Yamaha TRX850 Enthusiasts Forum

Hi all,

Just working on my Father's TRX850 at the moment. I've noticed that when I am behind him on a motorcycle ride and he is on his TRX at WOT lots of black smoke pours out his exhausts. He bought the bike second hand, so we have little knowledge of what the last owner did to it in terms of jetting, but I suspect that the previous owner has put larger main jets in it, probably at the same time he put open pipes on the bike. Likely he went to large on the mains and now we've got unburned fuel pouring out the rear. He's also recently fouled two spark plugs, which is the reason I'm working on it.

My question is whether there are other common causes for the these bikes running overly rich that I should investigate before pulling jets and checking their sizes against OEM?

Thanks.

Common issue with the trx running rich.
Emulsion tube and needle wear on downdraught carbs always rears it's ugly head after 20,000km.
So tubes wear oval and you start to have fuel leaking it's way up past the needle earlier than is optimum and not atomising properly.
100's of threads here on the issue and an aftermarket kit fixes most issues like keyster and alike.

142.5 is your std main jet size and they are set rich from the factory to accommodate all climates world wide.
Plenty here is Oz go down to 140's or 138.5 due to the air density here but you'll find the 142.s work find in a colder higher density climate.

I'm familiar with needles wearing and the resulting rich condition from other bikes. Have ordered a rebuild kit

Carb rebuild kits came in the mail this morning. Pulled the carbs down and, as I suspected, somebody beat me to it and has played around with the main jet size. I replaced the 148mj with the keyster 142.5mj so it should be better than it was! No idea why it had 148mj's in there, I guess somebody was told to jet up when putting on aftermarket pipes on and had a day with it.

I guess my questions are, which needle clip should I start out with? The Keyster needle has 3 notches, as opposed to the 5 of the needle I pulled out. How many turns out on the pilot screw is a good starting point?

Thanks.

Conrod has a good std carb explanation on his thread with results using an O2 sensor plus an example on checking and setting float height.

Completed a carb overhaul with the Keyster kit. Wow what a difference. Starts better, runs cleaner, revs out now. Just not quite sure on the float level height. Where is the thread for setting the float level?