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Dave,
That
is a strange one. I've had rpm drop from things like blown intake hoses, but the
rpm didn't come back until the hose was fixed.
Most
of the problems you listed might cause an rpm "wall", but not a repeatable
drop. This might not make much sense, but it sounds like something giving under
pressure to me. What could "give under pressure", but then return to normal when
pressure is removed? Something like a radiator pressure cap, a pop-off
valve....an O ring?
The
other thing I might suspect would be something in the ignition tuning that's
triggered at the peak rpm. A timing thing?
See
if retarding the ignition changes things.
Just
SWAGs.
Regards,
John
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