dlp@akgua.UUCP (D.L. Philen [Dan]) (11/08/85)
Having had several questions regarding what a Unisyn is, I submit the
following:
I just assumed that everyone that had experience with an SU also knew
what a Unisyn is. A Unisyn is a venturi gauge. They are about 19 or 20
dollars and can be found at most auto supply stores or in some of the
magazines like Road and Track. Try the auto supply stores first.
It has a tube with a float in it attached to a plate that contains an
adjustable venturi. You disconnect each carb (linkage), remove
the air cleaners and place the Unisyn over the intake port
of the carb. It is adjustable for either side-draft or down-draft set-up.
Next you adjust the venturi to get the float to reside somewhere in the center
of the tube. Now go to the other carb and adjust the throttle control
on the carb to match the same float height reading that you got on
the first carb. Now both carbs are adjusted so they are drawing the
same amount of air each. ie. they are balanced in regards to their
suction. This is important because the carburation (misting of the
gasoline) depends on the pressure and suction in the carb.
Now you are ready to adjust the rich/lean mixture by screwing up
or down on the jet nuts. Lift the piston with the piston lift
pin if one is fitted to the carb, or insert a small screwdriver blade
sideways under the piston and slightly twist the screwdriver to lift
the piston a little. The engine will speed up if too rich, slow
down if too lean, or speed up slightly and then return to original
idle if set just right.
Hope this helps you. I have had my Unisyn for about 18 years and think
it is great. Try it. You'll like it.
From Dan's English Car Garage -- Press on Regardless!
akgua!dlp
PS. If you really have trouble adjusting the idle or rich/lean, then
check the bushings around the butterfly valve. In older SUs these are
frequently worn and they allow air to be sucked in past the butterfly
(throttle) valve. The result is that you can never get it set right
because you can't set the suction properly. These can be easily
rebushed. I can recommend a shop if anyone is having this problem and
is interested. (Check for leakage by squirting some motor
oil around the throttle shaft where it comes out of the carb. The presence
of blue smoke in the exhaust is a sign that it is leaking.)