My TTR50(13mm Mikuni VM carb.) will not start without the choke on. After less than 10 min.I can shut off the choke and it will keep running. The idle speed screw has to be set a little on the high side to keep the bike running. The air mixture screw has absolutely NO effect on my set idle speed. Otherwise the bike does run ok. I live at 3900 ft above sea level in the Mojave Desert. Do you think the bike is running very lean? What size main jet and pilot jet should I use. Please Help.

I would say, if it hasn't been jetted, then it should for that altitude.

I have no idea what to, but there is a company on eBay, from the USA that specialises in re-jetting. I have seen them advertising jetting kits for the TT600, so it might be worth a look there.

Thanks for the responce! Yeah...Now I need to find out what size pilot and main jet to buy. Do you know where I can go to find this info.?

My TTR50(13mm Mikuni VM carb.) will not start without the choke on. After less than 10 min.I can shut off the choke and it will keep running. The idle speed screw has to be set a little on the high side to keep the bike running. The air mixture screw has absolutely NO effect on my set idle speed. Otherwise the bike does run ok. I live at 3900 ft above sea level in the Mojave Desert. Do you think the bike is running very lean? What size main jet and pilot jet should I use. Please Help.

If the bike is stock then it should be ok as is. At extreem altitude there may be slight variations. Don't forget at altitude your bike carburation will actually go rich, due to the lack of air density, but the same lack of density will reduce the vacume in the carb so reducing the draw over the jets so it 'should' self compensate to a degree just HP figures drop off.

It may be worth cleaning your carb before you go further.

Yeah, Thanks. Ill make sure I clean it first and than go from there.

Some Yamaha owners say that these small carbs are way to lean from the factory. I've heard that people who get larger main and pilot jets eliminate the need for the choke.

Yeah, Thanks. Ill make sure I clean it first and than go from there.

Some Yamaha owners say that these small carbs are way to lean from the factory. I've heard that people who get larger main and pilot jets eliminate the need for the choke.

You can't eliminate the choke. Every single engine on the planet needs a slightly richer mixture to start, then is leaned out for emmisions, economy, noise (yes really). The people who don't need choke are just wasting gas.