Hi, I recently bought a 1975 Suzuki RL250.  It ran but not well.  I found that the main jet was plugged.  After carefully cleaning it out and blowing it dry with compressed air, I reassembled the carb, gassed it up and it started easily.  It runs for a while but looses power in a couple minutes and then stalls.  it will not restart until it cools down.  Any suggestions as to how to adjust the carb?  It has a 130 main jet.   I'm running modern synthetic oil at 40:1.    

Thanks, I'll check those out.  I understand that a 140 main jet was std and I have a 130.  But like I said in the original post, the bike runs worse now than before when the main jet was plugged with debris.

I'm actually unsure of the year.  frame number is RL250-19298.  pls see pics.

Thanks, so I guess my bike is a 1975.

Okay n thanks.  I see your a Montesa rider.   Nice Bike.  I sold my 99 Cota 315R and decided to ride vintage....  

if it dies out after a couple min run time then insure proper fuel flow from tank and that it is vented properly.. there is a screen in the fuel petcock as well you know.

This kind of problem can be due as already said a blocked tank breather or something in the ignition breaking down under heat, what sort of spark do you have immediately after the breakdown?

good suggestion on checking spark.  Will do.  Thanks!

still have spark when it stalls.  The air screw seems to have little effect on the engine.  When I bought the bike, the gearcase was full with oil, but after only a little running the gear case oil was way low.  I'm running 50:1 syn oil now but it is still smoking.  No visible leaks for crankcase oil.  Could the crankcase oil be causing the smoke and carb issues?  Thanks

To burn that much oil you could send smoke signals to Alaska ! Gear / engine oil smells completely different from 2 stroke when burning, generally this will not stop the engine running, and if it did you wouldn't be able to restart without a new plug.

I'd go with suggestions above, either poor fuel flow, or electrics breaking down with heat.

sounds like crank seal has gone

Okay, now my RL250 is running fine for a few minutes then it stalls or only barely idles even with a full throttle, and choking has no effect.  At this time I notice the breather tube from the rear right hand side (clutch side) has spewing out crankcase oil.  The airscrew has little to no effect when the happens.   Does this mean the crankcase seal is leaking?  thanks

My bad regarding the crankcase oil.... I'd overfilled it.  Still have the same problem stalling after a few minutes of riding though.

I drained and refilled the crankcase with the proper level of oil.  Now the bike seems to be running without shutting down!  Thanks everyone for your help!