Engine Trouble Starts for a few seconds then dies.

[Frankfort MB's]

going bad when hot is either head gasket leak or stator coil.
plane your head truly flat using sandpaper on glass and make your own head gasket out of hi temp gasket material. throw that junk aluminum one away!
stator coil output can go bad when hot if the screw head to stator frame connection isn't clean metal to metal.
also stator winding can be broken but touching enough to conduct while cold but not when hot.

I'd listen to Jag.... He knows a lot more than me

 

[Louie]

Tightened down the head real good, I don't think that's the problem.
Ran it for about 10 minutes two days ago then died. Today won't even start.
Took the plug cap off and tried to start just to see what it sounds like. It sounds exactly like when I try to start it with the cap on the plug. This makes me think it's actually an electric problem.
Had a spare CDI and put that on, didn't do anything.
As I said before, the magneto and CDI are both giving me correct resistance values. Should I try replacing the magneto?

 

[Louie]

scratch that, just tested the magneto again and I'm reading around 30-50 ohms now. Going to replace it.

 

[Deleted member 12676]

http://www.dragonfly75.com/motorbike/testCDI.html

 

[Louie]

SUCCESS!
Here's what happened:
Tested the magneto again while it was still in the engine, was getting around 60 ohms. This is way off of what it should be. Took it out intending on replacing it with one from a new engine I have. Tested it again while it was out of the engine and got the right resistance value again (370 ohm). This made me pretty mad but I went ahead and replaced it with the new one anyways. It fired up and was really strong! It didn't loose revs any more when I disengaged the clutch. Rode it for a good 15 minutes without any problems.
I didn't notice any oil in the magneto case originally, but the darker color of the original magneto wrap compared to the new one worries me a bit. I will probably replace the gasket or seal or whatever is leaking.

 

[crassius]

some of those coils are almost black right out of factory (guessing it depends on what they paint them with to keep out moisture)

old one may be good with a bit of soldering on the ground at the upper corner of the armature

 

[gary55]

some times you just gota throw parts at them and enjoy your growing stock of spares.