750 big pin with 46 sbn carb? Does it work?

whennotsailing

whennotsailing

Location
grand rapids/holland/Grand Haven MI
Anyone have carb settings for this combo? Stock 750 big pin with stock exhaust, vortex flame arrestor. I can't make it run well. Tried 150 main/120 & 122.5 low speed. n/s and pop-off is unknown at this point. Runs okay on top end, but throttle it down and/or turn it off and I can't restart. Big hesitation from 1/4 ish throttle to full speed then takes off like a bandito.

Thoughts on:
High low speed jets
needle and spring
pop off
turns

Is anyone running this combo of engine and carb? Results? Too big a carb for a stock 750? Is a 38 mm single too restrictive?

Please and thank you.
 
fastgtfairlane

fastgtfairlane

Location
Charlotte, NC
I'm running a single 46 in a ported small pin. I believe I'm running a 2.0 n/s with 95g spring @ 24psi. 125 pilot 150 or 155 main. Also running a 61x flame arrestor. 1 turn out both screws
 
fastgtfairlane

fastgtfairlane

Location
Charlotte, NC
You definitely need to know what your popoff psi is. Not just the spring and n/s size. The listed psi in the mikuni manual isnt always what your psi will be. It's what it should be. Many times its 2-3psi lower than the manual. I've had to stretch the springs a little to get the psi correct. The popoff will affect your jetting. An indication the popoff is too low would be blip the throttle right off idle, let off, then get back into it. Itll gurgle or blubber then accelerate. If your popoff is too high, it will hesitate and stumble and sound like it ran out of gas for a second then accelerate. Check the sbn manual I posted a pdf file for the different spring identification.
 

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sturat

Location
WA state
if it passes the snuff test for WOT i would just turn the low screw in 1/4 at a time and check, retest, go the other way if needed. If youre hitting throttle and it's bogging and then eventually clearing out, you're rich on the low. If you have the same issue you may need to go up a spring or adjust the spring itself to get the desired pressure. Sounds rich to me.
 
whennotsailing

whennotsailing

Location
grand rapids/holland/Grand Haven MI
I've read and seen on some other threads that a 46 sbn is weird combo to be running and difficult to tune.... I definitely have some more fussing to do, but wondering if I am better to just pick up a proper carb. Hoping someone chimes in that is running this exact combo
 
S

sturat

Location
WA state
whennotsailing said:
I've read and seen on some other threads that a 46 sbn is weird combo to be running and difficult to tune.... I definitely have some more fussing to do, but wondering if I am better to just pick up a proper carb. Hoping someone chimes in that is running this exact combo
Click to expand...
my guy told me the 46 leans out on the high end and has seen issues because of it. The big pin (imo) needs duals. The 40i are what came on there originally and are some of the best carbs mikuni ever made. They are just hard to come by these days. Dual 44s are a pretty standard addition and can be had for about 600 bucks or so. If it were me I would be looking for 40i's.
 
whennotsailing

whennotsailing

Location
grand rapids/holland/Grand Haven MI
sturat said:
my guy told me the 46 leans out on the high end and has seen issues because of it. The big pin (imo) needs duals. The 40i are what came on there originally and are some of the best carbs mikuni ever made. They are just hard to come by these days. Dual 44s are a pretty standard addition and can be had for about 600 bucks or so. If it were me I would be looking for 40i's.
Click to expand...

That confirms what I've read. I want to stick to single carb as there isn't a ton of room in the super chicken, and I don't want to complicate any further then I need to. I just want reliability. I guess I'll start looking for a 40i.

Next question if you know... what about running a single 38 mm mikuni cdk II ? I have a few of those laying around.
 
S

sturat

Location
WA state
whennotsailing said:
That confirms what I've read. I want to stick to single carb as there isn't a ton of room in the super chicken, and I don't want to complicate any further then I need to. I just want reliability. I guess I'll start looking for a 40i.

Next question if you know... what about running a single 38 mm mikuni cdk II ? I have a few of those laying around.
Click to expand...
I have a set that just needs some light love. Rebuild kits and a quick scrub. $375 shipped and they are all yours. Airbox, carbs and manifold. 177570226_424720012292018_5391672170688167744_n.jpg

These are set and forget. I wish I had a ski to use the on but I'm running low on everything.

edit: a stock cdk238 will have that thing feeling like a gutted pig
 
veedubs

veedubs

Location
Dallas, GA
Definitely look into your needle and seat/spring combo. You need to be at 25-35psi pop off. That thing will run just fine on a single SBN 46. Follow the Mikuni tuning manual to get it dialed in then you won’t have to touch it again.


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S

Storbeck

Location
Minnesota
I have a friend who had a 750 bored to 780 in an x2 that he used for racing, aftermarket single carb intake with a 46, and he had the leaning out issue. It was only on long pulls, short bursts it ran great. His was a small pin motor but it was not stock, I'm not sure what sort of porting it had.

He switched to dual sbni40's and really liked them.

I don't think the issue you're describing is due to the single 46mm carb, it sounds lean on the low speed to me but hard to tell with just a description on the internet.

I do think your concerns about the leaning out are valid, although only if you're planning on doing long pulls of wide open. I think there are a lot of people running around with single 46mm carb on a 750 with no issues, but they are probably not doing long wide open pulls, or they have a combination that for whatever reason doesn't have the leaning out issue. I suspect not all single 46's have that issue, my guess is there is some sort of unfavorable resonance in the intake manifold/case/carb that is sometimes there sometimes not....but hell if I know.

You said big pin with stock exhaust, but this is in a superchicken so it's not clear what "stock" exhaust you're using.

To me, with any stock motor combo I doubt you're giving up a lot using a single stock 40mm carb. But again, I don't know.

I would also be a little bit concerned that you could have a big pin motor with really high port timing combined with an exhaust system that is a complete mismatch to that.
 
whennotsailing

whennotsailing

Location
grand rapids/holland/Grand Haven MI
Storbeck said:
I have a friend who had a 750 bored to 780 in an x2 that he used for racing, aftermarket single carb intake with a 46, and he had the leaning out issue. It was only on long pulls, short bursts it ran great. His was a small pin motor but it was not stock, I'm not sure what sort of porting it had.

He switched to dual sbni40's and really liked them.

I don't think the issue you're describing is due to the single 46mm carb, it sounds lean on the low speed to me but hard to tell with just a description on the internet.

I do think your concerns about the leaning out are valid, although only if you're planning on doing long pulls of wide open. I think there are a lot of people running around with single 46mm carb on a 750 with no issues, but they are probably not doing long wide open pulls, or they have a combination that for whatever reason doesn't have the leaning out issue. I suspect not all single 46's have that issue, my guess is there is some sort of unfavorable resonance in the intake manifold/case/carb that is sometimes there sometimes not....but hell if I know.

You said big pin with stock exhaust, but this is in a superchicken so it's not clear what "stock" exhaust you're using.

To me, with any stock motor combo I doubt you're giving up a lot using a single stock 40mm carb. But again, I don't know.

I would also be a little bit concerned that you could have a big pin motor with really high port timing combined with an exhaust system that is a complete mismatch to that.
Click to expand...

Points well taken. I couldn't begin to tell you what the exhaust came from.. Some other kawai sitdown p.o.s. All I know is it's purple and I was able to make it all fit with parts from a couple other exhausts I had laying around. If memory serves me correct it was a mismatch of oval vs round from exhaust mani to head pipe that necessitated the change.

Super chicken riding is nothing but long full pulls, especially when friends hop on it.
 
S

Storbeck

Location
Minnesota
In that case I would be careful listening to people who are set up for short bursts, and also comparing to people who have aftermarket head and exhaust and such. Unfortunately that will be most people. Your application is probably more like a couch and not many of them will be running a single carb.
 
veedubs

veedubs

Location
Dallas, GA
I run dual 46’s on a big pin 750 in one of my skis and it’s works flawlessly. It has clean bottom end power and can be ridden pinned as long as you want without issue. What you have can absolutely be used on your setup it just needs to be tuned properly. Will 120/150 jetting work? I don’t know. You need to get the pop off correct then go from there. Then tune with a tach.


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