I translated this from a swedish site
www.braigasen.seThe owner is a real carb-guy
Carburetor with fuel screw
Most common on 4-stroke carburetors (except Mikuni HSR). A fuel screw is always located downstream of the throttle and is pointed
Low speed jet
Start up and warm the engine. Find a comfortable idle turn using the idle screw. Set the fuel screw to 1 turn.
Slowly open the fuel screw (or mixer screw *) - one quarter turn at a time - wait a while for the engine to react. If the speed increases significantly, adjust this down using the idle screw. Continue to open the mixing screw until you notice that the rpm is starting to drop. If the rpm does not decrease even though you have reached 3-4 turns ... Change to a larger low speed jet!
If the speed instead drops as soon as you open the mixing screw more than 1 turn, then screw it in instead. If the speed does not drop even though the screw is almost closed ... Switch to smaller low speed nozzle!
With the right size of the low-speed nozzle, you should have found a rpm hump somewhere between 0.5 and 3 turns.
You have the right low speed nozzle if you find a rpm hump somewhere between 0.5 and 3 turns on the mixing screw
Needle jet
Replace the main jet with one that you drilled to 3mm. Move the throttle needle to the leanest position (clip at the top). Insert a button pin on the twist grip. Then you attach a piece of paint tape to the twist gripts fixed part and mark where you have 1/2, 3/4 and full throttle. Run on 1/4 throttle on any of the higher gears on medium rpm preferably in easy uphill. In this test, the engine will get too much fuel at a certain throttle. It is when this happens that gives us an indication of whether the needle jet needs to be replaced or not.
If you can give more than 3/4 throttle (... maybe even keep full?) The needle jet is too small. Switch to a larger one!
If the engine gets drowned already at 1/2 throttle, the needle jet is too large. Switch to a smaller one!
If the engine is drowned at 3/4 throttle, the needle jet is correct.
Needle
Finding the right throttle needle is perhaps the biggest challenge when it comes to jetting. Mikuni has manufactured thousands and thousands of size-specific needles over the years. What separates all these needles is 1) the diameter of the first straight piece 2) the length of the straight piece before taper, ie before it starts to taper and 3) the conicity, measured in degrees.
In this situation, we are content with adjusting the needle we have. It has five positions where position 1 (top) gives the leanest mixture and where position 5 (bottom) produces the richest mixture. To test the best needle position, you can test how the engine behaves between 1/3 and 2/3 throttle at different speeds and loads. Another way is to see which needle position gives the best acceleration from about 2000 rpm up to half throttle. Both of these tests are best done in light uphill.
Main jet
Now that you have found the right low-speed and needle jet, it's time to move to the high-speed circuit. Choose a test road where you can stay on full throttle.
Give full from 1/2 throttle and see if it responds well and wants to rev. If it doesn't, try plugging the air filter to 2/3 (... or give it a little choke). If this gets better, you will mount a larger jet.
If the engine fails to turn on and you think it is doing well on full throttle then test to hold full and then turn off a little on the throttle, not much just a little. If you then feel that power increases, well, after all, the main jet is too small.
If you want to make sure that you have chosen the right main jet, you can take the help of a good friend who sits further away on the test road with a timer. When he gives the start sign, you accelerate from stationary up through the gears until you reach the point where he stands and watches you. You change the main jet and make another run. The fastest time gives you the right main jet.
Please complete with a plugchop…