Ok, i just checked the fault finding guide in the haynes manual and i'll list anything pertinent that i don't recall you having checked.
POOR RUNNING AT LOW SPEED
1-low battery
2-plug cap or wires
3-bad coil
4-cdi...hopefully not !
5-pilot jet or air passage clogged
6-floats out of adjustment
7-leak in intake manifold
8-tank breather hose blocked or pinched....very common...did u check that?
9-valves out of adjustment
10-cyl head gasket
That should give you some ideas to look at. I didn't include things that were extremely unlikely or serious internal problems like piston rings and all that because obviously it's going to more than likely be something simple. And if it was something like that the only way you're gonna know is to have the dealer do a complete diagnostic. good luck.
