Hi Steve,
If the engine was rebuilt, check for bronze valve guides. I have
seen this cause your problems many times over the years. If the guide
clearance is too tight to the valves stem they will bind when hot.
If using bronze guides the phosphorous bronze guides are much then the
silicone bronze type. Their clearance needs to be slightly greater then
the steel guides to start but they wear better. My last problem was an
MGB that ran great around town but five minutes on the highway and it
would lose cylinders one at a time, get off the highway and two miles
down the road it would cool off and be ok again. When I pulled the head
you could see the score marks on the valve stems. After replacing the
guides and machining the new ones to slightly larger clearances no more
problems.
Wayne
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