QuestionI recently purchased a 76 yamaha DT250 that runs good. After riding around town a little I noticed a slight noise (best I can describe)a constant scraping like a bearing was going out. So I checked trans oil, it was full but smelled of gas so i changed it. rode it again same noise. Im' tearing down the motor to inspect for myself any specific help would be great. TIA!
AnswerNick, the trans oil is isolated from the crankcase portion where the crankshaft rotates by a seal on that end of the crankshaft. Apparently the seal has failed, causing dilution of the trans oil with fuel vapors.
Generally, this kind of problem also allows the trans oil to be sucked into the engine on the intake or transfer portion of the piston stroke. Often the transmissions are sucked dry causing transmission gear/bearing problems.
I am guessing that your bike still has the oil injection system intact? IF so, then the problem may just be in the trans. If not and it is not pumping injector oil into the engine, then it can cause piston seizures, on top of the other failures related to lack of lubrication.
A full teardown is going to be in order. You will have to have to replace the outer seals and have the crankshaft main and rod bearings checked, too.
IF you haven't taken one of these apart before, take careful notes or digital photos of the parts as you go.
There are a lot of different length fasteners holding the cases together. I don't know if a special case splitter jig is still required to separate the cases, or not.
Check with www.vintageiron.com or other vintage 2-stroke websites that specialize in the Yam DT series machines.
Bill Silver
