Let me see if I've got this right,
5000 rpm, WOT, and the engines drop dead. Back off the throttle, and the
engines spring back to life.
If so, that sounds like vacuum imbalance across the venturi piston.
Depending on the nature of the
problem, it can cause the piston to pop fully up at too low a throttle, or
cause it to drop closed at
a wide open throttle. I'd suspect the latter from the symptoms you've
described. It's easy to install an
air filter assembly incorrectly, blocking the atmospheric breather to the
underside of the piston, that will
drop the piston. Too restrictive an air intake will also cause a loss of
vacuum signal to the piston,
dropping it.
This web page is a pretty good one on dealing with the Hitachi SU's installed
on Datsuns.
http://home.istar.ca/~datsun/att06.htm Good reference web page, probably worth
printing
out and giving to them.
Also, as Carter pointed out, the SU is darn touchy with regards to fuel
pressure. It's quite possible
these carburetors are being overpumped, though I doubt it would cause the
running problem
you've described. It's simple enough to rev the engine up while parked, hold
it up for a little
bit, and see if fuel starts spraying out the bowl vents. FWIW, I had to use a
pressure regulator
on my Spitfire, because the oem type fuel pump that worked fine with the ZS
over-did the SU's.
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