The first thing I thought of when I read this post is the spark plugs. I've seen this happen before in 4-wheelers and in cars and trucks. Check the spark plug gap. Sometimes parts stores will tell you that their plugs dont have to be gapped or that they are preset from the factory. This is a common misconception. ALL PLUGS have to be gapped. Even if(big if) they do pregap them, in shipping they can be bent in from being banged around. This may not be the case and you may have already done this, but its the first thing that came to mind when I read your post. I took this from the service manual for the roadstar(that I got by reading the posts here, thanks guys). Hopefully it might help. Good luck, Nick

CHECKING THE SPARK PLUGS ADJ
2. Disconnect:
• spark plug cap
3. Remove:
• spark plug
CAACUHTTIUONNG: :
Before removing the spark plugs, blow
away any dirt accumulated in the spark
plug wells with compressed air to prevent
it from falling into the cylinders.
4. Check:
• spark plug type
Incorrect ’ Change.
Spark plugs
Model (manufacturer)
DPR7EA-9 (NGK)
X22EPR-U9 (DENSO)
5. Check:
• electrodes 1
Damage/wear ’ Replace the spark plug.
• insulator 2
Abnormal color ’ Replace the spark
plug.
Normal color is medium-to-light tan.
6. Clean:
• spark plug
(with a spark plug cleaner or wire brush)
7. Measure:
• spark plug gap a
(with a wire gauge)Gap Should be 0.8 ~ 0.9mm(0.031 ~ 0.35 in)
Out of specification ’ Regap.
8. Install:
• spark plug
NOTE:
Before installing the spark plug, clean the
spark plug and gasket surface.
9. Connect:
• spark plug cap
10.Install:
• fuel tank
• rider seat
Refer to “FUEL TANK” and “SEATS AND
SIDE COVERS”.