Help with high speed jetting

[BoLongo]

Seadoo specs large high speed jets in the Mikuni setup for the 787. They do that to prevent the backyard mechanic from turning the high-speed adjustment screw in and leaning out the fuel mixture which would result in high RPM engine damage. That is why the setting for the high-speed adjustment is set to 0. You can only make it richer you can’t make it leaner

I want to be able to have the same stock fuel mixture when I turn the high-speed screw, 1½ turns out. I will have to lower the size of the high-speed jet, so the high-speed screw becomes active. Turning it in will make it leaner and turning it out will make it richer.

From what I have read one turn equals one jet size up or down. My boat comes with 140 high-speed jets so I would lower the high-speed jet size to 138. I am not sure if this is correct. Any suggestions on a good starting jet size?

I am hoping someone will share their experiences with high speed jetting.

 

[BoLongo]

A little more information. I’m OCD about being able to monitor and control a non-computer managed engine. I will have EGT gauges so I can monitor internal temperatures to prevent piston damage. I have been all over Group-k and other racing sites. I do not want to create a racing engine. I want to make my two 787s run as efficiently as possible. I know the engineers at Seadoo are coming from the point of view of making the engine run as rich as possible to prevent engine damage. I want to get a little closer to the line between running too lean and running too rich. Why? because I am OCD like that.

I understand that adjusting a Mikuni carb can be a complicated matrix between low-speed jets, low-speed adjustment screw, pop off pressure, needle and seat size, high-speed jets and high-speed jet screw and not to mention fuel pump pressure and volume delivery. Changes to any one of these will change how fuel is delivery at different points of the RPM range.

My boat came with external fuel pumps, and I have converted to the Mikuni high volume pumps which feed each carb individually. I’m still trying to figure out if I need to increase the pop off pressure because of the newer pumps.

Before I make any changes to the jetting, I will establish a baseline exhaust temperature using the stock setting other than the high volume fuel pumps which I have already converted. From there I will slowly lean out the carbs noting each change in exhaust temperature. 80% of the time I will be in the 5000 to 7000 RPM range; so I’m focused on the high speed jetting.

I do not want to buy ten different sets of jets, so I am trying to estimate the best set of high-speed jets to start with. At this point, I am going to first try 135 jets and see where that gets me.

Any input would be appreciated.

 

[BoLongo]

I just asked the same question in the Seadoo PWC section so; please disregard this thread.

 

[Coastiejoe]

Locking this thread at OPs request..