Need help! Yahmaha 250 2-stroke

this might help you Identify

http://dropbears.com/y/yamaha/yzseries74.htm

Looks like a DT, it was an on/off road bike. Had lights and turn signals.79-82 is about right. Based on what you say and the way it looks. It could be a lot of things. Somethings weak so it can hang on and idle but not due much else. Its not odd.
Hows the compression when you kick it? Does it have reeds?

77-79 DT 250 enduro. Make sure all circuits in the carb are clear. Pilot circuit especially. Blow them out with a bit of air using a rubber tipped nozzle to seal against the passage openings. Then verify they are clear using a can of aerosol brake or carb spray.

Spray through all passages using the little straw on the nozzle of the can, and make sure fluid is squirting through the passage by watching for spray coming out the opposite end of the passage.

Some of the Mikunis from that era had a little O- ring on the tube that holds the main jet. Remove the tube, and check the O- ring. If it is bad, fuel will be drawn around the tube, instead of being pulled through the main jet, making the bike run rich, and never wanting to clean out.

Just checked the parts fiche. Shows no O-ring . Check the condition of the choke plunger as well.

1M1 prefix = 1977-78 DT 250.
http://www.off-road.com/dirtbike/yamaha-vintage-dirt-bike-id-guide-2179…

Your head is off so I assume you mean the cylinder. Get that dirt clean off as you don't want it getting into the motor as you lift the cylinder off. Spray wd-40 around those studs and let it do its work. Don't be in a hurry.

I assume you removed the 4 cylinder bolts that are located inside those 4 holes in the top of the cylinder? If it still does not come off, use a block of wood and a hammer and try tapping upward on the cylinder without breaking off any of the fins. Maybe try tapping upward on the exhaust port