I have a TY250a that I have a stalling issue with. What happens is it starts normal 3rd kick,runs well at lower rpms and during a hard accelleration (say between sections) it justs loses all power but will usually stay running or will start right back up. This issue happened yesterday as I had been riding typical section practice and stopped to talk for about 10 minutes and when accellerating after starting it fell flat again.It has a new Mikuni B&J carb, Power reeds,air filter. I have pulled the carb off many times after these occurances and it and the jets are all clean as new. The gas cap is venting fine also. I do notice that the fuel hose between the petcock and the filter always has a good sized air bubble in it and does not have a high point in it to trap the air. I have yet to look at the points.

Filter partially choked?

Try a new spark plug.

Do these thinks have points ?

If they do change the condensor as well.

I would defenitely look toward the gas side before anything else.:Tap fron the thank OK, as idmcc_sec suggested fuel filter or maybe floats not well adjusted in the Mikuni.

You can run the motor until it start to loose power immediately shut the tap and the motor and check if there is gas in the float chamber.

I had the problem with one of my TY's and it was a aftermarkt petkok.Fuel was coming but under power,uphill etc.it lost the power.I went mad,i changed everything and then back.and then i changed to a originla Yam petkok and it was okay.

Chrs:Mike