94 XP 657x Reverse Jetting

[tej98]

I know this forum isn't geared towards Seadoos, but I'm looking for some information that seems a bit harder to find now. I've got a 94 XP with the 657x and have read about reverse jetting on these smaller motors that seems to give better low end response which is what I'm looking for. Currently I have a set of proks on it and carbs setup like so:

65g springs
1.5NS
77.5 lows
140 mains

Plug chops look great and peaks around 6740 rpm just under 54mph, but low end response is still lacking and almost feels like it's too rich. I read about @wmazz experiences with reverse jetting curing lacking low end response on these smaller motors and wanted to give it a try.

I'm thinking of starting reverse jetting with:
115g springs
1.5NS
97.5 lows
120 mains

Does this seem like a good starting place? Or should I bump the NS to 2.0?

Thanks in advance for any input, it is much appreciated!

 

[kevbo]

The whole "reverse jetting" phenomenon occurs easily on mikuni 44 and larger carbs although it makes them very rich around 1/4 throttle.. The main reason it works at all is because many standup riders fan the trigger and racers never spend much time there. Trying it on a 38 means doubling the size of your pilot. Thats awfully rich at 1/4 throttle. I wouldn't recommend blindly falling for it. Tune the carbs so they throttle well without being too blubbery off idle. Make sure the main jet and top screw is a little rich of peak rpm at full speed.

 

[wmazz]

I developed "my idea" of reverse jetting with a high pop-off
on a 95' GTX because they had a bad problem with dribbling
fuel out of the main jet at low rpm. The 38mm SBN-i carbs had
the smallest main jet booster inside diameter (7mm) of any
oem carb. Increasing the pop-off was my solution combined
with the approximation of 4 low jets for each step increase of
a spring. I figured this out in 1996'.

(Sea Doo improved the dribbling by reducing the 1.5 N&S to
a 1.2 N&S on the 720)

In this case the stock jetting was:
132.5 main
75 low
1.5 N&S
80 gr spring
1-1/4 pilot (low speed) screw
0 hi speed screw.

The reverse jetting concept was shared by a Mikuni engineer. Add
the main (132.5) + low (75) = 207.5 and any combo of 207.5 will
work. I have used that idea on watercraft, MX bikes, and quad atv.

When you use reverse jetting with a high pop-off pressure, the
increases in throttle response and engine power increase without
having any problems with loading up. On the 95' GTX, and many
other watercraft the power increase feels like an extra 100cc just
by rejetting the carbs.

The perfect mod for the average user, or racer that needs more
power and acceleration without loading up while riding for recreation,
or on the starting line.

In this case, when you increase the spring from an 80gr to a
115gr spring, you need to increase the low jet 8 sizes.

So:
112.5 main
95 low
1.5 N&S
80 gr spring
1-1/4 pilot (low speed) screw
0 hi speed screw.

If you want to fine tune those specs a little further (but this can
be used with oem jetting), reduce the return line jet size to a #35
low jet (from a Mikuni VM round slide) into the "Y" fitting of the
return line, set the pilot (low speed) screw to 3/4, and lower the
idle to 1000rpm in the water. It will start the same, idle without
loading up, and improve throttle response, and rev out a little
further.


Bill M.

I recommend replacing the gaskets on carb work.

The 1994 XP also had a problem with cutting out in a hard
corner.

 

[tej98]

I developed "my idea" of reverse jetting with a high pop-off
on a 95' GTX because they had a bad problem with dribbling
fuel out of the main jet at low rpm. The 38mm SBN-i carbs had
the smallest main jet booster inside diameter (7mm) of any
oem carb. Increasing the pop-off was my solution combined
with the approximation of 4 low jets for each step increase of
a spring. I figured this out in 1996'.

(Sea Doo improved the dribbling by reducing the 1.5 N&S to
a 1.2 N&S on the 720)

In this case the stock jetting was:
132.5 main
75 low
1.5 N&S
80 gr spring
1-1/4 pilot (low speed) screw
0 hi speed screw.

The reverse jetting concept was shared by a Mikuni engineer. Add
the main (132.5) + low (75) = 207.5 and any combo of 207.5 will
work. I have used that idea on watercraft, MX bikes, and quad atv.

When you use reverse jetting with a high pop-off pressure, the
increases in throttle response and engine power increase without
having any problems with loading up. On the 95' GTX, and many
other watercraft the power increase feels like an extra 100cc just
by rejetting the carbs.

The perfect mod for the average user, or racer that needs more
power and acceleration without loading up while riding for recreation,
or on the starting line.

In this case, when you increase the spring from an 80gr to a
115gr spring, you need to increase the low jet 8 sizes.

So:
112.5 main
95 low
1.5 N&S
80 gr spring
1-1/4 pilot (low speed) screw
0 hi speed screw.

If you want to fine tune those specs a little further (but this can
be used with oem jetting), reduce the return line jet size to a #35
low jet (from a Mikuni VM round slide) into the "Y" fitting of the
return line, set the pilot (low speed) screw to 3/4, and lower the
idle to 1000rpm in the water. It will start the same, idle without
loading up, and improve throttle response, and rev out a little
further.


Bill M.

I recommend replacing the gaskets on carb work.

The 1994 XP also had a problem with cutting out in a hard
corner.

Thanks Bill, I really appreciate it! Would you recommend jumping from my current 65g spring to the 115g spring? I currently have the 65g springs installed due to the proks.

Or would it make the most sense to just try your recommended specs here for the 115g spring, but up the jets a bit to match my original total fuel flow of 77.5+140=217.5?

So maybe something like this for the proks?
117.5 main
100 low
1.5 N&S
115 gr spring
1-1/4 pilot (low speed) screw
0 hi speed screw.

Thanks in advance again!

 

[wmazz]

Thanks Bill, I really appreciate it! Would you recommend jumping from my current 65g spring to the 115g spring? I currently have the 65g springs installed due to the proks.

It will run great with the specs I provided, with the Prox's or the
stock FA.

Or would it make the most sense to just try your recommended specs here for the 115g spring, but up the jets a bit to match my original total fuel flow of 77.5+140=217.5?

It is important to have a baseline like the stock jetting, or the
recommended jetting for a pipe and try to follow my procedures
as close as possible for best results.

Because so many people think they need to change their pop-off
with a lighter spring, I think it is important to mention this:
The Mikuni Manual does not say "if you replace the stock FA, lower
the pop-off psi"

I prefer not to lower the pop-off. If your prox's did cause a lean
low end bogg the manual recommends reducing the pop-off.
But I would rather rejet the low jet because the low jet directly
controls the acceleration. Not the pilot (low speed) screw, not
the pop-off.



Bill M.

The specs assume you have a normal impeller for the application.

I strongly recommend not to use plug chops while tuning.

On Yamaha and early Sea Doo SBN carbs with 115gr springs, I
do not have solid advice. If you do not have some proven jet specs
from a pipe manufacture, then my only recommendation is to start
with "known good" low speed jet and spring combos like a
75 low, 80gr spring and a 1.5 N&S or 120 low, 80gr spring, and
a 2.0 N&S. Then fine tune the low speed and pilot circuits with
the idle drop test and modify as necessary for baseline jet specs.
specs.

 

[tej98]

It will run great with the specs I provided, with the Prox's or the
stock FA.

It is important to have a baseline like the stock jetting, or the
recommended jetting for a pipe and try to follow my procedures
as close as possible for best results.

Because so many people think they need to change their pop-off
with a lighter spring, I think it is important to mention this:
The Mikuni Manual does not say "if you replace the stock FA, lower
the pop-off psi"

I prefer not to lower the pop-off. If your prox's did cause a lean
low end bogg the manual recommends reducing the pop-off.
But I would rather rejet the low jet because the low jet directly
controls the acceleration. Not the pilot (low speed) screw, not
the pop-off.



Bill M.

The specs assume you have a normal impeller for the application.

I strongly recommend not to use plug chops while tuning.

On Yamaha and early Sea Doo SBN carbs with 115gr springs, I
do not have solid advice. If you do not have some proven jet specs
from a pipe manufacture, then my only recommendation is to start
with "known good" low speed jet and spring combos like a
75 low, 80gr spring and a 1.5 N&S or 120 low, 80gr spring, and
a 2.0 N&S. Then fine tune the low speed and pilot circuits with
the idle drop test and modify as necessary for baseline jet specs.
specs.

Cool, thanks Bill, much appreciated!

 

[wmazz]

The whole "reverse jetting" phenomenon occurs easily on mikuni 44 and larger carbs although it makes them very rich around 1/4 throttle.. The main reason it works at all is because many standup riders fan the trigger and racers never spend much time there. Trying it on a 38 means doubling the size of your pilot.

The idea of using the low speed as a main circuit reminds
me of the "Fish" Carburetor.

Bill M.

 

[bored&stroked]

What am I missing here? It seems like you guys are posting normal jetting, not reverse. Low speed has to be bigger then main to be reverse?

117.5 main
100 low

112.5 main
95 low

97.5 lows
120 mains

 

[wmazz]

I suppose you could call it big low jet specs.

When the same rules used on the Sea Doo are applied
to a 44 or 46 sbn give a 140 low jet and a 130 to 140
main.