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Carburetor Jetting

the perfect jetting

Before you begin your search for the perfect jetting, you need to be sure that there are no problems with your machine that will make it impossible to tune your carburetor. Take some time to go over the check list I've prepared. You'll save yourself some time and money in the long run.

  1. Clean and oiled air filter
  2. Good compression
  3. Crankcase or vacuum leaks
  4. Blown head gasket
  5. Operating power valves
  6. Cracked or broken reeds
  7. Silencer packing oil soaked or blown
  8. Clogged carb vent hoses
  9. Weak spark
  10. All of these problems can affect carburetor tuning

Basic two stroke carburetor tuning is a skill that should be mastered by anyone that has a two stroke bike or ATV. There are many informative articles on the net related to two stroke carb jetting and tuning. My method of two stroke carb jetting is aimed at training you to recognize when your two stroke carb needs to be adjusted and what changes you can make to accomplish the task. The best way to tackle a job or learn a new skill is not to cut corners, but to grab the bull by the horns and try to learn something. I can sit here and try to make this easy for you by giving you a list of jetting specs for your bike or I can get rich selling jet kits that I think will work, but the results will be the same , you will learn nothing. It's time to get fuel on your hands and use your head. When you’re done, you will know when your bike is lean or rich and you'll know what to do about it.

Basic Tools

You'll need some carb cleaner or even better, a dip tank. You can get the dip tank at any auto parts store. You can also use it to clean power valves and other small parts. Compressed air is a must! You'll also need a small flat-head and a #2 phillips head screwdriver. Main jets can be removed by a 6mm nut driver or a flat-head screwdriver.

Step by Step

The first step is to completely disassemble and clean your carburetor. Take care not to lose any of the small parts. Use the compressed air to clean all of the passages in the carburetor. Inspect and record the sizes of the main, pilot and needle jets. Compare the sizes to the stock factory sizes. If you bought your bike used, the jets may have been tampered with and jetting may be a long way off. You can find the stock specs in your service manual or you can call your local dealer. Take a look at the needle and seat. Check for wear. Wear will appear as a ring or depression on the needle. Replace as necessary. Set the float height according to factory specs and reassemble. Reinstall. Your jetting should be fairly close and now you can begin to tune your carb. If you don't have the stock carb, you have no way of knowing what stock jetting is.

Some Guidelines

Perfect fuel air ratio is 12.5/1. Any more air and we term the mixture lean - Any more fuel, and we term the mixture rich. A rich mixture makes a very distinct sound and smokes excessively. Start your bike and bring it to operating temperature. Next, engage the choke. Hear the difference? This is what a rich mixture sounds like. A rich air fuel ratio won't hurt much. Not that this means it's a good thing, but it doesn't have the same piston melting power as a lean mixture. Lean is heat and excessive heat destroys engines! Therefore, we have to be careful not to let our engine run lean for any length of time. Lean engines will be slow to respond and at times will pop and backfire.

Carb School

We've established our baseline jets and I'm going to assume that your bike is running fair. NOW WE'RE GOING TO SCREW IT ALL UP!!!! When I first started tuning carburetors , I would talk to other racers, call service shops, ask my friends, I even let my dad help one time ( one time!!!! ) All of this effort got me one thing ......FRUSTRATED!!! I began to realize that you can't explain how a carb sounds or feels. I also realized that most of the people I was asking were as clueless as I was. Hence the screw it all up method of carb tuning was born. This is how it works. Remember those sizes that we recorded--the main, pilot and needle jet. We are going to buy a series of jets in both directions of the standard jets and you’re going to keep changing jets until you find the perfect combination for your riding style and riding conditions.

What to Buy

Buy six main jets- three larger and three smaller than the stock number. Buy six pilots- three larger and three smaller than the stock jet size. Buy two needles- one leaner and one richer. These jets will cost you between $30 - $70 depending on where you get them. It may seem like a waste of money, but you need jets to jet your bike. If you need jets and can't find them, give me a call and I can help.

The Main Jet

Start with the air screw out 1.5 turns and the needle with the clip in the center position. Turning the air screw in will make the mixture rich at, and just after idle. Turning the screw out will make it leaner. The clip on the needle can also be moved to calibrate the jetting. If you move the clip up, you lean out the mid circuit. Down makes it richer. The main jet is most felt in the 3/4 to full throttle range. Change main jets in large steps at first to get a feel for what it does to the power of your bike. Listen to the way the engine sound changes. Look for changes in exhaust smoke. A word of caution here. As the part number of the main jet gets larger, the jet is richer, smaller and the jetting gets leaner. Remember not to make the main jet so lean that you create excessive heat and damage the engine.

Needle Clip Position

Needle position is most felt at 1/4 - 3/4 throttle position. Try the clip in other positions and try the other needles. Feel what it does to the bike. Do the same thing with the pilot. The pilot will be most felt from idle to 1/8 throttle.

Some Hints

Temperature, humidity and altitude are the most important variables to take into consideration when jetting your carb. You may have your jetting perfect for a given condition but, as soon as any of the variables change, so should your jetting. Here's an example. If you live in New England and start riding your bike in April. The engine is responsive at all throttle settings and makes tons of power. The temperature is a cool 50 - 60 degrees and the relative humidity is about 65%. Perfect riding weather. Now ride the same bike with the same jets in August when the weather is 98 degrees and the humidity is close to 100%. The bike smokes more than normal and isn’t as responsive as it was in April. Why? As I said previously, the perfect air fuel ratio is12.5-1. When the humidity goes up, the air thins out. If you have less air, you need less fuel. Lean the carb. Some bikes are more sensitive than others to these changes. The same thing goes for altitude. If you live or ride in the mountains, you need to go leaner. An important thing to remember is if the weather starts to get cooler and the humidity drops, you need to go back to the richer settings or your running the risk of a blown engine. Another thing to remember is the stock jetting is a little on the rich side.

Ken, You Can't Be Serious!!!

This system of jetting may seem a little seat of the pants to some of you, but it works if you’re trying to learn how to tune your carb. You can pick up any service manual and get all the long explanations and principles of carburation if you think that's what it will take. The scope of this page is not to tell you, it's to try to teach you. It's been my experience that most of us learn more by our own mistakes and hands on experience than by reading formulas and theories. Remember, proper jetting is the cheapest power you can buy.

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