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43 Posts
Discussion Starter #1
Hi folks,

So I've had my SM610 for a few months now and it's a beautiful machine with one problem - it (I believe) is running very rich.

The smell of unburned fuel is noxious and soaks into my backpack, it pops on decel. and sometimes is hard to start.

It has a Leo Vince pipe and has supposedly been jetted by the dealer.

How do I get down and fix this thing? Do I need to buy new jets to try out or can I just mess around with a fuel screw? Any specific, helpful suggestions will be highly appreciated. I hope to work on it tomorrow!


615 Posts
here's from another post in another section (forgot who posted it as i copied it to a word file)

from thumpertalk

You think you did everything right. Bought the most expensive pipe.
Paid extra for that jet kit. Followed all the instructions to the
letter. If you were lucky, the machine just might actually perform
better—except, of course, for that pesky off-idle stumble or the giant
flat spot in the mid-range, or how about the severe top end miss (must
be the darn rev limiter!).

Experience tells me that almost every hop-up anybody has ever done to
an ATV has produced at least one point in the rev range that
carburetion is considerably worse than stock. Why? Because all you
have is a piece of paper that gives you recommended jetting settings.
And unless you happen to be lucky enough to ride in those same
conditions, there will be some point in the rev range where you will
be too rich or too lean.

Jetting is providing the engine with a combustible mixture. The ideal
combustible mixture ratio is 14.7 parts of air to one part of fuel,
with the most power being produced around 12-13:1. While a motor can
(and will) operate on a mixture that is considerably richer or leaner,
power output falls off. If you happen to go leaner and ride it hard,
you may end up with an over-heated motor, or worse, a seizure.

Also be aware that carburetion is measured at throttle position
settings. It has nothing to do with engine rpm or transmission gears.
So telling the pipe manufacturer or boss that it skips in third. but
not fifth gear is totally useless information.

Did you know that your fuel pre-mix ratio (two-strokes only!) affects
your jetting? A carburetor jet flows X amount of fuel and air at a
given time. In that fuel is, say, 32 parts of fuel and 1 part of oil
(32:1). If you change your pre-mix ratio to 20 to 1 because you are
afraid of burning up your motor, all of a sudden the amount of fuel
has been decreased by 37.5 percent. And since it is the fuel and NOT
the oil that keeps your ring-ding cool, you run even leaner and
hotter! Same theory applies to four-strokes as well; the more fuel
entering the engine, the cooler the piston will be. The oil and water
cooling systems are not designed to cool the piston; only the little
bit of fuel that is mixed with the incoming air charge prevents your
motor from seizing. ONLY after the heat has been transferred through
the piston to the rings and then to the cylinder, will the cooling
system get the chance to do its job.

In a stock engine, the factory has spent a considerable amount of time
and money trying various jets and needles to come up with jetting that
not only passes the EPA (Environmental Protection Agency) regs, but
allows the machine to be operated at roughly factory rated output
without overheating and blowing up. When you, as an owner, change
anything to do with the intake that would remove factory built-in
restrictions to air flow into the engine or, exhaust changes that
would do the same for air flow out of the engine, then you will need
to re-jet.

Why? A carburetor is designed with fixed size main and slow (pilot)
jets. The jet needle attached to the bottom of the slide is fixed at a
certain height. Only the idle mixture screw is adjustable. If you have
increased air flow as outlined above, the increased volume will still
be mixed with the same amount of fuel as before, resulting in a lean
mixture. If you replace the main jet with a larger numbered jet, the
jet's internal hole will be larger, thus flowing a greater quantity of
fuel at 3/4-full throttle. If you raise the position of the slide's
jet needle by lowering the jet needle clip, you are allowing more fuel
to rise out of the needle jet at a given part throttle position which
is generally 1/4-3/4 open. If you replace the low speed (pilot) jet
with a larger numbered jet, the internal hole will be larger, thus
flowing more fuel at very small openings of 1/16-1/4 throttle.

Even if you popped for the extra expense of a jetting kit, don't
expect your jetting to be "spot on" unless you are willing to
experiment and try different jets. Why? Say you install the main jet
the jet kit recommends and it seems to run OK. Is it truly the best
for your machine in your riding conditions? It may not be, unless you
experiment by going up a jet size at a time until your machine
exhibits a stumble at full throttle, indicating a too rich mixture.
Then by dropping back one size you can be confident that now you have
the correct jet for your machine in your riding conditions.

The same thing should also be done with the other fixed jets of your
carburetor (jet needle and slow speed pilot jet.).

So, how do you start? At the bottom. Then you jump to the top and work
your way down.

The idle mixture screw is the only externally adjustable carburetor
jet available and controls up to 1/8 throttle only. There are two
types of idle mixture screws. One type is called a fuel screw because
it regulates the flow of fuel into the idle circuit. This type of
screw is located ahead of the carb's slide tower (motor side) and is
most often found under the carb's bore and upside-down directly ahead
of the carb's float bowl. By turning the screw out you increase the
amount of fuel that is allowed to slip around the tapered needle and
into the carb's bore where it is mixed with air that has snuck under
the carb's slide.

If the idle mixture adjustment screw is located behind the carb's
slide tower (airbox side) then the adjusting needle regulates air flow
into a fixed flow of fuel intended for idle. By turning this screw
inward you are reducing the air flow, thus richening the idle mixture.
When the motor is up to operating temperature, set your idle speed
screw to a stable idle. Then use either your idle fuel or air screw to
obtain a stable idle. Reset the idle speed screw as necessary after
obtaining the correct idle mixture.

The main jet controls 3/4-full throttle only. Ideally you should start
very rich (large numbered jet) and test at full throttle. It should
skip. If not then you are not rich enough! Once you have your rich
stumble, back off one size at a time until full throttle operation
results in normal operation. (Note: If your ATV runs faster at 3/4
throttle than full throttle you are definitely lean on the main!)

The slide's jet needle controls 1/4-3/4 throttle. It does this by
passing upward through the needle jet. The needle jet is a long brass
tube that contains many small holes in its sides that air passes
through. Fuel from the float bowl enters this air stream from the main
jet and into the center of the needle jet where it mixes with the air
to create an emulsion. This mixture of fuel and air is then metered by
the height, taper and diameter of the jet needle as the emulsion
passes upward around the jet needle into the carb's bore where it
mixes with still more air to (hopefully) arrive in the motor in a
combustible fuel-to-air ratio.

If you have a soft hesitation, without a hard stumble, anywhere
between 1/4 and 3/4 throttle, chances are your needle is lean, so
raise the needle by lowering the clip. Conversely, if you have a hard
stumble, chances are the needle position is rich, so lower the needle
by raising the clip.

If you get very unlucky you might have to start playing with jet
needle taper which controls how fast the mixture increases as the jet
needle is raised. This would come into play if you were lean at 1/4
throttle, yet rich at 3/4 throttle. The length of the needle comes
into play here too. The diameter of the needle controls how much fuel
escapes around the needle while still inside the needle jet. The
larger the diameter of the straight section or "L" length, the leaner
the mixture. Or finally, the "L" length, which controls how much the
slide rises before the tapered part of the needle starts.

The slide cut-a-way controls the amount of air allowed to pass under
the slide at 1/8-1/4 throttle. It controls the transition from the low
speed (pilot) jet to the main jet-fed needle jet/jet needle. Replacing
the slide with one that has a smaller number (less cut-a-way) will
decrease the amount of airflow under the slide at 1/8-1/4 throttle
openings, thus creating a richer mixture at that throttle opening. If
you have a rich condition at 1/8-1/4 throttle and you can't go any
leaner, try a smaller cut-a way. But thankfully, jet needle taper,
diameter, "L" length and slide cut-a-way are usually not affected by
most simple pipe/air filter modifications.

The low speed (pilot) jet controls fuel flow at 1/8-1/4 throttle. The
low speed (pilot) jet is usually not affected by most simple pipe/air
filter modifications. However, a slightly lean low speed (pilot) jet
can raise havoc in the winter where its fuel is added to the total
mixture strength required to start. You may find going one level up
will help a winter cold start situation.

Finally your idle mixture is revisited if you have a deceleration
backfire situation. When you chop the throttle and use the motor to
decelerate, if you get a stream of backfires, try increasing your idle
mixture strength 1/4 turn at a time until the backfire goes away.
Note: If you reach a point where your idle mixture is 4 turns out (for
fuel type screws, NOT air type screws), try going up one size on the
slow speed (pilot) jet and reset your idle mixture screw to 1-1/2
turns out and repeat the process.

2,971 Posts
eh i never read it, so no offense taken

oh wait after i read it, its almost word for word what my CRF manual explains. If ya wanna know how to tune the FCR get a crf manual online in PDF... its a bible.

43 Posts
Discussion Starter #6

So I took my bike into my trusty shop and they have very little experience with Huskys.

They need to know the original jet size/specs so they won't have to spend too much time fussing around. Does anyone have this info and/or probably jet sizes for a Leo Vince Slip-on?

Thank you! The sooner the better as I dropped it off today and they're going to try to open her up tomorrow.

113 Posts
This part is wrong, at least the 37 percent math:
A carburetor jet flows X amount of fuel and air at a
given time. In that fuel is, say, 32 parts of fuel and 1 part of oil
(32:1). If you change your pre-mix ratio to 20 to 1 because you are
afraid of burning up your motor, all of a sudden the amount of fuel
has been decreased by 37.5 percent.

Changing from 3% oil in your fuel to 5% oil in your fuel decreases the fuel supplied by jsut a little more than 2%. Add in the viscosity change, and MAYBE jetting is now off by 3-4%
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