Your basic thumper carbYour engine is basically an air pump, and your carb meters how much air and fuel are sucked into that pump. Even though they may differ wildly in size, shape and design, all four-stroke carburetors have the same basic parts or circuits. Your slide cutaway (or throttle valve) needle and needle jet will all affect your bike's acceleration from one-quarter to three-quarters throttle, and this is the most important area for off-road riders since we spend most the time at these throttle settings. Due to the hassle of making changes to these circuits, these are the most neglected areas of tuning.
Too rich jetting (too much cutaway, needle positions too high, too large a needle jet) can make your bike lunge and hard to control. If it's too lean in this area, the bike will feel really flat and down on power, but will respond quickly to changes in throttle position. It also may detonate (ping) under a load. Pinging can also be caused by too little octane or winterized fuel (oxygenated, blended with additives), so keep in mind any fuel changes if your bike suddenly starts detonating in otherwise "normal"conditions.
You main jet is probably the most talked-about circuit, and it's as critical to get right on a four-stroke as with a two-stroke. The main kicks in at half throttle and takes over metering duties as you hit full throttle. If your main is too rich, the bike will sputter and surge as it tries to burn all of that fuel. Too lean, and the bike will run flat or have a flat spot in the powerband. A severely lean main will cause your bike to seize just like a two-stroke. It's better to be slightly rich on the main than slightly lean, because it will run colder.
Yamaha four-strokes have an accelerator-pump circuit. This system squirts a stream of raw fuel into the carb venturi every time you wick the throttle. Think of it as the four-stroke PowerJet carb-it richens the mixture to run best at lower engine speeds, yet allows a leaner top for more over-rev. If you radically modify your engine (flowed head, hot cam, etc.) you may have to richen this circuit slightly, but it's otherwise not something you mess with for mere weather or altitude changes.
Your pilot jet (or slow jet) controls the idle circuit, or from zero to one-quarter throttle opening. The pilot jet and airscrew control the amount of fuel and air going into the engine at slow engine speeds. It's very important to tune these circuits because they control throttle response and starting. The pilot circuit has a major affect on how well your four-stroke starts-or refuses to start-after a fall. At every event we attend, there is always some four-stroke rider who comes into the pits with his bike revving wildly. Invariably, this rider will say that his bike is hard to restart after a stall, so he turns up the idle adjuster so it won't die.
That's like jumping from the frying pan into the fire. Thumpers are only hard to start when they are jetted poorly or when the wrong technique is used. The rider who turns up his idle is only perpetuating the myth about thumpers being hard to start. Most manuals (and this magazine) will tell you that you should not touch the throttle when you kick a thumper. Well, turning the idle up is mechanically opening the throttle, right? You will make your bike even harder to start. You have to fix the problem, not the symptoms of the problem.
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, soyou 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.
Bike won't start after a crash
Bike runs on or won't idle down when throttle is chopped
Bike won't start when cold outside
Bike sputters/won't clean out at high RPM
Bike coughs and stalls in slow turns
Bike hesitates or bogs over deep whoops or G-outs
Bike starts but won't take throttle without sputtering
Bike suddenly starts sputtering/gas flows from vent tubes
Bike runs hot/feels slow and flat on straights
Bike coughs and stalls when you wick open throttle