Other JM781 set up discussion

[N3vrSat1sfied]

Main Jet = 110 H
Pilot Jet = 125 L
Needle/Seat = 2.3
Spring = 95
Pilot screw = adj 3/4 turn out
Main Screw = adj 1.25 turn out

I'm ordering a set of 122.5 and 120 Pilot jets to see if that helps to clean up the low end. I'll update this when they arrive, I have them installed and tested.

My setup is as follows:

Gas Octane = 93
Altitude = sea level
Motor type/setup = bb781 61x/62t, 185 psi, 38cc domes, b pipe w/ mod chamber, MSD enhancer, #ZERO lightened flywheel, Pro-Tec intake manifold and Mikuni 44mm SBN carbs

My setup is nearly identical only difference is I am running stock 64x intake manifold with Main Jet = 115 H, Pilot Jet = 120 L Same springs, and needle and seat. Im like a 1 1/4 turn on top and bottom

 

[kevbo]

^^good info

Yea i plan to stick to the 38s for now...bigger carbs n bigger pump will come once i go to 898.

Ill be running race gas with my setup

A pair of 44s will make the pipe hit a little harder. They will require a richer pilot/popoff so expect around 15% higher fuel flow. The main circuit jetting can vary greatly so do not blindly use someone elses specs. I have a high port 701 that needed 140 mains with the top screws 1/2 turn out.

 

[circafwsk8r]

ill let you know in two weeks..

id like to get a flywheel lightened real quick so i can install it all together

hey man your in IL i got my flywheel lightened by Tim at z force racing. give him a call my flywheel is still going strong. dont remeber what it cost but it was cheap enough. $50? maybe.

 

[96superjet]

My setup is nearly identical only difference is I am running stock 64x intake manifold with Main Jet = 115 H, Pilot Jet = 120 L Same springs, and needle and seat. Im like a 1 1/4 turn on top and bottom

Yea, I plan on lowering my Pilot jet to a 120 to see if it will help clean up my bottom end.

Although I have been reading the Mikuni SBN manual tonight and have come up with a few additional ideas. The manual includes a pop off chart that displays expected pop off values for varying needle seat and spring combinations. I've indicated in red my current pop off value of 20 psi.





My thoughts are if I increase my pop off pressure to 22 or 25 psi, that should restrict the flow of fuel and eliminate the blubbery low end richness that I'm currently experiencing.

 

[96superjet]

Figured I'd throw this out too as it seemed to be an important note and goes along with the setup that motozachl is running on his ski.

 

[N3vrSat1sfied]

Yea, I plan on lowering my Pilot jet to a 120 to see if it will help clean up my bottom end.

Although I have been reading the Mikuni SBN manual tonight and have come up with a few additional ideas. The manual includes a pop off chart that displays expected pop off values for varying needle seat and spring combinations. I've indicated in red my current pop off value of 20 psi.


View attachment 226053


My thoughts are if I increase my pop off pressure to 22 or 25 psi, that should restrict the flow of fuel and eliminate the blubbery low end richness that I'm currently experiencing.

Might be a great idea. I think mine runs pretty good as is, but I still think it could get a little snappier.

 

[stefan]

What kinde off comprssion a you Guys running with the JM engins ???

 

[N3vrSat1sfied]

What kinde off comprssion a you Guys running with the JM engins ???

with the stock 38cc domes about 185lbs.

 

[stagesrt4]

im around 230+ on 35cc domes

 

[swapmeet]

37cc right at 200psi...


And I run pump gas.

 

[stefan]

im around 230+ on 35cc domes

dam a you on pump gas or race ??? how mush timing a you running ?

 

[stagesrt4]

dam a you on pump gas or race ??? how mush timing a you running ?

will be running at least 100 octane...

motors still sitting on shelf... will be set at mostly stock timing setting(slight retard) with MSD enhancer

I also have a set of 37cc domes ill be swapping in if the 35s are too tempermental.

 

[96superjet]

UPDATE:

just pulled my 44 SBNs today and made the following modification:

Main Jet = 110 H
Pilot Jet = 125 L changed to 120 L (WSM brand not genuine Mikuni)
Needle/Seat = 2.3
Spring = 95

Ran even worse...

Earlier in the week had a phone conversation with Ed at Full Spectrum Carbs www.fastcarbs.com and he recommended that I change to a 1.5 Needle/Seat and a 65 gram spring. He also said that genuine Mikuni parts is the ONLY way to go. Hmm... sounds like I'm gonna be buying some more carb parts.

 

[zyxwvu44]

Bringing up an old thread but it has good info in it...

Has anyone ran a single 48 on one of these? If so, how does it compare to the 38's?

 

[SKI BONEZ]

I know this is a old thread but the topic is still always asked 38.s or go bigger

My 2 cents
If u are looking for amazing bottom end snap pull those 44 or 46.s and try a set of 38.s u will be amazed at the difference
Not saying u should do that on a big cc motor but the 781 Jm rips amazing on 38.s
And I’ve tested this on many other 845cc.s and under 38.s are amazing

Jet maniac didn’t recommend 38.s like someone said for a budget purpose it’s just the best carbs for the setup


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