Engine Trouble Engine stops after a couple seconds

[TomONaBike]

Hey. I'm not new, on my second bike, have done complete tear downs, solved blown gasket, bad magneto and other problems but I am at a loss. My engine was running fine, and then it backfired and slowed to a halt. Now it runs weak for a few seconds before it stops firing. I have trawled through the depths of this forum looking for solutions, and tried every last one that I could find. Let me review:

Spark test with plug against acorn passes but anyway I...
Removed kill switch
Cut crappy connectors and twisted black and blue wires directly
Cut off white wire
Resoldered blue wire and wire from magneto
Replace spark plug cap
Replace spark plug

Looked for air leaks (found none)
Removed cylinder top half and cleaned carbon deposits off top of piston
Sanded gasket and where it meets
Torqued down acorn nuts and plug

Disassembled cns carb, float working, jet in place, fills with gas
Idle screw in where I want it
Air/fuel screw at a good level
Poured out all the gas out of the tank and carb bowl
Replaced with fresh gas and oil at a precise 1:32 mix

And nothing. After it sits for a while, I can get it to run for a couple seconds then it silently quits firing or backfires once and is done. All I have left is to pedal and turn the engine over without hope of it kick starting back to life. I even tried running it down long hills, doesn't wake it up.

Any suggestions would be appreciated because I need this thing, have invested countless hours working on it, and I don't know what I have to fix or replace.

 

[crassius]

first 2 things that come to mind are a CDI unit that has lost its timing point, or a blown crank seal

 

[Purple Haze]

Sounds like an electrical problem. Beg, borrow, or steal an ohm meter and check the mag coil and cdi. The specs are on Grubee's website. If they check good, check the woodruff key on the magnet. If it is sheared or the nut holding the magnet is loose, your timing will be off. Hope this helps.

 

[TomONaBike]

Hey guys, thanks for the help. I thought maybe I put on the magnet backwards, and went to check it. On right, but i bet some people may have that problem. I was able to confirm that the key slot was at 1 oclock at TDC as specified and that the key and nut were properly installed.

Then I got out my multimeter and put it to the cdi unit. I got readings that absolutely did not match the posted readings. And my coil gave 220 kohm readings which sound low, but that was what all of the ones Ive ever had read. So since I dont have another cdi and I have been getting a spark, i tried swapping out the coil with an old one that I had repaired previously by resoldering that little wire.

Eureka! It worked. So I rechecked my original coil, it gave a closed reading now, which really confused me because it gave a good reading before, I recently resoldered the joints and was getting spark.

What I think I learned is that the coils can go bad gradually and intermittantly particularly with regard to having a load. If that is true it would explain why my engine would start and then quit and give spark when the plug was removed.

These things are still quite a mystery to me as almost every time this motor malfunctions I solve it without knowing how. lol

 

[Purple Haze]

Glad you got er goin. Those coils are pretty crappy, but the new ones are better, the ones with 4 mounting holes instead of 6 for the older ones. Be sure to make it watertight, water will fry these coils in short order.