What is the correct jet size for the top of a Mukuni?

Only of you have ran the engine out of oil...Sounds like the carbs may not be set correctly.. .Did a professional do the rebuild?..What exactly was "rebuilt".. were any parts replaced?
Sounds like the low speed (pilot jets/air screw passages may not be clean as they need to be??)
Please check the compression on each cylinder..remove the spark plugs... hold the throttle (carbs) wide open when you check this.. spin the motor for 5-8 seconds... you should get 120psi. minimum..(and less than 10psi difference).. if less, it might be time to actually rebuild the top end..(new pistons/rings/hone cyl.).. only bore it if absolutely necessary.. get it measured..

There are only 3 parts in a two-cycle carb that ever need to be replaced,,(unless they are actually broken) The needle and seat assy., the jet needle and the needle jet. ..see pic... parts # 41 = needle and seat assy. #6 = jet needle, #37 = needle jet.



The needle and seat regulate the flow of gas into the carb as the floats rise and fall..,, if they are damaged or leaking the carb will overflow with gas., the needle jet is a orifice with a specific size..The jet needle has a specific size, taper, and length..== The jet needle and needle jet work in unison to regulate the flow of gas from right off idle all the way to WFO..the needle jet/jet needle WILL wear over time causing the fuel/air mixture to become richer and richer and start fouling spark plugs..(this is more pronounced on 4-cycle ATV's)...
I hope this has shed some light on your carb situation...
Good luck...
exploded view of mikuni carb Google Search

standard carbys will cover a .020 oversize bore
melted piston is from detonation which is lean mixture or incorrect ignition timing
the oil will be a metered amount so it will be correct for all rpm
items like incorrect float level and air filter are the suspect causes

did you check the float height?

You have a mikuni carb. there are curcuits to the carb. the first that works from idle to 1/4 throttle is the pilot cicuit, from there you transition to the needle jet/and jet needle. That controls the 1/4 to 3/4 throttle and then the main' jet which controls WOT. you may have a air screw that would be before the throttle plate if it is after it is a fuel screw find the factory specs for the jet sizes and make sure all is correct. if not you isolate where the problem is and correct in a rich or lean adjustment' I race yamaha 2 stroke road racing bikes, it is not an easy task sorry to complicate you so much

If it's the Sreamin' Eagle Mikuni, it should come jetted close enough to take your bike to someone with an exhaust gas analyzer. That's the best way to get your card jetted correctly with the extensive amount of changes that you have made on this bike. I would advise that you take the bike to someone that has a dynometer that is equipped with an EGA and have the run the bike up. I would not want the mixture to exceed 14:1 at any time and preferrably run about 13.5:1. It's a bit on the rich side but it will help keep your engine running cooler. I've installed the carbs before but I cannot remember what jets they had in them. Sorry. Good Luck.

Not sure exactly what you are needing? Carb size? Jet sizes?Float adj? Also ,is this an '86 TT225S? If so the carb is a 40mm Mikuni, the Pilot/Slow Jet is a #42, the Main jet is a #118 Can you be more specific on model and what you are looking for if this is not what you need?

this is mixture buddy on the carb is a screw just fiddle with this screw normally you screw it all the way in and unscrew it say tops 3 turns out but just fiddle with it until it runs perfectly the pic below will show you buddy

I believe they are referring to the nozzle the needle goes into. A venturi effect is created by the position of the nozzle in the throat of the carb. The nozzle has holes in it and the fuel is sucked through the main jet and out the holes to mix with the rushing air as the needle raises. Beyond that I haven't got a clue.

Please rate this solution. Thanks!

sounds like the main jets are partly pluged, you must remve all brass jets and seats including needle jet and its slide soak in carb cleaner and blow out with compressed air only.. hold up to bright light to verify if all are clean. jim