Hi all, I Have a 1970 dt1 250. I’m gonna give you the whole deal lol! It started when it would idle fine and I could ride it for like 5 minutes then it would start dying and I would have to hold it wide open for it to stay barely running. Tried everything cleaning the carb, new, spark, plug ect Nothing worked so I finally rebuilt the top end, thinking maybe it was compression Ran fine for like a day or two, then went back to having to hold it wide open! Now it wont even start! It’s got spark, air, and fuel “that I can tell” Sometimes it’ll start for a couple cranks then die again! HELP!!

Originally it ran fine until warmed up, then had a problem. Have you verified the correct jets? If it won't currently start, shoot some starter fluid into it and try to start it. If it fires, then dies it's probably a fueling problem.

Im not shure if the correct jets are in it but iv never changed them iv had it for 3-4 yrs now with no problems till recently. But will definitely try starting fluid tomorrow! Thanks for the help!

Ok so i was able to get it started and running couldnt ride it much bc of snow it gets up to like 5500-6000 rpm then sputters. And when under load it sounds like it’s missing like jerking around?

I was poking through the tech library and found this amazing Yamaha publication. I don't know if it'll help, but it is a fascinating read
www.yamaha-enduros.com/index.php/service...69-yamaha-shop-guide

All, the creator of this site Gary, used to have a saying that "you clean your carburetor and then you clean it again". I may not be quoting exactly but there are a few keys to these old Yamaha and having an impeccably clean carburetor is at the top of MY list. If your pilot is not perfectly clear, it will not start. Needle is midrange and main is high end. That is slightly over simplifying but your carbs is the key to starting and clean running. In order: spark, timing, air leaks and crank seals are where I always look after carburation but I always start with making sure my carb is perfect.

Video with good audio would be great if possible so we can hear how it gets before it stops.
Would give us a good handle on what help to offer heading in the right direction.
In my opinion if it starts by fading away "not instant cutout" then it's a fuel issue.
If it cuts out instant or even cuts in & out instantaneously then its a spark issue.