You have gotten some good info already, but I will chime in because I have
likely made more mistakes than anyone else on this network.
BASICS...is where to start. When building an engine, you should always check
and mark TDC before the heads are installed. Assuming that the mark is
correct...Pull the valve cover and check the timing mark with #1 or #6 on
split overlap. (When the exhaust is closing and the intake is opening and
reach the same height it should be very close to TDC.) If that checks OK,
check the timing at idle and @ about 4000 RPMs, this will indicate how much
advance you are getting. I suspect that you should not have more than 35
degrees total. Recheck the valve adjustment. I set valve clearance at the
point in rotation where there is the most clearance. It seldom is in firing
position. A less likely, but possible problem is cross firing do to magnetic
field induction. Any time plug wires run long distances side by side,
induction can be a problem. On a Chevrolet,(1-8-4-3-6-5-7-2) in routing the
plug wires, place #1 & #3 between #5 & #7 and #2 & #6 between #4 & #8. Even
if you don't have a noticeable problem, improper routing can cause
detonation. Take a good look at the distributor cap checking for cross
firing. Phasing of the distributor can also create this kind of condition.
Make sure that the rotor is pointing at the proper cap tower when in firing
The kind of problem you describe, can also be a lean condition, a weak or
broken intake valve spring. While the valve covers are off, crank the engine
and watch the valve to make sure they are all functioning properly and take
a good look at the springs.
This may sound as if I think you don't know anything about an engine, but
that is not the case. This is just a kind of mental checklist I use when
chasing a problem.
Good Luck! Lets us know what you find.
Tom, Redding CA - #216 D/FCC