exhaust backfire..

[challngd73]

OK, mechanic had my car for a bit to work on some stuff.  I put a proform 950 on the car, it already has headers, intake, hei, but probly stock besides that.  I know that carb is too big for a relatively stock motor, but I am going to stroke the motor next spring and only wanted to buy 1 carb....So i got it back, and it ran great on the way home, much better than with the 750 eddy, but close to home it would backfire through the exhaust when i nailed it...fine at part throttle...I checked the timing and the mechanic has it set at 8 degrees..would that cause the backfire, or is it due to the carb being too big?  motor is a 72 440 btw

[MEK-Dangerfield]

You can take a look at your spark plugs. My first thoughts are it is too lean, or you have a leak in your exhaust system, but since you didn't touch that....

Your timing might need to be bumped up a little more before TDC, but I don't think that is your problem here. 

[challngd73]

do you think it is the carb?

[IMNCARN82]

air idle mixture screw.  Or maybe jet it down for now.

[Strawdawg]

is it getting a good shot from the accelerator pump when you pop the throttle open?

[challngd73]

yes it is. 

[Strawdawg]

I would start by setting the total timing to where you want it....then you might disconnect the vacuum advance if you have one and see if that has an effect.

idle screws should have no affect as you are not running on idle circuit when cruising...

[challngd73]

thanks i will give it a try.

[Supercuda]

Based upon the information available, the engine is a little rich, and the backfire is fuel igniting in the exhaust system. The accelerator pump shot is likely too much, and the jetting too rich. A larger carb than necessary doesn't help, either. Basic rule for backfires: carb/intake=lean; exhaust=rich. Other causes abound, but this addresses the common problems.

[ghjedi]

For what it is worth, I had a similar problem with backfire in my exhaust (I think it is technically called "afterfire") when I upgraded my carb to a Proform 850.  My car would backfire occasionally at full throttle.  I suspected it might be too rich, so I backed off the secondaries a bit (1/8 of a turn) and the problem went away.  Note that I did not touch the primaries, since the car did not backfire at all at idle or low rpms.  You might try the carb first, since it is a very easy change to make and you'd know pretty quickly if it works or not.

[challngd73]

For what it is worth, I had a similar problem with backfire in my exhaust (I think it is technically called "afterfire") when I upgraded my carb to a Proform 850.  My car would backfire occasionally at full throttle.  I suspected it might be too rich, so I backed off the secondaries a bit (1/8 of a turn) and the problem went away.  Note that I did not touch the primaries, since the car did not backfire at all at idle or low rpms.  You might try the carb first, since it is a very easy change to make and you'd know pretty quickly if it works or not.

thanks, I may try that.

[challngd73]

An update, the timing was actually more like 0...so I adjusted it to 12 btdc, and took it out.  Stood on it, and it shifted at 5500 with no sign of backfire.  Then I backed out of it and the rear end began howling something fierce.  Limped it home and have new problems to deal with now.

[ghjedi]

An update, the timing was actually more like 0...so I adjusted it to 12 btdc, and took it out.  Stood on it, and it shifted at 5500 with no sign of backfire.  Then I backed out of it and the rear end began howling something fierce.  Limped it home and have knew problems to deal with now.

challngd73, I feel your pain   

[Strawdawg]

An update, the timing was actually more like 0...so I adjusted it to 12 btdc, and took it out.  Stood on it, and it shifted at 5500 with no sign of backfire.  Then I backed out of it and the rear end began howling something fierce.  Limped it home and have knew problems to deal with now.

That was the reason I suggested setting the time to the desired max.  Retarded timing means that the a/f is lit late and will still be burning when it comes out of the exhaust port...this can cause a pop in the exhaust system...

But, that was the easy part...sorry to hear about the rear end howl.  Somedays, it never ends.

[challngd73]

yes, today is one of those days.