Sounds like you've either damaged a crank seal during reassembly, or there's an air leak around the inlet, as Eric says.

Often, a simple (albeit rather crude) way to check, is to spray a little carb cleaner behind the flywheel and the inlet manifold when the motor's running. If you notice a sudden change in RPM, that's your leak. The inlet rubbers can crack due to age, and also the manifolds have been known to warp over time. I wouldn't use any sealant on the inlet manifolds, but a little grease can help it seal against the reed cages for testing purposes.

It's very rare for a 1988 centre seal to leak, but it could be possible if the crank wasn't accurately measured, or you've pinched it during the rebuild, but the symptoms sound pretty classic. If you have no joy with the above test, it's possible it could be internal, so again, as Eric says, you need a leak down test.
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Andy.
NSR-WORLD.COM

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