Engine won't start, gas pouring out of air intake.

L

Louie S

Guest
Working on my 4th build.
After the bike sitting in the garage for a few weeks, I finally got around to getting it running again. Had it running just fine, put maybe 3 miles on it over a few days.
Replaced the magneto because it seemed the spark was a little weak and there was some gas in the magneto case because it had leaked from the petcock. Turns out i just needed to turn up idle speed. Kill switch also does not kill the engine.
Anyways, I went to start it, hadn't changed anything since the last time I rode it (a few hours before) and when i dropped the clutch in, the engine completely locked up, and gas poured out of the air intake. Tried again, and still won't turn over at all. I know the chain isn't jammed because it moves just fine when I disengage the clutch. When I drop the clutch in, the wheels just skid and the cylinder doesn't budge.
What's wrong?
 
Oops didn't sign in to post that.

Just opened up the cylinder and...cracked piston!!!!
 
Can anyone tell me why it would have cracked so i don't make the same mistake?
 
sounds like wiring problem in kill button, bad float or crap in needle valve, and would need a pic of piston to even guess about that - what did you do diff from 1st 3 builds?
 
I'll get a picture tomorrow, but essentially the hole where the wrist pin goes shattered on the inside of the piston and the shattered pieces went down into the crank case and jammed the flywheel. Got everything out and did a visual inspection. Nothing damaged except piston. Gas was pouring out because piston was stuck around TDC and was blocking intake manifold so the fuel just backed up through the carb and out the air intake.

I imagine my oil/fuel mixture was too lean for the break in and I may have riden it too hard without fully breaking it in.

Still not sure about the kill switch but I'll cross that bridge once the new piston is here and installed.
 
sounds like the circlip that holds the wristpin came loose - often, this will leave chunks in bottom that are hard to clean out completely
 
No the circlips were fine, although the wrist pin itself was sheared in half somehow. The part that shattered and jammed the flywheel is the cast holes in the piston that the wrist pin sits in.
 
I think the piston seized and the only give was the bottom of the piston when the Conrod Whent down.
 
pics would be nice indeed , but what oil ratio were you using? sounds like the carb was stuck open filled motor with fuel then from trying to start while full of fuel the extra resistance caused the failure , although i've never come across the actual pin failing usually the bearing .
if you plan on reusing the engine i'd split the case and make sure the main bearings are clean .
 
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