Help... Engine Problems only 1,675 miles on a 1977 DT100

1977 DT100 With Only 1,675 miles

I just purchased a used 1977 Yamaha DT 100. The motorcycle was purchased from the original owner with only 1,675 miles on it. When I got the bike it would start only for 3-10 seconds, then it would putter out. I figured that It would be a quick fix and that I only needed a carb rebuild. I rebuilt the carb, and it fired right up and idled very nicley.

I rode it around for about 9-10 miles and the engine seemed like it had little or now power on it (it would rev real high but wouldn't pull very quickly) I realized now that there may have been a previous engine sieze. So while making it on my way back to my house while going up a slight hill the engine had a loss of power and stopped. Then I couldn't kick it over again.


I Took out the engine and removed the cylinder head and the top of the piston looked like it had been beated up (not a good sign) and the underside of the head cover had some metal pieces which were hammered into it. I looked further near the bottom of the rod (where it connects to the crankshaft) and I noticed 3 small pieces of metal. They are about the size of a piece of rice. I removed the piston to check the piston bearing and the bearing pins were all in good shape. I'm assuming (???)that the bottom end rod bearing may have failed releasing the pins of the bearing near the crank, and somehow they got up the cylinder on top of the piston, causing it to sieze? I havent taken apart the case yet, to determine if this is the actual problem, but I dont know if it's worth my time to determine the problem.


Should I bother rebuilding this engine or just get a used engine? Do you think that if the bottom-end of the rod bearing failed and the bearing pins came out that it would have damaged other areas of the engine?

I'm bummed out, especially with a bike with only 1,675 miles on it. I cannot believe the lower rod bearing failed - I have never heard of this happening? Any ideas on labor costs to split the case and replace the parts and re-boring the cylinder wall?

Am I better off just getting a used engine? if so, does anybody have any recommendations on where I can get a used DT100 1977 engine? any ideas on prices?

Thanks!!!!!!!!!!!! & Happy Riding

Is the crank locked up? If not, what locked the engine up? Is the spark plug still in one piece? or has the electrode come out of it.

In all likelyhood the crank bearings rusted as a result of being in storage so long. The engine will start fine and run long enough for the bearings to disintegrate. Main crank and con rod are probably rusted also. Have seen this quite often in inproperly stored snowmobiles. The crank can be rebuilt if you can find a con rod kit for it.

Good Luck!

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