If you feel you need to change jet size first purchase purchase jet kit of ebay its cheaper than individual pieces:
QMB139 Main JetQMB139/QMJ157 pilot jets QMJ157 main jets 0. Clean the carb - take apart and use ONLY carb cleaner in every port then return needle in default middle position.
1. Close progression port how it shown on picture, you stop at point when you can no longer see the 1st progression slot edge.
Important after this step is done forget throttle plate adjustment screw exists you never touch ever again no matter what you read anywhere on the internet!
2. gently close idle mixture screw and unscrew it 2 full turns
3. unplug the electric choke and fire engine up
4. if it idles fine around 2000-4000rpm leave it alone for 5-10 to reach normal working temperature 80 Celsius
5. After engine hot plug back electric choke and wait another 3-5 minutes for it to close, you should see drop in RPM
6. engine should idle fine, if not then you need bigger bigger or smaller idle jet.
If you close IMS (idle mixture screw) from 2 turns out position and engine gains RPM this means idle jet too big
If you open IMS from 2 turns out position and engine works better then idle jet is too small
Find a right idle jet first before you proceed you should be running slightly rich when IMS in 2 turns out position.
7. Now forget idle circuit and go to your main jet do whatever magic you find online to determine your optimal main jet size. Usually its just running on WOT and checking which one gives you best top speed or look at the spark plug.
Now the rest of the adjustment depends on correctly finding the main jet size so take your time.
You might find a range of jets that make no difference in my case 88-98 were same for 50mm kit, so you don't perhaps i had faulty.
So in case you have same thing you cannot determine which jet to use go with the bigger one, its better to run rich than lean.
If you want exact match do this.
A. Find range of main jet sizes that works same without noticeable difference at WOT same top speed, same acceleration and no noticeable problems in way engine operation.
B. Install biggest jet in the range
C. Using only IMS make idle circuit lean by by closing IMS to the point when engine almost stalls but still idles (so idle circuit don't affect spark plug reading).
D. Install new spark plug and run at WOT for a mile or two, you can stop on lights and idle its okay since idle is lean it will not affect spark plug reading.
E. Check spark plug using online information or ask on forum if too rich wash the spark plug with brake cleaner and install smaller jet and repeat step D.
Now once you found correct main jet stick to it.
8. Make sure electric choke is closed, adjust IMS 2 steps out (it should not be be more than that but its not golden rule so half turn more is fine) then warm engine and adjust IMS until you get about 1750-2000RPM and IMS screw is between 1 turn out and 2.5 turns out, lesser the better but dont stress out about it.
NOTE You might need to change idle jet because main jet size affects idle circuit as well (at least on old worn out carbs)
9. Now time to test your setup put on center stand and open throttle as slow as you can until about 2000-3500rpm (progression circuit) engine should work good no sudden drops of RPM. You also need to hold it in that zone for few seconds to make sure its stable.
If your engine stalls then you need to change idle jet up or down, you u can use this test again
Close IMS a little and try step 9 again and works better - your idle jet too big.
Open IMS a little and try step 9 again and engine works better - your idle jet is too small.
Return IMS to 2 turns out and try idle jet that better suits your needs.
Adjust idle again start from step 8 and go to 9
After you got 2000-3500 rpm working without any stalls you good to go to mid range.
10. put on center stand and slowly go from 3500 to 5000 rpm range and make sure engine does not stall, most likely you be fine here.
If not then you SOL and have to figure something out with needle position i have no time to write about it because needle will affect almost every other circuit. You can ask me about it and meanwhile accelerate on WOT.
11. Once you test everything on the stand you need to go and test it under load on the street and do the same thing very slowly accelerating.
Also going up the speed and and cruising slowly on progression circuit.
What you ultimately want to get is even transition between all circuits without any sudden drops or increases of performance between circuits.
