The FIRST thing I would consider if I had a car that missfired at speed but
was fine when not under load is the fuel filter. When your TR is under load
the carbs open, & the vacume tries to suck more fuel from the tank. If it is
partially clogged the engine will stumble over its self. I would swap the
fuel filter, check carb bowels for dirt & float level, & clean out the
fuel bowel at the fuel pump all in one fell swoop.
Second thing I would do is check the cap, rotor & points. If the cap is worn
or the rotor is worn the may be a high resistance pathway that you see under
higher revs or load. If the points are history, they have been known to
cause the same symptoms. Once you are sure the distributer is OK, check the
timing. Severly retarted spark can cause a lack of power & some funny things
if you have lots of valve overlap.
Third {sinse you did all those things you said you did} thing I would do is
swap in a spare fuel pump. If your pump diaphram is history, you will not
get the fuel you need under load.
I would also look for water in the fuel {if there is enough to cause problems
you would have sen it in the sediment bowel when you cleaned it.
You did not mention if you checked your spark plugs. If one or more is on
its way out, it could cause the problem. a Bad spark plug connector could be
a culprit {I like to use those baklite ones from air cooled VWs}
If you still have not found the problem, get out the compression guage.
Good Luck!
TeriAnn
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