77 460 high rpm backfire

[Hodge_justin]

Recently rebuilt this motor. bored 60 over, edlebrock performer intake manifold, RV Cam, edlebrock 750 Carb, and balanced. I just set the motor in a few months ago and have been having some minor problems. When slowly revving the engine in park i start to get some backfiring (splatter) starting at about 3K rpm and up. The distributor is stock and i replaced the ignition box when i rebuilt the motor. I'm just a beginner when it comes to fine tuning the timing and the carb. does this seem like a timing/carb problem or a worn out distributor problem? It seems to run ok on the road but think that it is lacking power. Please help if you can. thanks

[BChance]

Recheck all your basics.. cap, rotor, wires, plugs, firing order, etc..check your dist internals.. find a manual and make sure all the internal ignition specs are in range..Use the Edelbrock tuning guide to work through the carb set-up..If it is out of the box, you have probably fouled your plugs by now...

http://www.edelbrock.com/automotive_...ers_manual.pdf

If none of that works..did you break the cam in right? Look for a flat lobe, bad rocker, broken valve spring or pushrod issue.....

[ArdWrknTrk]

Cam timing could be off too.

What cam and timing setting did you use?
Have you checked your total mechanical and vacuum advance?

FORDMUSCLE webmagazine: Timing is Everything - Distributor Curving for Maximum Power

Duraspark_distributor_recurve_instructions_index

Dude... Don't Be Retarded

[mark a.]

Is the backfire out the carb or exaust ?

[Hodge_justin]

Engine has vacum advance-working correctly, new cap, new wires, new rotorbug, new coil, new plugs, correct firing order, shadetree mechanic set the timing and the carb, and the backfire is only out the exhaust never the carb. Removed all spark plugs but none look too bad. Cleaned and replaces engine still runs the same. I have messed with the timing myself and haven't been able to improve the backfire at all. I have been told that when the distributor gets worn the shaft may start to wobble and throw the spark gap and duration off in the distributor. This distributor has about 130K miles on it. Sounds like it may be worth a try, $20 after core return. Sorry for the lack of info, any new thoughts after i've provided more details?

[cobraguy]

I had a distributor with a worn out breaker plate. When the vacuum advance would come on it pulled the whole plate sideways changing the dwell and causing all kinds of trouble. If the distributor dosen't fix it check your valve springs and valve train. First check the obviuos stuff, bent pushrods, backed off rocker stud, collapsed lifter, flat cam lobe. To check the valve springs try to rotate the spring on the closed valves by hand, if you find one that spins it is weak or broken.

[mark a.]

Does it have adjustable rockers ? Was lifter preload set when it was built ?

[65f100_352]

Do you have new exhaust? If you have a leak up by the manifolds it will make a pretty loud pop, at least mine does.

[Hodge_justin]

Replaced the distributor but did not correct the problem. had a mechanic take a listen to it, he said i had at least one exhaust valve not closing all the way. he is going to take a closer look this week and find out what all is wrong (bent valve, colapsed lifter, etc.). i do have a new exhaust but no leaks, the rockers are not adjustable, I can't tell you if the lifter preload was set right because i had the motor assembled by the guy who rebuilt it. Just a beginner at this stuff, tore into this project just enouph to learn i was capable of really screwing it up good.

[cobraguy]

Collapsed lifter and bent pushrods will cause the valve not to open all the way. Things that hang valves open are pushrods too long. Decking the block or shaving the heads, broken valve springs, bent valve stem, damaged guides. Adjusted too tight but yours are not adjustable.