Hmmm, don't suppose there's a chance you fitted the rings in upside down?

I'm not sure what problems this would create, but I'd suspect a poor seal would be one of them.

The other thing, is that the bike was probably setup for an engine with low compression, and with new rings that'll have gone up and altered your jetting requirements. I would also suspect the compression has changed slightly if there was any carbon build up on the old pistons.

Food for thought...
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