ok heres the dealio i have an 87 f150 4x4 with a 351w and a 4 barrel. i recently had my old motor drop a cylinder due to the last owner setting the timing off so far the timing chain skipped a tooth, then the valve hit the piston and then bent the valve and the pushrod. so i was running it around town on 7 cylinders since i still needed to get to work, in the mean time i had a new motor coming in. so i ran it like that for about 3 weeks and about a week before i pulled the old motor i started to get a backfire out of the carb at about 1/3 to 1/2 throttel which would go away at full throttle. i didnt think much of it because i knew the old motor had problems. so i got my new motor and i just put it in, started her up broke the cam in and timed her to 10 degrees btdc just like it calls for. now just to let you all know the motor has 9.5:1 comp and a bigger cam but nothing wild. so i go to take it for its first drive and the dingel berry starts to backfire out the carb just like with the old motor. so i double checked my timing checked my vac advance did everyhhing i could think of and so far i havent been able change the situation at all. im about to give up and take it to a shop but im a poor college student so its not really in my budget lol. the only old parts on the truck that cam off the old engine are the intake and carb and the distributer. i keep eyeballing my distributer but at the same time i know theres alot of things that can cause this, so if anyone has any ideas id really appreciate the feedback.

 

The only thing that will make an engine backfire or (throwup) through the carb is a burnt valve or a valve that is held open by too long a pushrod, or in the case of mechanical lifters, too tight a setting, thereby keeping the valve open. Then when the piston builds compression, and a valve is kept open by the above or possibly a timing chain that has been installed off time, you will get all that compression back through the intake manifold, and out through the carb.

A classic (throwup)...

Hope this helps. Start looking for the above problems.

Forddieselman....

 

I put my money on the dist. being bad or something in the timing, and unlike stated above a burnt valve is not the only thing to cause this. If the timing is retarted it will fire on the intake stroke thus causing it to back fire through the carb.