The front is up about 1/2", the rear is about 1/8" lower.
My question: is there anything here to worry about? What does
it indicate about my engine's health? What adjustment is
needed (if any)?
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For Zenith Strombergs the "piston" or air valve "rises to a height which is
governed by the magnitude of the depression (vacuum) in the suction chamber
(that's the part under the cap) balanced by the weight of the air valve (and the
expansion force of the spring, when fitted)." from Haynes ZS manual.
Don't know if SUs operate the same but it sounds like you are not getting the
same "balance" between the air valve and the suction chamber in both carbs.
Assuming the air valve slides freely on the guide rod and the springs are OK
(they don't usually lose elasticity unless visibly damaged according to Haynes),
then that would point to loss of vacuum in the suction chamber. It could be
plugged at the vent hole in the air valve (which would restrict the amount of
vacuum created in the suction chamber and result in a lowered piston) or the
diaphragm is damaged or has a pinhole and losing vacuum, which also would also
result in a lower piston.
I'd take the cap off and do some visual checks to make sure the air valve slides
freely, that the diaphragm is seated correctly (it has a location lip on the
underside that has to fit in a recess in the carb body), that the diaphragm
doesn't have cracks (you can gently roll it in your fingers to check ) or
pinholes (GENTLY stretch it while held up to a light). It should be supple and
have a smooth texture. Check the air valve vent hole (on the bottom on ZS
carbs) to make sure it's clear.
Good Luck
Bud 71TR6 CC57365
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