Main jet rich or lean?

I am trying to figure out if I have a rich or a lean main jet. At about 3/4 throttle it bogs down and loses power. It runs fine in any other throttle range. I've run a plug chop and it looked white so I figured it was lean. Also, do shops usually have a selection of main jets to choose from or will I need to order? Can I just drill out the main jet? My bike is a KE125 with a Mikuni carb.

The throttle position you're having probs with is only vaguely influenced by main jet size.

If your main jet was so small that it was causing distinct leanness down @ 3/4 Throttle,,,
It would be extraordinarily lean at Full Throttle/Max RPM.
Most likely it would mimic running out of gas.
.........................................
For the symptoms you are describing,,,,

Try Raising the jet Needle by lowering the clip.

If 1 or2 position changes dont do the trick,,
You very possibly have a problem somewhere.
Such as an Air Leak in the intake manifold system/cylider base gasket/crank seals,,etc,

or perhaps partial clogging of the needle jet.

It it starts Ok,,,and idle is stable,, that pretty well eliminates air leak possibilities .

Needle adjustment----is a LARGE increment.
Much more influence than the tiny difference of distance from one clip position to the other seems it would be.

That makes engine Very sensitive/responsive to Needle Position changes.

If You change the Needle Position and Do NOT get a distinct and significant Change in Engine Performance,,,
That's a very good clue that the needle's Jet is
clogged.

While Riding it at that "spot" where it misbehaves,,
try Very Slightly backing off the throttle a Little.
If you feel engine pull a bit harder as throttle is closed----thats a lean Needle or Needle Jet/ or clogged needle jet

You can TRY a heavy dose of Fuel Injection Cleaner added to fuel,,
but the needle jet happens to be the Least affected part of the carb by Cleaners.
TUFF for fuel cleaners to clean the needle jet Right because of how the jet works & is made.

Which means You'll probably need to remove carb,disassemble,and clean it throughly.

Carb parts DO Wear.
Needles and their jet experience Physical Friction during operation,,,and are thus prone to wear which produces a change in their calibration.

They always wear "RICHER".
A High Mileage carb will often begin to run rich in 1/2~3/4 Throttle range.

Yours sounds to be obviously LEAN at that throttle position range.

That symptom of Fuel Shortage is what indicates
the likelihood of the Needle Jet being clogged.

Start/Idle/Low Speed running OK?
=Pilot Jet OK
=Airleaks Unlikely

High Speed/Full Throttle OK?
=Main Jet "close enough" to correct

Bogs,Power Loss @ 3/4 Throttle range?
=Lean Intermediate fuel delivery-->>Needle/Jet

Can't be WORN,,,that produces RICH mixture which results in Stuttering,blubbering similar to running on choke or lightly fouled plug.

Logic of how it all works leaves Clogged Needle Jet as prime suspect.

Only other possibility would be a VERY Worn ThrottleSlide/carb Body.
But That condition acts very similar to Air Leak at idle & low speeds.

....................................
"Jetting" is Not a Repair Operation.
Whenever a engine starts running Bad or Different when nothing else Known has changed...
Changing Jets is NEVER the "answer".

Except for physical wear on some carb parts and areas,,,Jetting should remain Constant over the life of the bike.

Ya cannot "fix" anything that's changed about the engine's running characteristisics by changing jets.
At best,,that would only mask the underlying problem.

"Plug Chop" is pointless at anything less than Full Throttle under Full Load.
Even then,,,it's Nothing as simple as Black or White.

I suggest you go fool with the needle position 1st,,
then clean the carb(especially needle jet) if that doesn't solve the prob


Good Luck,hope ya get it sorted out and running good

If your plug is white it probably does mean that u are running lean and u will want to go up in your main jet size. Your local shop should have just about every jet size that you can think of, especially a dealer. drilling out your jet is a very bad idea because it is an extremely precise measurement for how much fuel is being put into your engine. also pointless because jets for mikuni carbs cost about $2.50 each. you should definitely ask the mechanic at your local shop a suggestion for a starting point on carb size depending on what modifications you have done to your bike. if u let more air in you will definitely need a bigger main jet and sometimes need to adjust the clip position on your jet needle. and a bigger exhaust you might need to go a little bigger but not much.

Try making it richer and if the plug starts to foul out or the oil burn s the plug then make it leaner

DO NOT drill out the main jet.
Shops do have selections of jets to choose from.
Contact me for a carb trouble shooting guide.
All of your questions will be answered.
Include your email address - Can't forward stuff through Answers.