Carb Jetting Issue or something else?? 125TC

[Slammysammy55]

Hello,

I recently rebuilt the top end on my 2024 125 TC, it starts good and can get to idle pretty well until I blip the throttle, it will then rev high and not come back to idle for 5-10 seconds(thinking a lean condition) but then it will start to bog out like it’s getting to much gas, hard to save it even when hitting throttle after that. If i get on and ride it, rides better than before, more torque and sounds crisp. Spark plug looks pretty clean( not white to be lean or to black rich) I switched from a single ring piston to a double ring per the OEM manual but am wondering if maybe i need to adjust the jetting? Not much of an expert so would appreciate any advice anyone has! I did try adjusting the clip position on the jet needle to a lower clip position to get some more gas and obviously cleaned all the jets out and carb as a whole. But didn’t improve much, also adjust the air screw/idle screw didn’t help a ton either way, either would rev up way to high or just die on me, couldn’t find a good sweet spot so just returned to OEM manual specs. Thanks anyone for taking the time to read this! Hopfully someone can help!

 

[Crashaholic]

I recently rebuilt the top end on my 2024 125 TC, I switched from a single ring piston to a double ring per the OEM manual but am wondering if maybe i need to adjust the jetting?

This shouldn't require changes in your carb settings.

it starts good and can get to idle pretty well until I blip the throttle, it will then rev high and not come back to idle for 5-10 seconds(thinking a lean condition)

First off all my experience is with vintage stuff however it sounds like your 125 has the same fuel system (mikuni carb, jet needle, etc) as the older stuff I'm familiar with. With that said, it sounds to me like an air leak between the carb and the cylinder, e.g. the intake manifold. I think you're correct in your diagnosis of a lean condition which could be due to an air leak. In the vintage realm we would do a leak down test to confirm this. If your bike has some sort of electronic fuel management system such as an ECM then maybe something went funky with it at the same time as the top end rebuild.

If i get on and ride it, rides better than before, more torque and sounds crisp.

This leads me to think the leak may be closing as the bike warms up. I've had this happen to me once. If you let the motor cool down to ambient temp does it act the same when starting it a second time?

but then it will start to bog out like it’s getting to much gas, hard to save it even when hitting throttle after that.

Between this comment and the one above, it sounds like its starving for fuel. If your Mikuni has a float bowl did you have the bowl off at any point and if so did the float height possibly get moved?