jackwabbit703

Hello,

I've re-adjusted the timing and idle mixture yesterday. While on test drive, coming to a stop off throttle, I heard medium but not LOUD back fire coming through the side pipes. No backfire when accelerating or under full throttle.

Am I running little lean and need to richen to idle mixture or have APT to run little richer?

ramair_bryan

When we were younger..........we prayed our cars would do that....
Likely the throttle blades are tipped in to much at idle, making an air leak. Or your chasing a vac leak around, and adjusted too much fuel at the low speed adj to compensate.
Or........maybe our prayers were answered, and your one of my old buds. God was running late..............

jackwabbit703

You are correct about when we were young comment.

I've adjusted idle mixture to run little richer on the driver side intake and no more backfire on off throttle coasting.

Thanks.

ramair_bryan

Yup................

MakoJoe

Hell I remember before locking steering we used to turn off the car pound the gas to see if we could get a huge backfire and blow flames out of the exhaust system One buddy of mine blew the muffler apart but it was a huge bang and there were flames LOL

Back fire is caused by a rich mixture or in correct timing. Many racers used to advance the timing to get a better fire but when they let off the richer mixtures of the carb and multiple carbs would back fire with too rich of a mixture. Used to see lots of flames on side pipes when I was young and growing up my old man spent almost every weekend at Lime Rock Park, Bridge Hampton and Pocono Raceways. My favorite part was watching cars let off the gas going into corners and blew flames out of the side pipes.

Your timing is off or you have too rich of a mixture if it backfires. Lean cannot cause it because there is not any excessive fuel to cause a backfire.

4 speed

my wifes 2000 SS camaro stock A4 kinda does that from day one. kinda blub blub pop pop. nothing big and shocking, just kinda cool sounding though.

of course its FI and computer controlled.

LS4 PILOT

mine will make a loud crack though the side pipes backing off a hard push on the throttle
even the Harley guys look ....hee hee hee....
very cool....
yup that teenage urge ..just won't go away......wifey says.....stop ! act like you have some sense .

Nope ..... can't do it

OMF

I you have a small exhaust leak at the engine this will cause backfiring....

7t9l82

even the harley guys look i pulled up alongside 2 harleys with drag pipes on i95 and both looked over. made me proud.

Super6

A backfire in the exhaust can also occur if the carb is too lean. The mixture won't fire in the chamber, but will ignite in the exhaust due to the hot gases from other cylinders.

BKbroiler

The same carburetor is feeding the other cylinders so how is the exhaust from those cylinders any more combustible?

Super6

BK,

Do you think that the A/F ratio is exactly the same in all 8 cylinders with a carbureted engine? There's a leanest cylinder, a richest cylinder and then the others that are in between. They are operating in a narrow range of A/F ratio, but there are differences from cylinder to cylinder.

I don't claim to know exactly how this occurs, I believe it is related to runner length differences or velocities. Never thought about it that much. Ever hear of an Edelbrock Tarantula intake manifold for Chevy big blocks? The carb flange and carb were rotated about 15 degrees toward the passenger side. That was an attempt to equalize the mixture by making the runner lengths more equal. I had one of these manifolds. Don't know if there was a small block version of it. I know that some Chevy big block carbs had little tabs on, I think, the downleg boosters or main venturi to richen up the mixtures on the far cylinders, maybe no. 8 in particular.

A lean mixture is harder to light off in general. The proof of the pudding is inducing a lean backfire myself and having the shoelaces literally burn of the top of my shoe when the flame shot out of my sidepipe. The cure for that was richening up the mixture. So yes, I know firsthand that a lean condition will cause a backfire out of the exhaust. The charge doesn't light off in the cylinder, but the 1200F temperature of the exhaust from the other cylinders does it later in the exhaust system.

Also, look at post no. 3 above. Jackwabbit richened up the carb and the problem went away.

oldalaskaman

not sure what exhaust you are using, or if you've just changed it...but...all other things being good, poster #8 gets my vote. try the simple things first.

jackwabbit703

Hi all,

Thanks for all the inputs. I have richen the mixture on the driver side of the carb since back fire was coming from the driver side side pipes. After turning the driver side mixture screw one half turn out, no more back fires.

Thanks again.

73, Dark Blue 454

What he said.

Look for a vacuum leak first.

Ebman

[QUOTE=jackwabbit703;1584855015

Yesterday. While on test drive, coming to a stop off throttle, I heard medium but not LOUD back fire coming through the exhaust pipes. No backfire when accelerating or under full throttle, please what can I do to fix this?

eboggs_jkvl

[QUOTE=Ebman;1607328856][QUOTE=jackwabbit703;1584855015

Yesterday. While on test drive, coming to a stop off throttle, I heard medium but not LOUD back fire coming through the exhaust pipes. No backfire when accelerating or under full throttle, please what can I do to fix this?[/QUOTE]


I'd suggest that you open a new thread, rather than a 10 year old thread. You might get a better response with your issue.

Elmer