Jetting instructions for dual 38mm BN carbs mounted on a reed valve engine. - Blogs - PWCToday

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Jetting instructions for dual 38mm BN carbs mounted on a reed valve engine.

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Note: These instructions have been modified for bn 38mm round body carbs on a reed engine.
It assumes you have the correct impeller for your particular set-up. Read the entire thread if you
do not understand

Idle drop test
(1) Perform the idle drop test on a trailer. Set the low screw set to a rich setting. For example:
1 ½ turns out, and warm up the engine (Sam you may need a leaner initial setting between 1
and 1 1/4 turns out)

(2) 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.

(3) Slowly turn the low screws inward (clockwise) 1/8 turn on the front, 1/8 turn on the rear until
the engine wants to die, or quits.

(4) The engine will run smooth out as it approaches a good idle setting, but as it leans out,
sometimes the engine will struggle to keep running. If it does, shut off the engine.

(5) Make a note of how many turns it takes to bottom the screws ( Initial Low Setting).
With duals average the front and rear carb low speed screw settings. This will be your
( Initial Low Setting).

(6) Turn the low screw(s) outwards 1/4 turn beyond the screw setting that caused the engine
to die ( Final low Setting). Restart the engine, and recheck the idle. If the idle speed has
increased, repeat steps 2 through 6.

(7) Did the engine continue to run with the low screw closed?
If so the pop-off is too low.

Increase pop-off by reducing the seat size, or try adding a 2nd gasket under the seat. Check
pop-off on both carbs, and try to set them close. Then repeat steps 2 through 6.

(8) Did the engine die, or want to die while you were turning the low screw in?
If so, how many turns out?

If the ( Initial Low Setting) turns out is greater than 3/4, the pop-off is too high.
Increase the seat diameter, or bend the lever up slightly. Check pop-off on both carbs, and
try to set them close. repeat steps 2 through 6. I do not recommend cutting springs.

(9) Continue adjusting pop-off until (Initial Low Setting) is 1/4 to 1/2 turn out.

(10) Test acceleration. Depending on your set-up, you may need to increase the low speed
screw an additional 1/8 turn out.

(11) (Final low Setting) should end up at 3/4 to 1 turn out.

Recognizing Problems
(12) How to recognize detonation. I strongly recommend you listen to your engine before
you adjust the main circuit. You may want to skip to "Adjusting the Hi speed screw," then
test for detonation before you begin test riding it, while the ski is still on the trailer.

(13) Your watercraft must be secure for acceleration, and wot. The area must be clear of
rocks that could be sucked into the pump while testing. The ski should be secured to the
rear of the trailer to prevent it from swaying back and forth. The depth of the pump will
determine the load on the engine. If the thrust from the pump is spraying on top of the
water surface, the pump is too shallow.

(14) Have you ever heard an old car or truck ping while going up a hill, you might say it sounds
like a bunch of marbles or large ball bearings rattling inside the engine. Unfortunately watercraft
are not that obvious. Most engines will radiate a faint metallic pinging noise, similar to one or a
few bee-bees rattling in a cylinder. Detonation will occur in the mid-range, or at wot.

Warm up your engine, test acceleration several times. Then accelerate slowly a few times and
carefully listen to the engine near 3000 to 4500rpm.

Next test the engine at wide open throttle (wot) for about 20 to 30 seconds. Repeat if necessary,
But not more than 2 additional tests.

If you need to continue listening for detonation, give your engine a little break. Depending on the
water pressure from the pump, you may heat your pipe enough to cause decreased acceleration.
Let the engine run for a few minutes using a series of short accelerations before you retest.

(15) If you hear detonation while static testing in a test tank, or on a trailer, it is nearly a certainty
the engine will seize.
In my experience, I have found that engine failures are a result of more
than one problem at a time. This is an example of when not to over estimating the effect of carburetion.
If the engine is detonating, adjusting the carburetion further will not cure the problem.

Adjusting the Hi speed screw
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.
Review step #13

(16) If you are going to test on a trailer I recommend you start at a known rich setting. 1 1/2 turns should
work. Hold the throttle wide open. Slowly turn the screws inward 1/8 turn on the front, 1/8 turn on the
rear until the engine begins to clear out, and sounds good.

(17) Stop turning the hi speed screws inwards at this point to avoid setting the hi speed screws too lean.
This is your (Initial Hi Setting)

(18) Avoid trying to get max rpm while on the trailer. This will lead to lean high speed screw settings.

(19) The result of steps 16 and 17 should be a transition from an obviously rich engine at wot, to an engine
that continued to increase in rpm, and the engine just began to run smooth, as the hi speed screws were
turned inwards.

(20) (Lean hi speed) If the engine never transitioned from rich or rough powerband to a smooth powerband,
then we need to set the hi speed screws to 2 ½ turns out and repeat steps 16 and 17. Note: If you are
repeating this test, there is no reason to turn the hi speed screws further than 1 turn in.

(21) (Lean hi speed) If step 20 failed, then it is time to drill the main jet larger. Use the ¼ turn is equal to
.01mm or .002” I would recommend starting with 4 jet sizes or .008” and repeat steps 16 and 17.

(22) (Lean hi speed) If you are repeating step 21, I recommend you set a goal for the hi speed screw adjustment
of ¾ to 1 ¼ turns out as a (Final Hi Setting) and consider ½ turns out as an absolute minimum.

(23) (Rich hi speed) If the engine continued to run rich at wide open throttle, despite the screw setting, test
pop-off. If pop-off is still within spec, hopefully the high speed screws will find a minimum position near ½ turns out.

(24) Retest the engine for acceleration and smooth from bottom to top if possible (some porting may prevent this)

Water Testing
(25) If you do not have a tachometer, do not lean the high speed beyond your (Initial Hi Setting) It is your
safety net.

(26) After you have found the hi speed screw settings that produces the best or smooth rpm, I recommend
writing the (Initial Hi Setting) and low speed screw settings inside your engine cover (hood).

(27) Turn 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 the final hi speed screw settings inside your engine cover (hood).

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 meter 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.

Tune for smooth power transitions and consistent performance in all conditions. Your engine should produce
rpm close to your max rpm within 50 - 60 meters.

Updated 08-15-2014 at 06:29 AM by wmazz

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