'90 Yamaha 650 SJ Won't Fire "Enough"

[SuperMein]

Weirdest thing. Looks like I'm intermittently getting spark. It'll crank and immediately fire once and then won't fire again. I've pulled the plugs and I've both seen spark, and not. It's not a fuel problem I'm almost positive because I've primed it and sprayed some thrust into the cylinders. Plugs smell like fuel. I tried a new plug (even though the ones in there are pretty new). Put a newer battery in. Basically all the easy stuff I could do, none of which helped.

I've got a paperweight 701 that seized up, so I'm using it for parts. It's ebox looks like ass, but I stole every piece of electronics out of it and put it in the 650 with the exception of the relay (CDI, regulator, coil). Same problem. It'll fire once, then just crank. Cranks hard, so its not the relay or battery. If I let it sit for 10-15 and try it again, it'll fire once, then nothing.

What's the next logical thing to test and how do I test it?

Thanks

 

[SuperJet-Dave]

Could check the resistance between the spark plug boots. I think it is supposed to be between 3500 and 4700 ohms. If not, could have an issue with the sparks plug boots, wires, or the coil. Not sure if the coil is swappable from a 701, but the boots and wires are.

 

[SuperJet-Dave]

Could check the resistance between the spark plug boots. I think it is supposed to be between 3500 and 4700 ohms. If not, could have an issue with the sparks plug boots, wires, or the coil. Not sure if the coil is swappable from a 701, but the boots and wires are.

The 3500-4700 ohm range is for a 701. Guess I cannot be certain it is the same for a 650. Although I would guess that they are. It is essentially the resistance across the coil and I would think the coils are the same. Happy to be corrected.

Also, in my opinion, the weirder and more inconsistent the problem (like yours) the more likely it could be the CDI.

 

[jeepcreepcj]

You could also try unplugging the stop switch to eliminate it, I have seen a corroded switch cause weak / intermittent spark. On a later rn ski unplugging the 2 pin connector near the Ebox, I believe w black and white wires, takes the switch out of the equation. Yours is likely the same or similar.

 

[mthomps]

Sounds like stator or cdi to me. Time to pull the flywheel cover. Also Cdi's from crusty boxes aren't a good way to rule out cdi.

 

[be_lain]

Check the woodruff key, it could be sheared. Happened to me twice with the same symptoms

 

[Rushford_Ripper]

Timing off?