Pit Bike strange cut out under full throttle

[speeding_ant]

I'm having a strange issue with my pit bike, where it will start cutting out under higher rpm as soon as I give it full throttle. It's not a 'splutter' cutout, it seems electrical. It just started doing it out of the blue.

It's a Keihin carb on a Lifan 125 CDI engine. I've pulled the carby apart and cleaned it, and it all looks good. I adjusted it to factory specs, and tuned accordingly. Spark plug is only slightly black, and seems to be getting full spark. I've cleaned out the tank, and it's running fresh fuel.

There's the stator, cdi unit and coil (and potentially spark plug). What do you think would be causing this odd spark cutout at higher RPM?

[Akzle]

are you sure its spark?

[speeding_ant]

are you sure its spark?

Not absolutely sure, no. The carb does look good though (no blocked air holes, and it's tuned nicely, seals and floats look good), and it doesn't 'feel' like a fuelling issue.

On top of that, the bike idles nicely and there are no odd sounds coming from the engine.

[speeding_ant]

I went through the 'loom' and checked that no wires were exposed and that connections were good - no problems there. The coil looks like a coil...

[Ribit]

Clean and gap the spark plug or The spark plug might be cracked and it's only faulty when its under high loads. Perhaps replace it to eliminate it from the problem.
After that check the electrical connections back from the plug to ensure the spark is a good as possible

[speeding_ant]

Clean and gap the spark plug or The spark plug might be cracked and it's only faulty when its under high loads. Perhaps replace it to eliminate it from the problem.
After that check the electrical connections back from the plug to ensure the spark is a good as possible

Cool - might replace spark plug and see what happens. I gave it a clean, no idea about what gap this engine requires but it's not exactly a high performance motor... Connections back from the plug are clean as a whistle.

Failing that, a CDI unit is $10 from ebay... would that be the next logical replacement?

[Dave-]

sass this guy too

http://www.ngk.de/en/technology-in-d...nition-cables/

[ducatilover]

I'd close the plug gap for a laugh and see if it helps

[Akzle]

Cool - might replace spark plug and see what happens. I gave it a clean, no idea about what gap this engine requires

When in doubt, gap with a hacksaw blade.

[speeding_ant]

Well, replaced and gapped the plug, same issue. I've found that it seems to rev out in neutral, but cuts out when in gear.

Is that a chain length or clutch/tranny issue?

[gammaguy]

I'm betting its the secondary coil

[Akzle]

Well, replaced and gapped the plug, same issue. I've found that it seems to rev out in neutral, but cuts out when in gear.

Is that a chain length or clutch/tranny issue?

could be clutchy.

what does "cut out" mean?

[Drew]

I'm betting its the secondary coil

I'm gonna bet it's the primary coil (charging one)....since I think the secondary coil is the one with an HT lead coming off it.

I suspect we're both betting on the same thing though.