Just a guess.
When you had it running "about right", it probably was. Given a hot cam
with alot of duration, there is still unburned fuel in the exhaust. If you
have a leak in the header, or exhaust manifold, even a pin sized leak, the
oxygen introduced by the header leak will ignite the unburned fuel in the
manifold, causing your sky high exhaust gas temp. (this is actually how air
injection smog pumps work to reduce unburned hydrocarbons in the exhaust).
My bet is the rear branch of the header leaks, and the heat is radiated back
to the head. When you richened the rear carb you actually made the problem
worse, because you provided more unburned fuel to exit with the exhaust
stroke. Find the header leak, lean out the carb & try again. I also think
your total advance may not be enough. Again, with the more agressive cam,
the engine may need more advance than the cooper centrifugal advance profile
can deliver. Too late a burn, hot exhaust gas temps.
David Riker
davriker@pacbell.net
http://home.pacbell.net/davriker/
http://community.webshots.com/user/fool4mg
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