Your 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 the most 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 may detonate (ping) under a load too. 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"
Your main jet is probably the most talked-about circuit, and it's as critical
to get it 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
Yamaha's new 400s 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's PowerJet carb - it richens the mixture to run best
at lower engine speeds, yet allows a leaner top for more over-revs. 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 in to 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) 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 Carburetor Jetting Tricks
Your bike's owner's manual is a great source for recommended jetting and tuning
tips. If you bought your thumper used and don't gave a manual, get one. Set
the idle speed as per your manual. If it won't start easily 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 an old one, so failure to properly warm the bike will result in
a too-rich setting. With the bike up to temp, adjust the airscrew so that the
bike runs and responds best to slight throttle movements. Now, kill the motor
and see how many turns out you have on the airscrew. Less than one, and your
pilot is too lean. More than two, and it's too rich. Install the next-size 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 after-market
pipe, air filter or even removed OEM baffles (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