How to tune MainJet/high speed screw on freestyle ski

[wmazz]

The idle drop test is how I have been tuning a carburetors low speed and pilot circuits since the early
1980's. The only thing that has changed is the the adjustment after the engine dies or wants to die.
On early Mikuni BN carbs the adjustment is 1/4 turn. But when the CDK II was released in 1986, the
adjustment was increased to 1/2 turn. The same 1/2 turn adjustment works for the Mikuni SBN, SBN-i
carbs. The reason for this change was the manufactures started using an fuel-air pilot circuit on the
CDK II, SBN, SBN-i carbs. The BN has a traditional low speed circuit.

A very common loss of peak performance is caused by the low speed (pilot) screw set richer than
necessary.

With the idle drop test you can determine if the low jet and spring is the correct size. On the the CDK II,
SBN, SBN-i carbs, the low jet, spring, seat diameter, and low speed (pilot) screw are all inter-connected.

Using the idle drop test rules, I was able to develop reverse jetting with a very high pop-off, using seats
from 1.2 to 2.0 and pop-off 32, 55, 76 psi. It is the style of jetting I prefer for all SBN, SBN-i carbs from
increasing the performance of stock engines to full race engines. But it does not work on carbs with
modified low speed by-pass holes (like Novi).

The only type of carbs I would not use this test on is the Novi, or any carb with a modified return line
circuit. Otherwise it works on all diaphragm carbs including weed whackers.

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 (when the engine acts
like it is about to lean out and die) yourself and make the adjustments, and document turns to bottom or
closed. Next, reset the low speed (pilot) screws to the setting where the engine died, and turn the low speed
(pilot) screw(s) outwards an additional 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 to count screw settings that suits you. Then write
the settings down and keep it for latter, and reset the screw(s) position.

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 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. 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.


Bill M.

 

[Zero Client]

Bill is spot on for tuning the pilot circuit. In the end low screw should never be over 1 turn or under 1/2 turn from closed. This would necessitate a change in low jetting for sure. I’ve had good luck tuning for 7/8 turn on the low screw following similar advice from lifer tuners.

Op original ask was for Main and high screw which is a different procedure like peak rpm, plug reads, egt 1280f, or AFR. Most of these not possible w watercraft though, although if running dry pipe I don’t see why you could not try egt or AFR. Never done or seen it tho.

For bottom end stroker motor I’d expect fuel needs in the low-mid range to be high, while wot would be low. This would actually follow the torque curve if you’ve ever seen a traditional 2-stroke dyno chart, say of like a cr500r dirtbike, torque drops off steeply at higher rpm with a bottom end stomper engine. So, aka reverse jetting is a must/plus, but on the high screw I’d expect a low turn count -maybe 1/4-1/2 turn in combination with a main jet rich enough to satisfy 1/2-3/4 mid throttle high output low rpm torque requirements of a bored out bottom end stroker motor.

Pay attention to your mid throttle mixture to know what your main and high screw need to end at as after 1/3 throttle, the pilot circuit becomes less involved.

Or ask your builder because they should know without actually touching your ski.


Sent from my iPhone using Tapatalk

 

[Quinc]

Quinc, Is your return lines modified to run a #60 to #100 jet?



Bill M.

Whatever the 46's return hole is stock. I haven't drilled them.

 

[maxboost]

@wmazz thanks a ton for the write up, very helpful read! I have "read" that when adjusting idle mixture, if you are too far in or out on your low speed screw one should adjust pop off and not the jet? I was wondering if you could give a brief explanation on how to obtain a good pop off or is it more trial and error?

Thanks to @Zero Client as well!

 

[wmazz]

To begin with, pop-off only measures the springs preload, and the spring rate controls
the transition from the low circuit to the high speed circuit. So IMO replacing a spring
instead of the low jet is a mistake.

Another thing many people don't realize is the CDK II, sbn, and sbn-i low speed screw is
actually a pilot screw for starting and idling. It does not adjust the low speed directly.

My rule of thumb for springs and low jets is 4 low jets = 1 step up in spring rate. For
example: 1.5 N&S, 80gr spring and a 75 low jet. In order for a 95gr spring to work and
still transition from low to high, you need to increase 4 low jet sizes. Then the new spring
will work: 1.5 N&S, 95gr spring and a 85 low jet.

Or you could use this rule of thumb: use ¼ turn = 1 pilot jet size.



Bill M.

I rarely consider a carb problem to be a pop-off related problem. The silliest of the
"pop-off" problems is: "It's hard starting because the pop-off is to high," BS.

 

[Storbeck]

On the left is the low speed screw from an oem 44 from a Yamaha 760, the right is low speed screw from oem 38 from some Yamaha 701 (no idea what exact motor it came from). I think the 38's might have either one of these on thier low speed depending on the year and model but I'm not sure.

Not only is the tread much finer pitch, but the taper on the needle is much shallower on the screw on the left. I think it's fairly obvious that one turn of the screw on the left is a much different thing than one turn of the screw on the right. Probably one turn on the screw on the right is equivalent to at least 2 turns on the screw on the left I would guess.

This is one thing that always makes me question people who say things like "it needs to be 1 turn out to run right" or "1/4 turn is equal to one jet size"

That is probably true of aftermarket carbs that (I think) use the same screws, but definitely not true universally.

 

[wmazz]

This is one thing that always makes me question people who say things like "it needs to be 1 turn out to run right" or

"1/4 turn is equal to one jet size"

First off, Speaking for myself, when I am rejetting a carburetor, I like to shoot for
3/4's of a turn out on the high speed, and pilot screws. I am not saying the screw
adjustments needs to be anything specific to "run right."

For example you could rejet a carb and end up with screw settings of 2 on the high
and 2 on the pilot and consider that good, but I would continue rejetting it until
I reached my goal.

2). Does anyone know what the difference between a 150 main jet and a 147.5 jet??
the answer is .001," that is all. Low jets are the same .001" or possibly less.

I use the same rule: 1/4 turn for 1 jet size for mains and lows. It might not be perfect,
but it is a good rule of thumb that will get you in the ball park.

To begin with, pop-off only measures the springs preload,
and the spring rate controls the transition from the low circuit to the high speed circuit.

I would like to make a correction. The above should have said:

To begin with, pop-off only measures the springs preload. The preload and the spring
rate controls the transition from the low circuit to the high speed circuit.

My rule of thumb for springs and low jets is 4 low jets = 1 step up in spring rate. For
example: 1.5 N&S, 80gr spring and a 75 low jet. In order for a 95gr spring to work and
still transition from low to high, you need to increase 4 low jet sizes. Then the new spring
will work: 1.5 N&S, 95gr spring and a 85 low jet.

If you raise the low jet 4 sizes, you need to decrease the main jet 4 sizes. I have applied this
style of jetting to 125 MX bikes and Yamaha Banshee's. I have been jetting with really high
pop-off pressures ~55 to 70 psi since 1996. I started jetting that way because the Mikuni
Manuals solution for fuel dribbling out of the main jet didn't work for my SeaDoo GTX
customers. But raising the pop-off did, and they gained a good boost in acceleration
and possibly power.

3). Those screws are a good example of nearly any carbs hi speed and pilot screw. Except
that is a pilot screw and not a low speed screw. Carburetors equipped with pilot screws,
have a low speed / pilot circuit that works different than the old Mikuni BN, or similar
old snowmobile carbs. I know they are labeled with an "L" but they are not low speed
screws.

4). OEM Yamaha Mikuni carbs are manufactured to different specs than all the other OEM,
or aftermarket Mikuni carbs.

I can only speculate the reason. I believe they are different because emission testing occurred
at an idle. Yamaha carbs have different low speed bypass holes and the throttle butterfly has
a steeper angle than all other Mikuni carbs.

Again; I believe the steep throttle valve angle and low speed by-pass holes allowed Yamaha
to run high pop-off with lean low jets.

And when they don't have any vacuum leaks, they start and run as expected, just like the other
brands of watercraft with starting issues.


Bill M.

 

[maxboost]

I would like to make a correction. The above should have said:

To begin with, pop-off only measures the springs preload. The preload and the spring
rate controls the transition from the low circuit to the high speed circuit.

@wmazz for example if I had 28 psi pop off pressure with a setup with a 115 gram spring and 28 psi with a 80 gram spring, would if yield the same results during “transition” or would one be slightly different than the other? Thanks

 

[wmazz]

I have never tried that before, But I can give you a technical explanation that
compares the 40 sbn-i, sbn 44, 44 Buckshot with normal jetting (no reverse jetting).

It would start with atmospheric pressure (pretend that it is always constant),
and that negative pressure (vacuum) is always a variable. In a carburetor negative
pressure occurs 2 different ways: Manifold vacuum (idle to ~3/8 throttle), and
negative pressure created by air moving through a restriction (venturi effect).
Approximately 3/8 throttle to wot.

Atmospheric pressure (constant) is what pushes down on the diaphragm and opens
the needle and seat (not pop-off psi). Atmospheric pressure pushes the fuel into the
venturi, and Atmospheric pressure pushes air through the flame arrestor and into the
carb.

Because negative pressure (vacuum) is a variable, the needle and seat may not
open because of a vacuum leak (on the carb base, intake manifold, etc.). Atmospheric
pressure remains constant, but their isn't enough manifold vacuum, so the needle
remains shut.

The same basic process occurs after 3/8's throttle, when manifold vacuum is disappearing
and in order for the carb to transition to the main jet, a new vacuum source must be
created by the venturi and air speed. This is a poor transition that creates a lean spot
from approximately 3/8's to 5/8's throttle. This same lean spot is common on all carbs.
(keep in mind that atmospheric pressure opens the needle and seat, and vacuum is
decreasing in this range)

Our diaphragm carbs normally use 2 venturi, to combat this lean spot. A main venturi and
a booster venturi (Bombsight). If you decrease the inner diameter of the booster venturi
the main jet (sbn-i) will transition sooner because it created lower pressure at the main
jet before a sbn carb will.

So the 40mm sbn-i will transition the easiest, the 44 sbn has the normal problems, but
the large diameter of the 44 slows the air velocity. The standard 44 sbn is at a disadvantage
to the 40 sbn-i because of the diameter. This air velocity problem increases with diameter.
So a 46 is worse than a 44, and a 48 is worse than a 46, and so on.

Finally our basket case is the Buckshot with almost no venturi at all, the buckshot has
a large lean spot and big problems producing low pressure at the main jet.

for example if I had 28 psi pop off pressure with a setup with a 115 gram spring and 28 psi with a 80 gram spring,
would if yield the same results during “transition” or would one be slightly different than the other?

It would be different, but I don't think you would see much of a difference on a good carb
like the 40mm sbn-i (and maybe not on a Yamaha carb).

The carbs that already have problems transitioning to the main jet would benefit from less
spring rate and larger low jets.

The preload or pop-off is the pressure required to start the spring in motion, and the spring
rate is the weight required to move the spring 1 mm. So the 115gr spring would require more
vacuum to open the same distance as the 80gr spring, while Atmospheric pressure remains
constant.

Pop-off is not what actually opens the needle and seat. Basically it is just Atmospheric
pressure and negative pressure (vacuum).


Bill M.

Keep in mind that the pop-off test is just a test and reference pressure.

The seat diameter, spring rate, low jet and pilot screw setting are all
inter-related. So in your example there should be a larger low jet
with the 115gr spring.

 

[Big Kahuna]

@Rowdogg50

 

[homemadejoe]

Hello all, question for wmazz, in the first post of page two you wrote:
The only type of carbs I would not use this test on is the Novi, or any carb with a modified return line
circuit. Otherwise it works on all diaphragm carbs including weed whackers.

Can you clarify what you mean by "any carb with a modified return line circuit"? Does this include carbs that have had the built in restrictors drilled out?
Also any chance you could provide jetting specs for those carbs that had the pop off pressures of 55 and 76? Just curious what jetting those pop offs allowed. Thanks for your write up in this post, very informative

 

[wmazz]

Also any chance you could provide jetting specs for those carbs that had the pop off pressures of 55 and 76? Just curious what jetting those pop offs allowed.

It is not really the high pop-off, it is the spring used. Because
how the needle & seat actually opens has little to do with pop-
off.

But you can make a generalization that greater performance,
and acceleration is possible with a high pop-off. You can not
say the same with low pop-offs, unless you incorporate reverse
jetting. The risk with reverse jetting and low pop-off is loading
up in a 5 mph zone, or bogging off a starting line, and that may
be acceptable for a racer, but not the average customer. That is
why modifications to the return line are necessary.

I started using reverse jetting with a high pop-off ( back in ~95'
or 1996) as a way to fix a customers 95' GTX. Not only did it fix
the problem of fuel dribbling out of the main jet boosters, but
it also felt like it gained 100cc from just rejetting the carbs.

A common set-up for X2's with an SXR800 engine is the 40mm
SBN-i jetted to FPP dry pipe jet specs: 152.5 main, ~75 low, 1.5
needle and seat, and 80gr spring.

We started racing with 132.5 mains, 95 lows, 1.5 N&S, 115gr spring
and pilot screw set ~1/2 to 3/4 turns out. Later we found that leaning
out the main jet to compensate for the richer low jet was not necessary.
So we ended up with 152.5 mains, 95 lows, 1.5 N&S, 115gr spring
and pilot screw set ~1/2 to 3/4 turns out, and gained even more
power.


Bill M.

A high pop-off (high spring rate with a small seat) isn't a disadvantage,
it is an opportunity to increase the power for average rider for very little $$

Keep in mind that the pop-off test is just a test for leakage
and a reference pressure.

Another myth is the engine will lean out because the N&S is too
small for a modified engine. We can thank HK for that myth.