Freestyle Need some jetting help

[Citizen]

Here is my setup currently -

Brand = Mikuni SBN
Model = Retail
Size = 46mm
Quantity = 2
Pilot Jet = 127.5
Main Jet = 130
Needle/Seat = 2.3
Spring = 115g ( i think )
Pilot screw = 1/4 or closed
Main Screw = 1/4 or closed
- - - - - -
Gas Octane = 92
Altitude = Sea level - 2180ft
Main type of riding = Freeride/freestyle
Would it pass the 10 second WOT test = Not sure what this is
Powerband = Want mostly bottom end, I mostly chase boats and jump wakes.
Motor type/setup = 784cc ? - 760 cyl bored w/ 85mm pistons, case porting, ADA girdled head w/ 35cc domes, Carbon fiber reeds, Reed spacer, R&D intake mani, R&D exhaust Mani, R&D Dry Pipe w/ water injection, ATP Epic ignition, Lightened flywheel, Modded waterbox.


I know that my screws should NOT be closed but it seems to run best when they are. I assume my jetting is way off but I can't afford to drop 100 bucks on a jetting kit to screw around till i find right setting. Does anyone have an idea on what my jetting SHOULD be so I can at least get in the ballpark and be able to use my screws to adjust from there.


As it is, it runs decent, very good high end but low end does not have much snap and midrange is ok I think. I have never had it tuned right so it is hard to tell how poorly it is running compared to how it should run.

I have done some searching but no one seems to have asked about my setup at my altitude. Thanks in advance for your help guys.

 

[Matt_E]

The altitude you're at doesn't change jetting much. You can compensate with just your hi/lo screws.
So find a similar setup as a starting point in the carb jetting database thread.

 

[Citizen]

Will do thanks Matt_E

 

[superjetkev]

hey i just jetted my 46s for my 6mm stroker and bpipe. 122.5 main, 115 pilots, 2.3 n/s, 80g spring 1 turn out bottom, 3/4 top.

runs mint! idles great and hits like a hammer. your jetting seems awfully rich to me.

good luck man

 

[Matt_E]

Citizen, I think that jetting of yours is way too fat.

 

[chadetroit]

just marking this so I can find it later. I have a 762 (84.5mm) with dual 46's mikuni, mod b-pipe, epic. Starting point I believe is 120 low and 140 high (need to check this when I get home) on the new engine. I'll let you know where I net out

 

[Citizen]

Thanks very much guys...yea it is def fat as it only runs half decent with screws closed. I plan to go with your setup superjetkev or something very close.

 

[Mark44]

Those carbs are way over sized for that particular motor, why 46's?

Mark44

 

[Matt_E]

Those carbs are way over sized for that particular motor

Mark44

Says who?
That's a 771cc motor. I run dual 46's on my 763cc with great success.

 

[Mark44]

r

Says who?
That's a 771cc motor. I run dual 46's on my 763cc with great success.

Take a look in the Mikuni manual for size too HP requirements.

Says me and Mr. Mikuni :nana:

I didn't say they wouldn't work heck a Holly 650 double pumper can work just saying they are over sized for the motor.

 

[Matt_E]

Have you actually read the manual or is your memory a bit cloudy?

1. Up to 80 hp - SBN 44
2. Up to 90 hp - SBN 46
3. 90 hp and up - Dual SBN 38, 44 or 46

A couple of points:
1. According to that manual, the dual 38's on the stock 96+ SJ motor are oversized.
2. Either Yamaha (and Seadoo and other manufacturers that have put dual SBN's on motors with less than 90 hp) or Mikuni is wrong.
3. I think it's a safe bet that a ported 771cc piped motor makes "90 hp and up".

In closing, I'm not sure what your point was.

 

[chadetroit]

So Matt what are settings are you running on your 763 with the dual 46's?

 

[Matt_E]

I am running dual 46 BlackJacks, which have better flow than standard 46 SBN's.

• 130 Pilots
• 107.5 Mains
• 2.0 Needle & Seat
• 95 gram springs
• 1-1/8 on the low speed adjuster
• 1 turn on high speed adjuster

 

[chadetroit]

Thanks for the reference. BlackJacks might be a winter Mod to my current stock 46's

 

[Mark44]

kept you busy for a while didn't it. LOL............ easy pray.

Mark44

 

[tor*p*do]

kept you busy for a while didn't it. LOL............ easy pray. [Mark44

what are you praying about?
do you pray at the shrine of Mikuni?

LOL, I think you meant prey

 

[superjetkev]

my motor is also 771cc its 81.5mm bore x 74mm stroke. and also running std 46s not blackjacks. its very close to the jet specs that art told me to try with this engine. i ended up with a 122.5 main instead of a 120 as he suggested. he also suggested a 95g spring but i only had an 80 on hand so went with it. works very well i dont think i will change it at all. hope that helps

 

[Citizen]

I wasn't able to get a 122.5 main but I got a 125...should still be better than the 130's I was running...also got 115 pilots and 80g springs. Thanks again for the help kev

 

[Pablo]

Vaughn,

You may want to get your hands on a 95g spring, but you're so close with the 125 vs. the 122.5 main it may be overkill to try a 95 spring and push your popoff too high (I'd still try it). Carb tuning is labor intensive and tedious, but in the end it makes or breaks a ski's performance.

 

[keith t]

this was my set up on 760cc engined jet with all the usual modifications (FP, total loss etc etc)
Type
Mikuni
Mikuni
Size
46mm
46mm
Main jet
120
120
Low speed jet
115
115
High screw turns
1 1/2
1 3/8
Low screw turns
1 3/8
1 1/2
Needle seat
2.0
2.0
Spring weight
95g (dull silver)
95g (dull silver)
Pop off pressure
35
35
Spacer
yes
Reed valves
carbon tech
Manifold
RIVA