How to jet your bike 101

I get alot of calls and emails about how to properly setup the carburetor on the pocketbike so I thought i would make a write up to help everyone out. First I think it is best to explain the basic fundamentals of a carburetor. There are alot of variables to tuning a two stroke but if you know the basics and know how a carb works it will help you out.

Elevation/temperature- at higher altitudes a bike runs richer because the air is less dense so you are getting less volume of air so you need to jet down. This is also true for warmer temperatures ie your bike will run richer when its warm compared to when its cold. Humidity also plays a part when the humidity is higher your bike will run richer so you need to jet down. It is much better to run rich and foul a plug then run lean and fry your motor.

Needle- The needle is what controls fuel flow at 1/8 to 3/4 throttle all things being equal ie gas mixture etc. Also remember the slide cutaway and needle diameter play a part in this as well but we really wont discuss that here since we are only covering the basics. To get to your needle unscrew the piece on top of your carburetor and pull it out. You will see a spring, and black round piece which is your slide. Inside this there is a needle with a E clip on it. To get how this works imagine a hose with water coming out of it. Now imagine a needle shaped object about the same diameter of the Inside diameter of the hose being inserted. Since the needle tapers at the end the more the needle is lifted out the more gas can flow. Now if you remove the needle you will see several grooves (5) which the e-clip is inserted into. Now if the needle clip is moved down towards the point it lifts the needle up more. I have attached a pic showing the needle and main jet so you can sort of get what i mean. Yes I know my ms paint skills are great.

Needle jet tuning- To effectively tune your motor it is best to run it on a slight uphill to put a load on it. So as you are riding keep throttle at about 25% and go to about 75% and listen to your motor. If you hear blubbering you are running too rich, excessive smoke also means you are running too rich. Any pinging noise will mean you are running too lean. When you go from 25% to 75% throttle if the bike sort of gasps you are running too lean. Moving the e-clip on the needle down towards the point lifts it up from the main jet and makes it richer. I would suggest starting at the bottom (richest) position and work from there. Now remember that 3/4 to full throttle is only affected by the main jet so if at wot you are running lean dont try to correct his by needle clip position as once the needle is all the way out of the main jet the amount of gas that can flow is determined by diameter of main jet.

Main jet-Main jet controls fuel at wot so if you are too rich its possible to foul your plug but only if you are at or almost to full throttle. Please do not confuse full throttle with rpms. Now before testing on a slight uphill I would recomend putting in a new plug so that when you are finished you can look at your plug for an indication of how your bike is running. So now ride your bike on a slight uphill and go wide open throttle and listen for any pinging (lean) (also feels like someone is kicking you from the back) or blubbering (rich). So at this point you can either jet your bike richer or leaner depending on your results.

On alot of carburetors you also have an air screw and pilot jet but our china bikes do not have this so I wont worry about a write up unless I get alot of pms about it. Also keep in mind that this writeup assumes that everything else on your bike is in good running order.