it an 88 tbi 350 rebuilt less than 10k ago, new pistons ring and valve job. it started burning oil after 3-5k and before i pulled the engine it barely ran. i took the engine apart and the 4 and 8 piston top and valves has alot of crud built up on them but everyone else is clean and normal. what would cause this rings? valve seals? head gasket? it used about a quart every few weeks and was running on 6 cylinders. any ideas. thanks

Could be a few things, you should have first checked your compression. But now try these,

check your heads and look at the exhaust valve and intake valves are they both "coked" up in the runner?

does the intake lifters rise the same amount as the other lifters?

use a valve spring compressor and remove the intake and exhaust springs and check for excessive play/bad valve seals.

thats all i can think of right now someone else might be able to tell u something else to check.

Thanks, Chas

Sounds like the block and pistons need to go to a machine shop to have them checked. May have excessive piston to cylinder wall clearance.