I have a 350 engine that for somereason was getting exhaust into the intake ports.New engine ,but had a lot of carbon buildup on valves.Will after checking timing numerous times ,and reomveing the intake about 5 times .I also checked everything else possible at least 3-4 times.I even went through checking the angle of the heads to the intake.I finaly find that the valve stem seals are not the correct ones.None are in place they are all floating up and down the stem. O well live and learn.