Help with yamaha chappy 2 stroke

[BlasterMaster15]

after workin on the blaster motor so much i decided to try to fix this old school moped and am absolutely stumped with how to fix it. it will only start with choke on and takes a good 25 kicks or a bump start pulling it down my whole block. the carb is perfectly clean the neddle clip is at the middle and its all stock. reeds are good, has good compression and has spark. it starts up and runs good for maybe 15-20 seconds and dies under throttle. it wont run without choke. it revs really high by itself i thought airleak and put rtv on the intake and it seemed to help. it smokes like a mofo when it does start and i dont know how many tuirns to set the mix screw. anybody have any ideas?
currently it wont start at all

 

[UncagedShadow]

did you try leak testing it? Might be an air leak in some other place. Sounds like it might be running rich. Now all the smoke could be from over mixing the gas, but if the smoke is white, I say crank seals all day.

 

[caseywoods5]

i built a 2 stroke mini bike and it was doing the same exact thing, turned out that the fuel line from the tank to the carb was too long and was pinched off. gas would come through it but very slowly so it wouldn't stay running long.

 

[BlasterMaster15]

can i leak down with my blaster leakdown tester?

 

[BlasterMaster15]

its thick white smoke and smells horrible

 

[Luni]

It honestly sounds like its running lean to me. The key statement is you have to choke it to get it to run, and it revs up on its own.

Have you checked the carb float valve? Make sure the float isnt stuck and it is getting enough fuel? Check the fuel line and make sure it flows freely.

 

[scotj77]

thick white smoke that smells awful is tranny oil. you are leaking from the oilseals on the crank to the tranny

 

[UncagedShadow]

As i stated above. I would say your running lean and have a crank seal leaking. An air leak is probably causing the lean condition and the clutch side crank seal is likey the one leaking. I beleive the blasty leakdown tester should work. might need some modding to fit it on the exhaust port though.

 

[Big D]

i agree with luni. i think it might be the float vaulve. thats what happend to my dirtbike and it turned out after sitting 4 years it stuck. my dad also buy vintage sleds, fixs them up, sells.. most of the time you see old sleds that run only with choke. its a carb problem

 

[Big D]

you need to clean your idle jet