If you didn't use a decent fiber gasket for the base, there's a high likelihood you have a leak. Also, despite the indexing collar (which often gets lost on rebuild) the vespa cases can end up very slightly twisted, leaving high spot-voids at the 12/6 position. When you rebuild it's always a good idea to check that the base surface is as close to perfect at the seams, before you install your engine peripherals. It's easily adjusted by loosening the case bolts and some taps with a rubber mallet.