Group,
Here's the next wrinkle in the old "barking carburetor" syndrome. The original
problem turned out to be that three exhaust push rods had off-set their
respective rocker arm sockets. I figured that I had over-revved the motor,
put them back in place and entered the HSR Historic Race at Road Atlanta last
weekend. During practice Saturday, my motor did the same thing, but this time
at a carefully monitored 5500 rpms.
My friend, Neil Estes, and I pulled off the rocker arm rack and found that all
four of the exhaust valves were sticking convincingly. We pulled the head,
hand-drilled out the valve guides with a 5/8th drill bit, reinstalled the
head, it ran fine and took second place in the race Sunday (as the lone TR-4
in a class of MGB's).
At the Saturday night refreshment session, the collective wisdom was that the
initial displacement of pushrods was caused by over-heating, not over-revving;
that the bronze guides had expanded within a steel head and deformed inwardly
against the valve stems and that this deformity remains after the head has
cooled.
The motor had 17 races on it when it first had this malady so too-tight
fitting valve guides is probably not an issue and, yes, I was having cooling
(over-heating) issues when the problem first occurred. I never associated
that with sticking valves.
What is the collective experience of the group with heat-expanded valve guides
which lose their memory when cooled. Quite frankly I am quite comfortable
trusting my Saturday night drinking buddies' thesis, but a second opinion
might help us all.
Thanks,
Richard Taylor
TR-4
Atlanta
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