Why will it start and then die out after 30 seconds riding and overflow with gas.

[lilbeanbikeboy2007]

So recently i had bought a new cylinder and piston and a reed valve kit with a pwk21 carb. I ported the cylinder, raised the exhaust roof a bit and lowered the intake a lot, like maybe 3mm. The bike wasnt running well with the reed and the new carb so i took it all out as well as the windowed piston and put the right one back in. Gas was streaming into the cylinder before i took it all off and im wondering if maybe my crankcase is flooded. Or maybe the piston is too small and i dont have the best compression. Either way i havent been able to get it run longer than 30 seconds. It only works after i drain the cylinder of all gas and turn it over a few times. Its too much gas and i never had a problem with my old nt carburetor. Theres so much gas getting pushed out of the intake and the engine usually dies with a very loud backfire. Im not sure whats wrong. Everything should be working but its not.

 

[mony0_3]

Seems the jet is too big. A #68 is ideal for most engines.

 

[Wyvern]

If you are flooding out at less than 3/4 throttle two things come to mind.

Pilot jet too big or out of adjustment.

Bowl float stuck and overflowing.

One way to determine if the float is stuck is shutting off the fuel valve when its started and seeing how it runs, you'll have at least a couple minutes before the bowl goes dry (usually more)

 

[Chainlube]

Bowl float stuck and overflowing.

One way to determine if the float is stuck is shutting off the fuel valve when its started and seeing how it runs, you'll have at least a couple minutes before the bowl goes dry (usually more)

The float could have a crack in it and has sank and/or the float needle is stuck and letting the fuel to run continuously.

Do you have a fuel filter?

 

[lilbeanbikeboy2007]

If you are flooding out at less than 3/4 throttle two things come to mind.

Pilot jet too big or out of adjustment.

Bowl float stuck and overflowing.

One way to determine if the float is stuck is shutting off the fuel valve when its started and seeing how it runs, you'll have at least a couple minutes before the bowl goes dry (usually more)

Im going to take it apart soon to see whats wrong but i really do think theres something wrong with my piston, while the carburetor could definitely be the problem.