Cr125r cut out at 0-100% throttle

[Twostrokepower!!!]

Hey my bike is recently rebuilt and runs so great throughout most of the powerband, has 4rs on it, airscrew is 1.7 turns out, always starts up and runs perfect! exept... Going from no throttle at all to Whacking it wide open it has almost like a cutout point, is this too rich or lean or something? Or is it just revved out? I feel like sometimes it has more revs to go, i run it way up in the revs so any info on this would be good, i race sunday and i cant figure out what this is, everything is fresh, (plug filter,gas,clean carb) I would appriciate any help, thanks!

[Jeram]

do you get the same issue if you go from idle to half throttle suddenly?
if it only happens when you go from idle WOT then it is you main jet most likely.

Ride the bike to get it up to temp then do a quick WOT run of a couple hundred meters , kill engine, pull clutch in, come to a stop... inspect plug.

If the bike seems to be running very lean at WOT dont do the WOT run until you first put a larger main jet in, this will reduce the chance or seizing during your plug chop testing

if light grey/white ash color go up a main jet, if dark or oil go down a main jet.

repeat the process again and see if it improves.
what your chasing is the tanned (coffee with milk) color.

plenty of photo examples online to reference against... literally must be thousands of internet threads and spark plug photo FAQs.


If its running oddly at all RPM and jetting changes arnt helping one bit Id be looking elsewhere like air leaks, broken reeds, carb float heights, dirty/clogged carb internals, fouled plugs, weak spark, the list goes on and on.
by it should just be the jetting thats off!

[Twostrokepower!!!]

Ok great, everything else is perfect, the bike is mint, its gotta  be the jets, or needle adjustment, Its jetted 168 main and a 40 pilot, Pro circut reccomends 165  main 52 pilot. It has a perfect plug color and starts and idles fine, seems weird to me, its very crisp sounding and doesnt sound too lean, idk what to do for right now though....

[Jeram]

your plug color is fine for what throttle %

[Twostrokepower!!!]

Wide open, but how do the jets sound?

[Jeram]

I cant say just by seeing the numbers, it depends of alot of variables

temp, altitude, motor mods, pipe, carb condition, air filter etc etc etc

try putting in a larger main, if that doesnt fix it, try putting in a smaller main...

If that doesnt fix it have a fiddle with the needle clip position

Then you will not only fix your bike but you will also learn alot in the process so you can do it confidently by yourself next time it happens

if jetting doesnt make the problem better or worse then its not the jetting.

[Jeram]

PS: try doing a 1/2 throttle plug chop and see if the results are any different.

this should tell you whats happening with the needle