Well, you have the ultimate questions. It is hard to fit carbs to an engine
that the manufacturer never tested.
I, too, would have started with the TR 2000 since the engine is probably
roughly similar. As you found, the similarity is not sufficient.
To do it right you need a dyno and see the SU piston travel under engine load
and then see what the mixture is at that moment.
The piston travel is important because you need to see if the pistons come all
the way up under full power. If the springs are too strong then the pistons
will always be too closed and you are effectively running a smaller carb than
the HS4. A tighter spring will tend to force the mixture to be richer than a
weaker spring because the venturi effect is stronger. In general, I might err
on the side of lighter springs since I rarely use full throttle.
Once you have the springs appropriate your your engine, then you can worry
about needle selection.
In general, the piston travel will be roughly proportional to accelerator
travel. If you are cruising, then you are probably using something in the
middle 1/3 of the needles,but it is hard to be sure exactly what.
If you have a dyno, you can see the exact needle position, and can know exactly
how rich/lean you are.
A poorman's and possibly dead one's dyno is a SECURE lift and use of the brakes
of the car. One author (not me) says you can rev up the engine on a lift in
gear and use the brakes to create resistance for about 30 seconds, then you
need to let the drums cool off.
If you are justifiably scared to try that, then you can remove the dampers from
the carbs and slowly increase the RPM's (out of gear) listening for any hint of
a stumble. When you find one, then that area is lean. Then you can use very
fine sand paper and a drill (to spin the needles) and thin-out the appropriate
area of the needle by 0.002, and try again. Once you can go through the entire
RPM range to about 3000 with no hesitation, then you taper the rest of the
needle about 0.001 per each 1/8" to the tip of the needle.
Then try it with more brisk throttle opening (1 second to half throttle, and
release before the revs get toooo high). If it stumbles, then further thin out
the appropriate area of the needles.
At this point it will probably work quite well in your engine.
See my needle selection database at
http://mywebpages.comcast.net/rhodes
Read the book "How to Build and PowerTune SU Carburetors" (I think that is the
name) by (I think) Des Hammill.
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