Kawi2strokes.com Forum

Guys,
I've had three triples now, that I'be rebuilt bolt for bolt and with the help of this community, had 3 great bikes. Now I'm stumped.... on of all things a friggin Yamaha Vino 1 cylinder 2 stroke 50cc scooter with CDI and the story is he did a 75 mile ride and a couple of mornings later, it wouldn't run at all.
It will not pull gas mixture thru the carb to run. I'm been through the carb and I'm 100% sure the carb is good. Even swapped for a new carb. It's getting gas, float level is fine. I can get the bike to run off carb spray, but it seems like it won't draw the fuel on its own. Reed valves look new, and no vacuum leaks.
As a 1 cylinder..... Do I need a leak down test?
I hadn't thought I needed to check compression because it starts and revs but I just can't see why it won't draw mixture on its own.

Any ideas?

See if the base gasket is leaking..?

Had that happen on a single cylinder 250 ... broken piston skirt exhaust side.

Yes, do the pressure/vacuum,tests, could be a seal,

Those small engines seize easily because they work so hard, and then have not enough compression to even start.

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1974 Kawasaki H2B 750, 1981 Yamaha XV750 Cafe, 1986 Kawasaki KDX200, 2003 Honda XR100, 2004 SDG140. 2006 Ninja 500R Turbo intercooled fuel injected.

Thank you, that's the direction I think we have to go! Head and jug coming off! Thanks again!

Taking the top,end off will not tell anything about the lower end sealing, prezzure/vacuum test first, then pull the top end.

Long shot.....

rubber piece between the carb and the cylinder...leaking ?
bowl gasket have a hole in it for an air port...if there's such a port and gasket doesn't have hole for it...well, no air.

Just thinking out loud...

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Busy doing more riding than wrenching.......

Pulling the carb and pipe off the engine, assuming it is a piston port engine, and using a mirror to look into the ports while rotating the crank slowly will get you to the point you will know if the piston is DOA. Pressure/vacuum teasting will confirm or deny the integrity of the lower end, and sealing, then, if it doesn't pass, take the top end off.

I've never seen a small engine work so hard it grenades itself, when properly tuned. Moto3 ran 125 2 strokes singles for a long while, they didn't blow up race to race, and they spun to 18,800 rpm's.

Guessing this 50 has an iron cylinder. Iron does not have the thermal expansion of the aluminum piston, so must have large bore clearances to begin with. The bore clearance only gets tighter as the engine works harder, plus iron cylinders don't dissipate heat like aluminium. Result is often a seize.
As a teenager I was frustrated with my Yamaha FS1 and this problem, so I machined both ends of the cylinder and hacksawed the fins off. Machined end caps for the top and bottom of the cylinder and wrapped a copper sheet around the end caps and silver soldered to the exhaust stub. Added a heater core radiator to the bike and just used thermal to circulate the coolant. I then could run .00075" of bore clearance instead of the .005" I had to run before as air cooled. Was so mechanically quiet too it was amazing.

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1974 Kawasaki H2B 750, 1981 Yamaha XV750 Cafe, 1986 Kawasaki KDX200, 2003 Honda XR100, 2004 SDG140. 2006 Ninja 500R Turbo intercooled fuel injected.