Topic: Cylinder/Piston issues: QT50

[Tugboat]

I've been helping a friend of mine with her Yamaha QT50 scooter. Was dying a lot - like it was running out of gas even when she'd just filled up. Pulled the carb and it was full of crap. Cleaned out the gas tank, installed new fuel line + filter, cleaned & rebuilt the carb and petcock. Put it all back together - still won't start. It's getting gas & spark though, so I checked the compression. It was 75-80 and should be 110-120. Pulled the head & cylinder off and, well, see below. This thing is a 2-stroke w/ oil injector (no premix) and has a YT60 head & piston to get a bit more power. Any idea what could've caused this?

[scottly]

I've been helping a friend of mine with her Yamaha QT50 scooter. Was dying a lot - like it was running out of gas even when she'd just filled up. Pulled the carb and it was full of crap. Cleaned out the gas tank, installed new fuel line + filter, cleaned & rebuilt the carb and petcock. Put it all back together - still won't start. It's getting gas & spark though, so I checked the compression. It was 75-80 and should be 110-120. Pulled the head & cylinder off and, well, see below. This thing is a 2-stroke w/ oil injector (no premix) and has a YT60 head & piston to get a bit more power. Any idea what could've caused this?

My guess would be that the oil injector wasn't working properly.

[crazypj]

even if there is oil in tank, if it has been allowed to run dry you have to re-prime pump, it doesn't self bleed
 There will be a small screw to allow bleeding (probably chrome, about 6mm dia, #1 Phillips)

PJ

[Tugboat]

Right on - the manual confirms the bleed screw. But the second sentence in the "bleeding" section is "Start the engine and run at idling speed" which of course I can't do. Any suggestions? I know to bore out or replace the cylinder, new piston, new rings, new gaskets. But then what?

[scottly]

Maybe pre-mix until you verify the injector is working? The worst case would be too much oil if it is.

[crazypj]

It will gravity bleed, just takes longer. (maybe 10mins?)

PJ

[bobjohnson]

Yep with 2 strokes its always good to premix until you can make sure everythings in spec. That definately looks like a lack of oil to me. She probably let the oil tank run dry by accident.

Actually thats why i like to premix anyway. Maybe im just ruined from my years of riding my little YZ80, but i dont feel confident with an auto-oil mix.

[crazypj]

It never a good idea to pre-mix unless you re-jet.
 oil doesn't burn like gas and creates heat, plus, you now have a lean mixture (jet was designed to flow a certain amount of gas, if you pre-mix, you have less gas flowing)

[scottly]

I would rather pre-mix, with a minimal amount of oil, than risk more of the same damage!

[bryanj]

Exhaust gasket leak or non standard silencer

OR not jetted to suit the big bore

OR timing different between 50 and 60

OR wrong plug

OR air leak on inlet