Hi there I have a 1999 TTR250 (USA). I have a big gun exhaust and snorkel removed. I just installed the acerbic 22l tank and Im getting great gas flow and ventilation to the carb. It seems I'm all the sudden running rich because when I pull off my air box cover it seems to run a bit better than normal.
I have a 147 main and 50 pilot. The bike ran great about a month ago and now after changing the tank, I'm having a rich condition. I checked the float height and its at the correct position. Bike always starts and idles perfectly so doesn't help with troubleshooting. When cold, the choke is needed, then once warm it idles smooth so I know that's at least correct.
I also did a complete carb tear down, soak, and cleaned all ports with a wire. Accelerator pump is pumping great. reset fuel/air mix jet to 1.5 turns out.
Yesterday, I couldn't get the bike to rev at all above just an idle with stock jetting or bigger pilot jet. It was sputtering so bad off idle I wasn't sure if it was lean or rich or timing or valves (until today when I reinstalled stock jets and took airbox off and bike ran way better up to 3/4 throttle.
Today, I leaned the needle to the highest position (closest to blunt end) and now I'm getting good throttle response and smooth acceleration up to about 1/2-3/4 throttle. Between 3/4 to full I begins to sputter a bit and sounds like it starving for air/getting too much fuel. I ordered the mikuni n102/221 small round main but I have run into a problem. The threading is WAY smaller than the teikei 147 main that's installed and it definitely won't fit. Has anyone else had this problem, and how could it be that everyone else seems to use these but on my carb it clearly won't fit the larger carb threads? Also, I have a mikuni vm28/486 52.2 pilot jet that doesn't seem to make any difference when installed. Also, after doing this I realized, should I have ordered a smaller main because of the rich condition. I cannot make sense of increasing air flow through the bike with the pipe and airbox mod but having to decrease the fuel...i could be wrong there.
The biggest improvement I've gotten so far is leaning the needle out and opening the airbox entirely and using stock jets (very curious as I did not have a rich problem about a month ago and I have a big gun exhaust...so more air coming in and out should require more fuel...not less...?)
If anyone has any pointers on the size of the mikuni main jet problem I'm running into, and or common symptoms for a TTR to miraculously start running rich (that isn't one of the above things I've checked) I'd REALLY appreciate some insight. My main curiosity is that the bike starts and idles perfectly no matter what adjustments I make to the carb (besides the idle screw of course).
Might not be the fix but just because it's happened out of the blue it might pay to check that the inlet stub ( the part that holds the carb to the head ) isn't leaking . Tends to happen a bit the rubber comes away from the inner metal
