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Jetting Basics, Part 2

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Jetting Basics
First off, jetting isn’t always increasing a main jet, arbitrarily changing the needle seat diameter, or
modifying pop-off psi to somebody’s standard. Jetting is all about creating or maintaining a balance.
This balance applies to any type of carburetor, and always begins with the pilot circuit. The pilot circuit
is flowing fuel 100% of the time, and the low speed circuit creates a balance between the pilot screw
setting, low speed jet, and spring rate. Of course the high speed is the sum of everything before it.
Therefore if you change the low speed jet, you must readjust the high speed circuit. The pilot circuit
may not have a significant effect on watercraft performance, but it has a long term effect on millage.
The pilot circuit is also an important diagnostic tool.

Balance is critical in the pilot & low speed circuits of our float diaphragm carburetors. For example,
Mikuni sells five springs for the SuperBN with different spring rates and lengths that range from 65gr
to 135gr. Mikuni also sells seven different needle and seats that range from 1.2mm to 2.8mm. That
means there are 35 combinations springs and seats, and a few of them will have a pop-off above 55
psi. Since there is always a balance between what people call pop-off pressure, and a low speed jet.
A certain low speed jet will always work. However, sometimes the low speed jet required to maintain
this balance may be larger size than the main jet.

fig 1

Needle seat
Spring rates
65gr 80gr 95gr 115gr 135gr
1.2mm 29psi 48psi 76psi
1.5mm 32psi 38psi 43psi 55psi
2.0mm 18psi 21psi 25psi 32psi
2.3mm 15psi 17psi 20psi 22psi
2.5mm 12psi 14psi 17psi 19psi

I have tested many fuel pumps in running conditions from idle to wot, and all the fuel pumps put out the
same flowing pressure despite significant size differences, 3psi at idle and 7psi at wot. As you can see,
the only combination that may have a fuel pump pressure issue is the 2.8mm Harley Davidson seat with
a 65gr spring.

The example of a Keihin CDK II with a mid range jet, the main jet is a 135, the Low jet is a 72, and the
mid range jet is a 45. When you rejet a carb, you begin with the sum of all the jets as a starting point.
In this case: 135 + 72 + 45 = 252. Nearly any combination of jets that maintain the sum of 252 will work
for this particular set-up. The actual diameter of these jets is 1.35mm, .72mm, and .45mm.

For this example; assume the low jet (72) will remain unchanged because it is part of a balance between
the pilot fuel-air screw and the lever arm’s spring rate. You want to increase the midrange jet
a bog or lean
spot at ¼ throttle. If you increase the midrange jet two sizes (from 45 to a 50), then you must subtract
two jet sizes from the mains (from 135 to 130) in order to maintain the sum 252.

{Time to readjust the main screw and calculate possible change}

Next Sea Doo GTi jetting example

717 carbs dribble out the main jet booster.
Specs: 1.2 needle and seat, 80gr spring, 67.5 low jet, 130 main jet, 1 ¼ pilot air screw, hi speed screw
closed. The sum of the jets is 67.5 + 130 = 197.5 total Rule of thumb: 4 low jets for every increase in
spring rate. In this case, the spring needs to be increased to a 115gr spring. The new low jet size is 87.5
(but sometimes I used a 90) 197.5 – 87.5 = 110 main jets. 110 + 87.5 = 197.5 total Mikuni low jet
diameters are 96% of the marked size. So a 100, is actually a 96, or .96mm.

I could use the rule of thumb and subtract 8 main jet sizes, but since Sea Doo’s normally have a closed
hi speed screw, I normally play it safe by decreasing the main jet 3 sizes, and open the high speed screw
¼ turn for every main jet size.

Just a suggestion: While the carbs are disassembled, use a granite flat surface, a flat piece of glass, or
a machined flat surface to flat sand the carburetor base. I use a reinforced 220 grit glued to the flat
surface with “3M Spray 77.” I would also recommend a supply of carb base gaskets, and/or Three Bond
1207b (Suzuki bond).

Idle drop test
Perform the idle drop test in a test tank, a trailer, or secured to the shore line. Set the pilot screws set to
some arbitrary rich setting. For example: 1 ½ turns out, and warm up the engine. Set the idle to a low
rpm. A good rule of thumb: You should be able to hold your hand behind the pump. If the pump pushes
your hand away, the idle is too fast.

Slowly turn the pilot screw(s) inward (clockwise) until the engine wants to die, or quits. Make a note of
how many turns it takes to bottom the screw(s). It will be easier to shut the engine off yourself and make
the adjustments, and document turns to bottom or closed. Turn the pilot screw(s) outwards ½ turn. Restart
the engine, and recheck the idle. If the engine rpm increases, and you can no longer hold your hand behind
the pump, I recommend readjusting the idle and the pilot screw again.

Next, test the acceleration and make note of any strange behavior or tenancy to quit on deceleration. Shut
the engine off and turn the pilot screw inwards until they lightly bottom. Make note of the number of turns
in. I count screw settings in half turns, use whatever method suits you.

I normally strive to achieve ¾ turns out on the pilot screw, and if the engine begins to die at ¼ turns out,
I consider that perfect. If the pilot screw position is at 1 turn out, that is a sign the low speed jet is too lean.
Another rule of thumb is to use ¼ turn = 1 pilot jet size. In this case I would recommend caution and only
change the jet two sizes richer, and then retest. If the engine is still running and the pilot screws are closed,
this is a sign the low speed jets are too rich. In this case I would recommend leaning the low jet 2 sizes.


Adjusting the Hi speed screw
Like many other people, I don’t recommend to adjust the hi-speed screw in a test tank or on a trailer.
That doesn’t mean it can’t be done correctly in a tank or on a trailer, but you need to be aware this
method will likely produce a lean high speed screw setting.

If you are going to test on a trailer I recommend you start at a known rich setting. 2 turns should work.
Slowly turn the screws inward until the engine begins to clear out, and sounds good. If you are working
with duals or a hi-speed screw without a “T” handle, you may want to turn in the screw 1/8 turn at a
time until the engine accelerates smooth from bottom to top.

After you have found the screw setting that produces the best rpm, I recommend writing it down, and
then turning the hi-speed screw out ½ turn. Finish testing on the water, by turning the screw in 1/8 of
a turn at a time, and do not exceed ¼ turns in.

When you adjust your hi-speed needle settings in a body of water, I recommended you find an area without
a 5mph zone.Water Test in 50 to 100 yard lengths. This only enough time to create a first impression, turn
around and go straight back to shore, and make the appropriate change.

Initially don’t worry about inspecting plug color. Your first priority is to achieve a smooth transition from
bottom to top. If the hi speed screw is rich, the power band will take off and then slow down near 5000rpm,
then accelerate to 6500 rpm or so. Repeat the process of short hops until the engine begins to accelerate
smoothly from bottom to top, and stop making changes.

Do not tune for max rpm on a short hop. The most likely result will be an engine that begins to run lean after
a wot run. The exhaust system will heat up; it changes the tuned length of your pipe, causing your engine to
lose some bottom end and top end.

Tune for smooth power transitions and consistent performance in all conditions.

Updated 07-23-2014 at 11:24 PM by wmazz

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