thanx in advance for any help. I have a 69 Z28 with a zz4 350. problem is that it fires into the headers from cylinder #8. if I pull #8 plug wire it stops the misfire so I know it's #8. I have new rotor and cap and plug wires since problem srarted. Have adjusted the hydraulic lifters. compression is oK, timing is OK. had heads ported/polished etc. last summer and it ran fine after that. how do I diagnose a bad hydraulic lifter or a valve that is hanging up on me?
It could be a sticking lifter or valve. When the engine is cold, the lifter piston should move when pushing down the pushrod. It should move 3/16" to a 1/4" freely. If it checks out good, then try pushing down on the exhaust valve stem and feel for sticking/binding on the return. Take a piece of flat bar, measure the distance from the valve tip to the rocker stud. Drill a 7/16" hole in the flat bar where the rocker stud measurement is. Now use the flat bar as a lever setup to depress the valve stem. Be careful to press on just the stem tip and not the spring retainer or you'll end up removing the head to retrieve a valve.