Tuning the OKO PWK Carburetor
Tuning the OKO is very different than the CVK style carb your scooter came with.
Setting the idle:
Pull the choke plunger out to full choke, then hit the start button. If the engine won't start, try giving it a little bit of throttle while you crank to lean it out.
Once the engine is running, close the choke and adjust the idle screw as needed to keep the engine running without giving it any throttle. Let the engine warm up for a few minutes.
Once the engine is warm, turn the air screw clockwise a quarter turn and see if the engine picks up RPM's. If it does, the idle circuit is lean and you are on the way to correcting it. If the RPM's drop, it is becoming too rich and you need to turn it back out counterclockwise.
Continue turning the air screw in 1/8 turn at a time in the direction that makes the RPM's increase until the RPM's level out (won't go any higher). During this process, adjust the idle screw as necessary to keep the idle low but not in danger of stalling. After each 1/8 turn, give the motor 10-15 seconds to respond to the new setting. Once you have found the area where the engine neither increases nor decreases in RPM's you are close to having the idle adjusted.
Is your pilot jet the right size?
Stop the engine and check to see how many turns out the air screw is. If the air screw is less than one turn out, you will need to switch the pilot jet to a larger size.
If the air screw is turned out more than two full turns, you will need to replace the pilot jet with a smaller size.
When you have reached maximum idle speed with the air screw turned out between one and two turns from bottom, you have the idle set up right (or very close). Blip the throttle slightly. If the engine stumbles, turn the air screw out slightly. If the engine "searches" or returns to idle slowly, turn the air screw in slightly. You are looking for that spot where the engine takes a little throttle, then returns quickly to idle.
Time for theory (not fun, but important stuff):
Different throttle positions are controlled by different sections of the carburetor.
Just off idle to ~1/4 throttle: From just off of idle to 1/4 throttle is the transition position between the idle circuit and the needle. Unlike round slide carburetors, the OKO PWK does not have different cutaways for the slide, so you have to use the right size carburetor to end up with an ideal tune.
If just off idle to 1/4 throttle indicates leanness, it means there is not enough airflow through the bore to pull fuel up through the needle jet when the idle circuit is at its maximum. The cure is to switch to a smaller sized carburetor.
If just off idle to 1/4 throttle runs "rich", it means that fuel is being pulled up through the needle jet too early due to extra vacuum created by too small a bore. The cure is to switch to a larger sized carburetor.
1/4 to 2/3 throttle: The position of the needle determines the mixture from 1/4 to 2/3 throttle. As the throttle is opened close to 3/4, the main jet will also be a determining factor. If the bike runs well at 1/4 throttle but not 3/4 throttle, check the main jet. If it runs poorly at 1/2 throttle, but okay at 3/4 throttle, adjust the needle height.
More than 2/3 throttle to full throttle: Running at or near full throttle, the mixture is controlled by the main jet. The right sized main jet will pull strong and smooth. If your engine runs okay near full throttle, but loads up or leans out after running at those openings, try adjusting the main jet size accordingly, as "carry-over" from too rich or lean a main jet can affect the idle after running the engine hard.
Time for a test ride:
Now that you have the idle set, take the scoot for a short test ride and see how it responds at about a 1/8 throttle opening. If the engine pulls smoothly without losing power, it indicates that the idle circuit and slide cutaway are correct.
If the engine stumbles a lot when opening the throttle, it indicates a richness on the cutaway or the top portion of the needle.
If the engine loses power upon opening the throttle, it indicates leanness above the idle circuit, either on the slide cutaway or the top portion of the needle.
If it seems like the idle and cutaway are correct, try throttle openings to 1/4 throttle and to 1/2 throttle. The engine should pull strongly and smoothly if the needle is correct. If the engine stumbles or is slow to respond to throttle openings, that is an indication of richness, and you should try lowering the needle by raising the clip a notch.
If the engine lacks power or "surges/searches" when the throttle is released, that is an indication of leanness and you should try raising the needle by lowering the clip a notch. Once you have found the needle position that offers smooth, clean acceleration with good power, letting the engine return quickly to idle when the throttle is released, try opening the throttle from 1/2 to 3/4 throttle.
If the bike is slow to accelerate or stumbles nearing 3/4 throttle, the main jet is probably too large. Try a slightly smaller size until the bike pulls smoothly with good power.
In Review - General Tuning / Troubleshooting Tips
The air mixture screw is only used to set the idle mixture. Once the idle is properly set, you won't be touching that screw again (ie. it has no effect on the main jet).
It is important to only adjust the air screw for the final setting after the engine is completely warmed up. The final adjustment should be between one and two turns out. If less than one turn out, replace the pilot jet with a larger one. If more than two turns out, replace the pilot jet with a smaller one.
After adjusting the air screw for optimum idle, blip the throttle just slightly. If the engine stumbles, lean the mixture slightly by turning the air screw out. If the engine "hunts or surges" when the throttle is released, or is slow to return to idle, turn the air screw in a little to richen the mixture.
Always tune the carburetor from the "bottom, up". Start with the idle, then check the needle, and last, the main jet.
With the idle tuned correctly and the clip on the needle in the right "notch", if the carburetor has a lean spot just off of idle, the carburetor is too large for your application. If just off of idle, you have a rich spot, the carburetor is too small for your application.
Based on experience, these OKO PWK carb sizes work well for the GY6. When in doubt, the smaller carb will generally be easier to tune.
150cc-170cc = 24mm-26mm
170cc-180cc = 26mm-28mm
180cc-205cc = 30mm
A properly tuned carburetor will pull strong and smooth at all throttle openings and return to a smooth idle.