I've been fiddling with this for awhile and there's about a million random hits about motor stuttering with various solutions.
I've finally nailed down the actual "cause" but I don't have an explanation of why or a definite way to fix it.
So here's the story:
2004 Bayou 250. Suddenly started stuttering under load. Idles perfect no matter what.
Found the air filter had disintegrated. Rebuilt the carb, Replaced the air filter.
Ran great for test drive. Next time tried to use it and it stutters to death after a short warm up.
Couldn't find anything obvious. Tore the carb apart again. Checked every orifice again. Reassembled and ran great again for test.
Next time tried to use - warmed up and started stuttering.
Checked the air/fuel jet adjustment (again), couldn't get a change. Changed the gas (again), no change.
Changed the plug, nope. Getting good fire and advised by a lifelong dealer/mech that the coil almost never goes bad.
Finally decided the CDI is probably going bad. Checked the connector, also cleaned up all the other connectors. Ran fine for a whole day.
Yesterday it did the same thing, stuttered, spit and misfired to death.
As I was limping it back to the shed, I was fiddling with the wires and suddenly it fired back up smooth.
I continued messing around and realized it was the front (right handlebar) brake lever!
If the lever moved just a bit it would practically kill the motor, but pushed all the way out or pulled inward at all it was fine.
Really worked with it today, If you pull the switch wires loose and the motor will stutter and try to die under throttle, but if you connect the wires it runs fine, but the brake lights stay on.
I checked the switch with a meter and apparently it's actually a potentiometer not a switch, but the readings are ridiculous.
almost no resistance fully open, a little more, then HUGE resistance then completely open at full depression.
So I'm guessing that the system has to have a little resistance to detect it's "OK" and the brake lights remain off.
But it makes no sense that this thing kills the motor.
Any insights from someone who's more knowledgeable about these?

 

Brake light switches are off or on, not potentiometers. I would guess the regulator is overcharging and causing the CDI to slowly cook hence the misfire but, with a light on, the voltage drops back a bit and the CDI is happy again. Test the voltage at the battery with the fault on, if it goes over 15v as you rev up you have a faulty regulator.

 

Well that certainly made more sense than anything I came up with. Sure enough it's about 14.4 on the battery when off. Key on drops to about 13.5. Cranked up it returns to about 14.4 with the brake switch wires connected and lights on.
With the brake switch wires apart so they are off the voltage starts climbing up to around 18 or more.
Time for a voltage regulator!