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Jetting Basics, Part 1

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The article will be based on the assumption the watercraft is in good running condition before changes
are made. The simplest method is rejetting an oem set of carbs with the intention to correct a problem
or to increase acceleration. The worst mistake is to perform changes to your jetting or attempt "reverse
jetting" as part of a rebuild, or in conjunction with other pump, and, or engine modifications without first
developing a functional baseline to work from. I also recommend the use of a 20:1 premix with a high

quality oil during testing and preferably to use it as your everyday premix.

Do not attempt jetting if your impeller has leading edge damage, or if the pump is not properly sealed
to the hull. This is critical with any watercraft (especially a Yamaha or Kawasaki), do not make the
mistake of under estimating the impellers effects the carburetion. It is important the impeller to engine
combination is close, before attempting jetting changes. Do not to overestimate the effect of carburetion
when tuning for a specific rpm. Often the largest rpm gains will come from properly adjusting the low
speed (pilot) screw.

Use the idle drop test (under a load) to adjust the low speed screw, to determine if your low speed jet,
spring, needle and seat, and low speed (pilot) screw settings needs to be changed. Use my rule of thumb
for jetting carbs until your screws end up at (3/4, ¾), (1, 1), or (1 ¼, 1 ¼), and by doing so, when the
carburetor screw settings change, you will be able to judge if the weather, location, or a mechanical
reason is the cause of the new screw settings.

I recommend the use of a tachometer, such as SeaDoo P/N 295 000 100 Digital/induction type tachometer.
I used this type for many years. However this tachometer is not waterproof and must be properly sealed in
a zip-loc bag. Recently I found this inexpensiveand improved version of a tiny tach. The resolution is only
+\- 60rpm, but this is accurate enough for jetting. I recommend you do not get caught up in tweaking every
last rpm possible.

I recommend you begin with expected results by creating an rpm goal based on four factors: (1) What rpm
does your pipe make peak HP on a stock engine. (2) Exhaust duration of the stock engine. (3) Exhaust duration of your ported engine. (4) The tuned length of your pipe. Determine the pipes tuned length by measuring the pipes centerline length from the piston to the end of the rear cone (where the stinger begins).

Most aftermarket wet pipes I have measured, range from 1050mm to 1095mm. I will use the 1095mm length
as an example. Pipe manufactures know where their product made peak horsepower at, and most of them
supplied an rpm goal. If your engine is ported; peak HP rpm is shifted upwards with increases in exhaust port
timing. Once you have collected the data, use this Modified formula from Blair's "The Basic Design of Two Stroke engines."

39,567.5 * Exh Port Duration / Tuned length = Peak HP rpm
-> 395567.5 * 180 deg / 1095mm = 6504 rpm

Kawasaki 650 and 750, and 800 engines come in three basic porting configurations.
Kaw 650 Exh Port Duration is 178 degrees
Kaw 750 SX, SS small pin Exh Port Duration is 180 degrees

Kaw 750 Xi, small pin Exh Port Duration is 186 degrees
Kaw 750 big pin Exh Port Duration is 186 degrees
Kaw 800 Exh Port Duration is 186 degrees

Kaw 750 SXi Pro big pin Exh Port Duration is 192 degrees
Most Kawasaki 750, 800 port specs are similar to the SXi Pro.

Now apply the longer exhaust durations to the same formulae and notice where the peak horsepower rpm
was moved to: 39,567.5 * Exh Port Duration / Tuned length = Peak HP rpm

-> 395567.5 * 186 deg / 1095mm = 6719 rpm (+215 rpm).

--> 395567.5 * 192 deg / 1095mm = 6936 rpm (+432 rpm).

As a goal, I recommend that your impeller/pipe/engine set-up to exceed your peak HP rpm by +100 to 200rpm.

If your peak horsepower rpm goal is 6936rpm, your engine should accelerate to ~6900 rpm. Your ski should
reach that rpm easily, and in less than 50 yards. If you’re estimated peak HP rpm is greater than your actual
water test rpm by approximately 500 rpm, your engine is in danger of a detonation seizure. In this situation
it is better to put the ski on the trailer, than to try making up the difference with better jetting. In this situation,
I recommend you to find an impeller one size less than you are using (if possible). Normally significant gains in
rpm are not achieved by jetting. Significant gains in rpm and speed is achieved with correct impeller pitch and
nozzle size. Modified engines are prone to failure when the propeller pitch is too great.

When your jetting is correct, you need to begin making small changes to your impeller. As you make changes
to your pump, use my rule of setting your screws to (3/4, ¾), (1, 1), or (1 ¼, 1 ¼), and by doing so, when the carburetor screw settings change, you will be able to judge when your jetting needs to be changed for your new pump set-up.

Recognizing Problems
I would like to take this opportunity to discuss recognizing problems while you are working or testing.
I recommend the day before you plan to work on your watercraft; you take your watercraft for a ride,
put it in a test tank, or run it on a hose. I am recommending this because I want you to look for signs
of a vacuum leak. Like hard starting (greater than 1 minute, after it has been sitting for a week). Another

symptom is a engine that dies while it is idling, and you should suspect any engine that has a fuel primer.
If or when you remove the carburetors. A wet carburetor base gasket is a sign is caused by a vacuum leak.

When working on a Yamaha, many mechanics will not remove the carburetors from the two-piece
mounting plate. I recommend that you do, and look for wet gaskets on the reed plate and carburetor
base gaskets. Nearly every Yamaha with Mikuni’s, Kawasaki with a Keihin, and rotary-valved SeaDoos
I have worked on had signs of intake manifold vacuum leaks due to the manufacturing process. Pay
attention to watercraft with aftermarket flame arrestors missing the bracket between the carburetor
and the cylinder head. Mikuni mounting flanges tend to crack without the bracket and some intake
manifolds too.

The vast majority of SDK II, SBN, and SBN-i carburetors were equipped spring rates and seat diameters that
produce high pop-off pressures. These engines were designed to run in this configuration, and if you need
to reduce the pop-off pressure, that is a sign of a problem.

During a carburetor disassembly inspect the hi and lo fuel screw positions, and write them down. No fuel
screw should be turned out beyond 2 ½ turns out. This is a sign of a problem, or possibly something else.
Look for vacuum leaks. Inspect the fuel screw o-rings, and replace if necessary.

When a carburetor is rebuilt, use genuine Mikuni, or Keihin parts (especially if you do not have a pop-off
tester). That includes needles and seats. Always replace the o-ring that seals the needle and seat. It will
save you time, and money in the long run.

Inspect the lever by using a small screw driver to lightly depress lever at the pivot pin to see if it is
loose. If so tighten the lever around the pin with a set of needle nose or duck bill pliers during assembly.
During reassembly set the lever on Mikuni carbs level with the casting. Keihin carburetors need the lever
set to 1mm below the float diaphragms gasket surface.

If you are fortunate to have use of a test tank or trailer to static test your ski with, I recommend you take
the opportunity to listen to your engine. It is best to listen to it at a slow idle.

The first symptom I want you to listen for is rusted main bearings. If the engine radiates a soft Roarrr
sound, you need to pull the spark plugs and rotate the engine by hand. If you feel pitting in the bearings
as you turn over the engine, you need to stop. The Roarrr sound will get louder with rpm, but so does
everything else, and it becomes difficult to hear.

The second symptom is detonation. If your engine has a history of engine failures, I strongly recommend
you listen for it before you rejet the carburetors. If you are running race gas, you may want to run a gallon
or two of pump gas while static water testing. Your watercraft must be secure for acceleration, and wot.
The static water test area must be clear of rocks that could be sucked into the pump while testing. If your
engine has a history of engine failures, this is a good time to switch to 20:1 premix with a high quality oil
during testing.

Updated 11-19-2020 at 02:40 AM by wmazz

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