Jetting of Carbs

General Jetting Tricks

You bike's owner's manual is a great source for recommended jetting and tuning tips. If you bought your bike used and don't have a manual, get one. Set the idle speed as per your manual. If it won't start readily using the manual's technique, your pilot jet is the likely culprit.

Whether your bike is air- or water-cooled, you should start it and get it up to race temperature before tuning the pilot circuit. A hotter engine will run leaner than a cold one, so failure to properly warm the bike will result in a too-rich setting.

With the bike up to temp, adjust the air screw so that the bike runs and responds best to slight throttle movements. Now, kill the engine and see how many turns out you have on the airscrew. Less than one, and it's too rich. More than two, and it's too rich. Install the next-sized pilot and repeat the test.

Most off-road bikes are jetted lean to meet emissions standards, so you will likely want to richen these circuits, especially if you have gone to an aftermarket pipe, air filter or removed OEM baffles (in pipe and/or airbox). If you remove the muffler diffuser, you should toss the airbox stuffer too, or the airbox won't be able to draw enough air to feed the engine. Most aftermarket companies will give you recommended jetting, so use this as a baseline.

Under most conditions, about the only time you will need to go leaner on an EPA-legal four-stroke is because of altitude. Air is thinner at higher altitudes, so it contains less oxygen and your jetting will be too rich. You will want to go down a size on the pilot, one or two on the main and lower the needle a position (raise the clip).

Cold air is denser than warm air so it holds more oxygen. On cold mornings your jetting will be slightly rich, but thumpers are less susceptible to changes than two-strokes. Where you might change the pilot on a two-stroke when it's really cold, an airscrew adjustment will suffice on a thumper.

The same is true for barometric pressure. As the barometer rises, the pressure compresses the air, and your jetting will be slightly lean. A falling barometer causes a rich condition, but thumpers don't care about the weather as much as two-strokes. Overall, the Yamaha thumper is jetted almost perfectly from the factory; however, it is very picky about its air filter. Do not over-oil the filter and do not expect it to start immediately after oiling the filter. Let it sit overnight (and not in the cold) to allow the carriers to evaporate. Better yet, keep spare filters in a plastic bag so that you never put a freshly-oiled filter in the bike on race day. Modifications throw stock jetting out the window, so this troubleshooting guide will apply to most four-strokes.

Thumper Troubleshooting

Thumper Troubleshooting

Bike won't start after a crash Pilot too lean Idle set too high Improper starting procedure Bike wants hot start button

Bike runs on or won't idle down when throttle is chopped Idle set too high Air leak intake or engine Pilot too rich (when bike is hot)

Bike won't start when cold outside Pilot jet too lean Air filter is over-oiled Motor oil too thick for temperature

Bike sputters/won't clean out at high RPM Main jet too rich Air filter is over-oiled Spark plug has debris on electrode

Bike coughs and stalls in slow turns Pilot too lean Idle set too low Valves set too tight Decompressor is set too tight, so turning the bars engages release slightly

Bike hesitates or bogs over deep whoops or G-outs Float level too low Carb vent tubes blocked Main jet splash shield not installed Float level too high, gas is trapped in vent tubes (install T-vents)

Bike starts but won't take throttle without sputtering Pilot jet too rich Water in fuel Debris in main jet

Bike suddenly starts sputtering/gas flows from vent tubes Stuck float check valve Debris in gas or carb

Bike runs hot/feels slow and flat on straights Main jet is too lean Fuel octane low, causing detonation

Bike coughs and stalls when you wick open throttle Needle too lean Slide cutaway too lean Pumper circuit blocked or too lean