NUMBER ONE: GETTING THE FEEL

The MXA test crew can't explain how to "feel" if your bike is jetted properly. This comes with experience and sensitivity. But, a properly jetted bike has immediate response and runs smoothly throughout the rpm range. A rich machine will have sluggish throttle response, run rough under a load and load-up in off throttle situations. A lean machine will respond erratically, produce less power and ping under a load.

NUMBER TWO: CIRCUIT TRAINING

Carburetors have different circuits to supply the engine with the correct ratios at various throttle openings. The main circuits are the pilot jet, needle and mainjet. The pilot is for one-quarter or less throttle settings, the needle meters one-quarter to three-quarter throttle and the mainjet manages fuel flow from one-half to full throttle. The pilot, needle and main work at different times, but, in truth, they do not really start and stop. They overlap to some degree.

What's the point? That pinging could be caused by too small a mainjet, too high a needle clip position, too lean a slide cutaway, a dirty air filter, an unseen air leak or old gas.

NUMBER THREE: STOCK UP NOW

The air screw and needle can be adjusted manually, but the pilot and mainjet need to be replaced to make adjustments. That means you need to carry spare jets in your tool box. The first step is to write down the jets that are currently in your carb and then obtain spare jets that are two sizes richer and two sizes leaner. If you have to go further than two jet sizes, your bike needs more jetting help than the average Joe can supply.

NUMBER FOUR: SKIP THE SLIDE

As a rule of thumb, don't mess with your bike's slide cutaway. The only 2003 bike that the MXA wrecking crew changes slides on is the Suzuki RM250. It is best to work on the air screw, needle, pilot or main before bothering with the slide. Why?

1. Slides are expensiveapproximately $100.
2. In most cases the slide is not the problem.
3. Adjusting the needle and air screw should correct all but the most serious problems your slide is creating.

NUMBER FIVE: GET A PLUG READING

Tuners gain a great deal of jetting information by looking at the spark plug. You can't read a spark plug if you have cruised back to your pit. You need to run the bike wide open, in as tall a gear as you can manage, and then kill the engine with the throttle wide open. Don't idle the engine back to your truck. Kill it and coast. Remove the spark plug and look at the plug's insulator (the white porcelain part). As a rule, it should look like coffee with cream. Dark brown or sooty black readings mean the engine is rich. A gray, ashy or white reading indicates a lean running engine.

The limitation with plug reading is that it only gives an overall view of how the engine is running. Today's clean burning lubricants also make plug reading a tougher job. Plug reading is not as easy as it was in the old days.

NUMBER SIX: HOW TO TEST YOUR JETTING

Get your gear on, warm up the bike and ride in a straight line from one end of an empty field to the other, shifting through as many gears as possible while still running through the complete power band before shifting. Don't just hold it wide open. Roll on the throttle in each gear and concentrate on the throttle position, the sound of your motor and your relative position in the field. Don't come to a complete stop at either end of your run. Slow down to a few miles per hour, turn around and roll the throttle back on. You are trying to get a feel for how hard the bike pulls, how far it goes in each gear and how responsive it is. Make as many passes as you need to get the feel.

NUMBER SEVEN: TESTING THE MAINJET

The carbs mainjet affects how the carb works from half to full throttle. When the mainjet is working to its fullest, the slide is opened all the way and the needle is pulled as far out of the needle jet as possible. After you have made your test runs with the stock jetting, install a mainjet that is two sizes richer (it will have a higher number than what you removed). Do your runs again. Pay particular attention to how the bike feels. If your bike is reluctant to rev out as easily as before, then you are too rich. Go back to the pits and change to the next smallest mainjet. Keep doing this with different mainjets until you feel a noticeable improvement in power. Once you have the mainjet set, move on to the pilot jet.

NUMBER EIGHT: CHOOSING A PILOT JET

The pilot jet has the greatest influence from idle to 1/4 throttle. To test your pilot jet, follow the same drill as with the mainjet, but this time install a pilot jet that is two sizes richer than your stocker. Do your test runs again and pay particular attention to how the bike feels when you pull out of the slow turns. If your bike is too rich on the pilot, it will feel sluggish and unresponsive until you are past 1/4 throttle. A perfectly tuned pilot jet will help your bike accelerate off the line and out of corners. Repeat the pilot jet test until you get to the one that feels the best.

NUMBER NINE: THE AIR SCREW IS AN AUXILIARY PILOT

The air screw, which is found on the left side of the carb, is an adjunct to the pilot jet. The externally adjusted air screw controls the amount of air that is fed into the pilot circuit. It is adjusted by turning it with a screwdriver. Turning the air screw in restricts the air flow and richens the mixture (turning it out leans the mixture by letting more air in). How do you adjust the air screw?

1. With a warm engine, bring the bike up to a fast, steady throttle setting (approximately 1800 rpm). Get a friend to hold the throttle perfectly still or set the slide stop for a fast idle.
2. Once the engine is running at a fast idle, turn the air screw in (clockwise) until engine rpm drops. Then, slowly turn the air screw out until engine rpm picks up.
3. Stop turning the air screw once the rpm reaches its peak. Peak rpm is when
4. the engine runs the cleanest and fastest.
5. The typical air screw adjustment is from one-half turn to two full turns out. If the air screw is turned out more than 2-1/2 turns, it is a warning that you should switch to the next leanest pilot jet (from a 32.5 to a 30, for example). If the engine has peaked before turning the air screw a half turn out, switch to the next largest pilot jet.

NUMBER TEN: WHAT ABOUT THE NEEDLE?

Your carb's needle affects throttle settings from 1/4 to 3/4 turns. Luckily, you don't need to replace the needle to adjust it. It slides up and down inside the carb. How far it slides into the carb determines how rich or lean the mixture will be. To test needle settings, go back out into the field and make a couple of test runs. Now, return to the pits and move the clip on the needle down two notches, then go back out. The needle's clip position is the quickest and fastest improvement you can make to your bike's jetting. Don't be afraid to move the clip up (leaner) or down (richer). You spend most of your racing time in this range.

It is not uncommon for tuners to recommend switching to a different needle. Before you make this swap, try all five clip positions on your needle. The difference between one needle and another is often as little as one clip position. Save yourself some money by testing every option on your stock needle. If you find your clip position at the very top or very bottom, then you need to go back to the mainjet and pilot jet and try a different setting. When everything is in harmony, your needle clip position will be in one of the middle three notches.

NUMBER ELEVEN: START ALL OVER

Once you have your bike perfectly jetted, pray that the weather never changes. Weather, altitude, gas quality and track conditions will all put new demands on your jetting. Loamy dirt requires more fuel than hardpack (try raising the needle one notch). Tracks above 4000 feet need less fuel because there is less air at altitude (go down one size on the mainjet). Jetting is an art, but you aren't trying to paint the Mona Lisa; you just want it to hang straight. For that, you don't need a paint brush a screwdriver will work fine.

Reprinted from Motocross Action Magazine August 2003

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