There has been some talk of the difference between the 88-89 MC18 model MC18's

Some seem to sweep the servo, while others don't. It looks like the PGM-II does (1989 R5K) and the pre PGM (1988 R2J) doesnt.

Try removing the petrol tank, start the engine and then gently blipping the motor to over 3k watch the servo motor. If the servo motor sweeps from LO to HI then there is nothing wrong.
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If I have to take the carbs off once more...