Bayou 250 -- electrical gremlins

[v8unleashed]

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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.

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

[Frank Angerano]

When you say the spark is too week have you checked it ? Taken the spark plug out and held It against the engine while cranking?

[Twilight228]

Out of curiosity, how do you draw the line between normal, and "weak" spark?  Purely visual (consistent blue), or tester?

[Frank Angerano]

It’s a healthy spark that you don’t need to have to look hard to find.   Fresh plug in the bike rest it aginst the head and you should see and hear the spark.   

[JacobSlabach]

I've been wondering this as well.

[Twilight228]

Thank you!

[Frank Angerano]

👍🏻

[v8unleashed]

Yes, I've tested the spark.  I don't actually think it's weak, I'm just wondering out loud since everything else seems to have been covered.  Just got an inline plug tester, and it seems fine with that too.

Does anyone have one of these or similar bike, and could confirm for me the voltage while cranking, at the two leads going to the coil?  Just take the two leads off the coil, plug in a voltmeter, and crank it over?

[Frank Angerano]

If you have spark at the plug and it’s clearly visible then that’s good.  The pick up coil delivers the spark at the exact time it’s supposed to and that may be giving you a problem.   But before you get into all of that double check the bike does not have a rev limiter that could be giving you a problem or just bad fuel? Carburetor is 100% properly adjusted ? Have you tried to make adjustments?