Raising the piston, or lowering the jet, will richen the mixture. A thin
blade screwdriver can be used to lift the piston 1/32", to check the
mixture. The SU tool kit has metal rods, that can be placed into the
piston oil resevoir, and lift the piston, with the air cleaner in place.
You can tune by adjusting the jet height, and listening to the resulting
engine exhaust note. Correct mixture is indicated by a regular, even
exhaust beat. An irregular beat with a splashy misfire and colorous
exhaust indicates a weak mixture; too rich a mixture produces a rythmic
misfire and a blackish exhaust.
Bob D. wrote:
>
> My TD's carbs have no piston lifting pin, just a hole to poke something
> through. It's rather difficult to precisely lift the piston this way on a
> hot engine. On the other hand, it's very easy to move the jet down by way of
> the jet lever. Now it's said that lifting the piston weakens the mixture.
> Lowering the jet would richen the mixture. Has anyone heard of a tuning
> procedure involving moving the jet instead of lifting the piston? (I'm
> thinking here something along the line "moving the jet down a certain amount
> should have such and such affect on the engine rpm".)
>
> Just wondering.
>
> Bob Donahue (Still stuck in the '50s)
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