Pachymon

12 years ago i bought a 1973 mustang mach1,it had a problem that it still has today,it backfires through the carb when i press hard on the gas pedal.I was at auto mecanics school when i bought the car, i asked all my teachers,did everything they told me,asked pros,did that too.Now i'm finishing a 11 years resoration project ,the car is almost finished ,but it still backfires through the carb.Here's a list of what i have done to try to solve this problem:Rebuilt carb,changed carb,changed ignition wires,replaced points by pertronics ignitor,changed distributor,adjusted vacuum advance,rebuilt the engine (replaced timing chain,did not change cam or lifter).I am beginning to think it might be a bad cam or a bad lifter! Any help is appreciated

67mustang302

The accelerator pump shot on the carb is too small. What carb is it? Autolites had a linkage that could be adjusted I think, as do Edelbrocks/Carters, and Holley type carbs use pump cams. You can also change squirter sizes as well. Basically when you whack the gas the accel pump isn't delivering enough fuel to cover the transient lean until the main circuit starts to deliver fuel.

At least, I'm 99% sure that's your problem.

Pachymon

I forgot that ,i also oversized the acceleration jets,they were .028 and now they are .031.Besides from increcing fuel consomption,it did't change anything.the old carb(i still have it)is an autolite 4300,the new carb is an holly 650 double pump mecanical secondary.Thanks for your help!

Pachymon

I also noticed something strange...when i whack the gas pedal,at the precise moment of the backfire timing falls from about +15 BTC to about -3 After Top dead center for a split second and then ,the motor accelerates and the timing goes back to normal.

67mustang302

Changing squirter size shouldn't have a bearing on mileage, changing the pump cam would though. Sounds like you have an ignition problem though.