fuel in, or not giving it enough timing. I had been taking the approach of dropping the fuel first, but now I am starting to slowly advance the timing to see if it helps.

So far, it is hard to tell. I am going to take the timing back out and see if the fuel alone will do it.

ANOTHER QUESTION:
Will fuel burning in the headers significantly raise their tempurature?

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Jameel Qazi
#'s 67 SS
65 TPI / 700R4 El Camino