I have had some small block chevy intake valves tulipped my latest is a 350 with #2 #6 & #3 intake valves pistons are fine combustion chambers in good shape didn't go lean all cyls are black like it was rich has anyone have a idea what causes this

What kind of valves are they as well as size and how much time on them. Spring pressure, etc.

Prolonged periods of running just on the edge of detonation/high cylinder temps will result in tuliped valves. Detonation not quite severe enough to blow head gaskets etc., so it goes un-noticed. Timing and lean air/fuel mixture would be the most likely causes. It's not only possible, but likely, that it IS lean at some point in the operating range. The fact that the chambers/valves are black right now means very little relative to the conditions during which the damage is occuring.

motor is a 350 vortec all factory stock

just found out this is a JASPER MOTOR with approx. 1 hour of run time on it customer & the installer had a fallin out now I find myself in the in the middle

A marine based unit and not out of a truck?

Wannabe

yes marine motor

Ive had a 2 buddies with cruisers with Merc small blocks do the same thing,was told it was the low octane/87 and didnt have a good grade stainless valves.

What RPM are you running at WOT?

I had the same thing happen on my 730 hr. 1995 350 Mag. Most likely a bad fuel, age, and detonation issue coupled with a few injectors getting gunked up. It culminated in the #1 exhaust seat failing and sharing the crunched up pieces with #2 & #4. While doing so it punched a hole in the head near the #1 spark plug breaching a coolant passage.

At least half if not more of the intake valves were tulip'ed.