I'm at a loss. I have a Bayou 250 I picked up a while back that SHOULD run, but won't. Overall it's in really good shape.

Initially thought it was a carb problem, so we played with the carb, and actually got it running at one point. Started up and idled repeatedly. Took it out of the garage and it had good power, ran strong. But halfway up the driveway, it cut out, and won't start up again. Carb seems to be fine, and it is certainly getting fuel. It won't start even with starting fluid.

Only thing I can think is the spark is too weak, or it's not sparking at the right time. Timing looks good. Valve clearance is good, everything inside looks mint. Changed plug, battery is good (on a charger). Checked ohms on the stator (within specs), the crank sensor (within specs), and the coil (within specs). Actually bought a donor bayou 220, and tried the coil and CDI from that. No change. I read somewhere the harness can get frayed somewhere along the frame, but I don't see anything obvious. Cleaned the main ground. Has good compression, over 140.

One interesting thing. Input from the battery to the CDI is 12v. Output from the CDI to the coil will rise to NEGATIVE 120 or so DC volts (not AC) when cranking, then slowly falls off. It's this way with both CDIs. Is that normal? Is it supposed to be negative voltage? The leads to the coil are different sizes, presumably to prevent reversing polarity, so it seems to be wired properly.

On the CDI wiring harness:
BK -- goes to coil neg
G/W -- goes to coil pos
R/Y -- shows 12v when key is "on"
Bk -- 0v when "on," 10.8v when cranking
Bk/W -- from crank sensor

I'd appreciate any ideas, I really don't know where to go from here.

 

That sounds like a polluted and / or gelatinized "gas factory" with jets that do not let anything through.

 

After changing the plug, what color and how much spark did you have at the plug?