Sounds like a carb issue.
First off, make sure all your air bleeds are clean. Then I'd take a hard look at the accellerator pump circuit(s).
C keep in mind it could be several diiferent things...
You might try some need bigger squirters, The accellerator pump (s) have cams, there are 2 holes on them, try moving the screw to the next hole and see how it does, you may need to go up to a 50CC pump kit.
Its kinda hit and miss, try one thing if it doesn't get any better, then put it back if it does then leave it alone and move on and try something else.
Another thing to try would be to richen up the primary main circuit - go up a couple jet sizes and see how it responds if it gets better go up 2 more.
If you have a vacuum advance make sure it hooked up to a ported vacuum source and not manifold vacuum, also bumping up the timing a tad will help bog.
Goodluck and let us know what it took to fix it>Cp