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1991 yz 125 weak spark help!

7K views 5 replies 3 participants last post by  itsmygame 
#1 ·
I recently purchased a 91 yz 125 that appears to have been uncared for for some years. It started when i bought it but didnt run very well. I found silicone jamming the reeds open, exhaust unsealed from the barrel amongst many other minor faults. Ive sorted these problems and the bike now runs fine (although not very fast) for around 20 mins and then cuts out. There is only a very weak spark and i believe this could be the problem. The wiring had been redone with ultra thin cable so ive rewired the bike with the correct gauge wire. Ive tried a new plug, no difference.

Any suggestions of what to check? If there was a resistor for the spark plug where would that be? HT plug? Does anyone know what the resistance of the coil should be? Does anyone know how to test the stator?
Help would be greatly appreciated!!
Thanks, Harry.
 
#4 ·
Ohms Test Needed

If you cannot find anything for this, I'd search for an ignition coil. If the windings get hot and expand, they may create a resistance greater than what the plug needs to fire. You may check timing too. It could be off enough to start but not enough for correct spark at the right time (low power).
If the rotor was welded, I'd have to ask if the magnets are still strong enough to create proper coil excitement (low voltage to the ignition coil). Just some ideas, no revelations, sorry.
 
#5 ·
The flywheel was welded together (very badly) and obviously not balanced properly, it looks like the flywheel magnets have collided with the smaller coil on the stator plate at some point (not sure if its the primary or secondary). I've ordered a new flywheel along with a manual. Hopefully this will tell me the resistance readings for the coils. Should be here tomorrow!! Cheers guys.
 
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