Get A Plug Reading
You need to run in as tall of a gear as you can and then kill the engine with the throttle wide open. Don't idle the engine. Next, remove the spark plug and look at the insulator, (the white porcelain part), as a rule it should look like coffee & cream (TAN). If its dark brown or sooty black, your motor is running rich. A gray ashy or white reading indicates it's running lean. A plug reading only gives you an overall view of how the motor is running.
Note: This should be done with a new plug after your bike has been warmed up.
The Main Jet
The carb's main jet affects how the carb works from half throttle to full throttle. After you have made your test run with the stock jetting, install a main jet that is two sizes richer, (the number on the jet will be higher on a richer jet). Do a test run again, pay attention to how the bike feels. If your bike is reluctant to REV out like before, then you are too rich. You will need to go to a leaner, (smaller jet). Keep doing this until you feel a noticeable improvement in power.
The pilot jet has the greatest influence from idle to 1/4 throttle. If your bike is sluggish or unresponsive until past 1/4 throttle, that is a sign your pilot jet may be too rich or too lean. To test the pilot jet, follow the same procedures as the main jet, remove the stock pilot jet and install a richer, (larger size), pilot jet then repeat your test. A properly tuned pilot jet will help your bike accelerate off the line and out of the corners. Repeat the steps going either richer or leaner until throttle response is crisp and clean.
The Air Screw
To adjust properly, the engine needs to be warmed up. Once warm, bring the bike up to a steady throttle setting, (about 1800rpms), and get a friend to hold the throttle steady. With the engine running steady, turn the air screw clockwise until the engine RPM drops. Then slowly turn the air screw out until the engine RPM's pickup. STOP turning the air screw once the RPM's reach it's peak. Peak RPM is when the engine runs it's cleanest & fastest. The most common air screw setting is from 1/2 turn to 2 turns out.
Note: lf your air screw is more than 2 turns out, it is a sign that you should switch to a leaner pilot jet. If the engine has peaked before 1/2 a turn out, it is a sign you need to go to a richer pilot jet.
Your carb needle affects throttle settings from 1/4 to 3/4 turn. Start out with the clip in the middle setting, make a few test runs, then change the clip down, (this will richen the carb), two positions. This is the quickest and fastest improvement you can make to the bikes jetting. Don't be afraid to move the clip up, (this will lean the carb). The needle setting is important because most riders spend most the there riding time in the needle range. Try all the clip positions on the needle to see what runs the best. If you find your clip position at the very top or bottom, you will need to go back and change main and pilot jets.
A properly jetted carb will have the clip on the needle in one of the middle three notches.
Quick Tips For Small 4-Strokes
Please Start tuning with a Clean & Properly oiled air filter in place.
Please test ride the bike, you cannot properly jet/tune a bike sitting on the stand.
- If it bogs or dies right off from an idle when you turn the throttle, your pilot is too lean. You need to get the Pilot tuned in so it will come clean off an idle BEFORE moving on to other adjustments.
- Your main adjustment is your Main Jet. Once you get your plug close with your main jet, fine tune from there.
- When fine tuning your adjustment screws, turn them all the way in, and back out, 1 and a Half Turns, for your starting point.
Before Changing Any Adjustments On Your Carb
- Make sure it is perfectly clean.
- Make sure you can see light through all the jets.
- Make sure you have a Clean Air Filter.