I have a TY250 (Yamaha Trials) -1974 that is in great shape. It starts up fine, revs, as well as idles just fine. When I place in gear and accelerate it bogs down. It's like it's choking for fuel. I can hold the choke down and this helps a bit, but eventually dies. Does anyone have any idea what might be causing this. I've been through the fuel system about three times already. Taking all advice as would like to get this thing up into the hills and start riding the ridges... thanks for any help you can provide..

You state you have gone through, but did you remove the jets and pass a fine wire thoough them? Many have been fooled!

Insure the tank is venting and flow to the carby is good as well, just thoughts!

If it restarts fairly quickly it would point to fuel, if not, ignition.

OK assuming all jets vents etc are clear and that the jetting slide etc is correct is it on "electronic" ignition or points? if points swap the condenser as it's surprising how often these let you down and don't just assume that if it's a new one it's ok.

You don't give us many clues. Have you recently acquired this bike? What altitude are you riding at ? What carb is fitted and what are the jet sizes, cut away on slide, needle setting ? What plug are you using and what is the gap? Has the ignition system been changed ? What state is the air filter and exhaust system? Is the reed system good? Are you using pre mix and what ratio ?

Fill in the blanks and I'm sure we can all help.

Tony

Crankcase seals cause this. Easy to do.