I do not know anything about the Manco. But in all honesty, the Tec motor on the Yerf has a very short life span and any $ for carb repairs made would be better put towards another motor IMO.
I bought a new Harbor Freight "Predator" motor right out of the box for $107 out the door, and gained about 2-3 MPH over the Tec with port matched intake and exhaust manifolds. (28 MPH TEC VS. 31 MPH HF)
http://www.harborfreight.com/engines...nia-69730.html
It's almost a drop in replacement motor.
http://buggymasters.com/forum/showth...highlight=3203
http://buggymasters.com/forum/showth...highlight=3203
Then you can get a new jack shaft and I would also get a 9 tooth sprocket to gear it down. That should take care of your issues for relatively low cost.
http://buggymasters.com/forum/showth...highlight=3203