I recently had the dealer put a hot seat performance speed kit and a larger 42mm mikuni carb on my 2001 scrambler 500 4x4. It drives fine as long as I give it enough throttle, but sputters when driving slow through trails. After I have been driving for about an hour or so when I stop it idles like crap and sometimes even dies. This carburetor didn't have a fuel pump and it had to be hooked up prime. All I know is I am not happy with the way it runs at slow speeds and the way it idles. Not to mention the smell the riders behind me have to bear. When I back up fast I smell the exhaust and it almost makes me choke! You would think I was driving a 2 stroke! Anyway can anyone tell me what the problem might be? Did the dealer just jet it wrong or is it maybe because of the carb? Any info you could give me would be helpful because I don't want to take it back to the dealer because I had too many problems with them before and they had my quad for a whole month before I could drive it again!

 

Sounds like you hit the nail on the head. If you check your plug, it's probably black. I would start looking at the carb, for a stuck float.

Rainman

 

You are not alone. I am having the same problem on my 39mm carb on my Scram400. It bogs and dies at low RPM but is fine at higher RPM's. I am getting pretty fed up with all this jetting crap! Dealer is taking care of it. Something between the pilot jet and the needle jet...