125 Dies before powerband!

[Jopiz]

Hey guys.

My bike (1991 kx 125) has got a new symptom and its not running good. Current jetting 158 main, 50 pilot and 16mm float height. Carb cleaned. Free of blockages. Does not spit any ooze but smokes a little.  Fresh piston and good compression. New seals aswell (did a leakdown test with super results, no pressure loss over 15 minutes at all). Reeds are brand new. No leaks. Everything is in mint condition. Spark is super good.

For some reason she dies just before hitting the powerband, or actually is on powerband for 1-2 seconds and completely dies. Then if i wait 5 sec and she's running again. On road or on stand, no difference. Please do experienced mechanics have any advice? I personally feel she's not getting enough fuel. I changed fuel line so its not clogged up. Filter should be okay too.   

-J

[bigbellybob]

could be the gas cap. the vent can get clogged

[Coop]

Start with the easy stuff, change the plug and try again. Just because the spark looks ok doesn't mean the plug isn't fouling under load.

Also check the pipe for blockages. My 125 started first kick after a fresh rebuild but was gutless under power. Turns out I had a mouse nest (and a dead mouse, lol) in my pipe. The bike rips now.

[Jopiz]

Yes that could be. I gotta get rid of filters and all kind of stuff from the fuel line and put just a clear hose. It feels like the float bowl is not filling fast enough to feed the engine which sucks it empty in a few seconds. I have checked floats and they are ok. The needle valve is closing and opening properly.

The pipe is not clogged, im 100% sure. Plug is also new. Spark jumped like 1/2 inch when i tested it. It sure is firing. No coat of carbon, color of insulator was brown on the other side and white on the other side. Wouldnt misfire lead to fouled plug rather than 5 seconds of stalling?

Edit: When i do WOT it gains speed nicely and hits the powerband for that 2 seconds. Then it stalls. I can pull the clutch and roll for a couple of seconds (Engine completely dead, like killswitch was on). I release the clutch and she's running again.

-J

[Coop]

One other easy/free (albeit perhaps unsafe) check that's worth a shot is disconnect the kill switch and try it. Faulty kill switches can reek havoc.

[DoldGuy]

Jopiz,

Replace your spark plug cap. I know it sounds silly, but I had one with very similar symptoms. It would heat up as the RPMs would rise, & create an open circuit, coast to a stop & it would fire right back up. Tried a new CDI, Stator, & Coil only to find it was the silly cap.

Hope its as simple as this, keep us posted!

DoldGuy

[dirtjunkie85]

I agree with Doldguy. its gotta be an electrical problem if you have checked everything else. i could be a CDI problem i cracked the CDI on my KTM 250sx and it did great for a while but then would die all of the sudden. i swapped the CDI and it ran great. start with the plug cap first though and then go to more complicated things.

[Goat]

I too have had a bad plug cap before. It's a cheap retarded fix. Also check all your connections are tight and covered. They could be grounding out anywhere from the frame or with each other.

[Jopiz]

Replaced the cap and now she pulls through the gears in no time! I thought the cap was good but it certainly wasnt. Thanks for your help guys! 

[Coop]

Awesome! Yeah a lot of times the eye ball method doesn't always work to tell if a part is good  .

[Goat]

That's awesome. Glad it was an easy and cheap fix.