POLL: Best carbs for ported 701-760?

[Quinc]

@egbrig @Jr. @waxhead

Any guesses on what 1 turn of the tuning screw is equal to in jet sizes? I tried getting this information from Mikuni and mikuni distributors and no one knew.. Also talking about the corse threaded screws.. I have one carb that is so fine threaded I can turn the screw out 8-9 turns before it comes out...

 

[Pro-pulsion]

I would be interested to know this too. I know it comes down to a lot math lol. Trig out the angle of the needle but to know the size of the hole it closes off is the real question. Maybe try coloring the screw point with permanent marker to see where it stops and measure that diameter? Might give some starting measurements on calculating jet size equivalents. I think this one without it already being done by Mikuni is going to be a math intensive answer lol.

 

[Jr.]

@egbrig @Jr. @waxhead

Any guesses on what 1 turn of the tuning screw is equal to in jet sizes? I tried getting this information from Mikuni and mikuni distributors and no one knew.. Also talking about the corse threaded screws.. I have one carb that is so fine threaded I can turn the screw out 8-9 turns before it comes out...

The screws are mixture screws. They regulate mixtutre from the jets into the airstream. Im going to again ref you to read the discription as stated in the SBM manual. As its somewhat complex as to the function of each component.

P

 

[BruceSki]

I always heard they will only meter properly from .5 turn to 3 turns. Anything after that and you need to go up a jet size.

I have No valid proof of that though.


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[TheBuzzard]

1 turn of the screw is equal to about 1/2 a jet size if that. it aint very much. when I first started figuring this stuff out, I had some taper bore 46s that came on a running ported 701. it had a huge lean bog and I ended up turning the low screws out 3+ turns to get it to not bog. I thought i'd just throw a 1 jet size bigger in there and get the screw turned in. 1 jet size bigger got me at 2.5 turns out. I had to go up like 3 jet sizes from 130 to 145 to get the screws back down to 1.5 or so turns out. I had to take the carbs off and on a few times to get it right. if you want your jetting to be right, you have to take the carbs off a bunch of times and try it. everybody want to just have somebody tell them what jets to put in there. IT DOESNT WORK THAT WAY! you can get a starting point but it probably isn't going to be spot on. aftermarket carbs have course threads and oem has fine threads.

 

[Quinc]

1 turn of the screw is equal to about 1/2 a jet size if that.

Perfect! Sadly I have changed jetting more times then I can count (tried to carb tune a bad epic ignition) and will probably never buy a used carb again..

 

[Pro-pulsion]

That is the learning curve of proper tuning lol. I think anyone that has a true drive to tune their ski well has been through the re & re of carbs about a bajillion times lol...myself included.

 

[TheBuzzard]

and for the knuckleheads that list their jetting as having 127.5 jets in there is a joke! you either wasted money on 2 half sizes that weren't necessary or thats just what you had laying around. you could have put a 125 or a 130 in there and turned the screw a red snatch hair one way or the other

 

[Vortex]

and for the knuckleheads that list their jetting as having 127.5 jets in there is a joke! you either wasted money on 2 half sizes that weren't necessary or thats just what you had laying around. you could have put a 125 or a 130 in there and turned the screw a red snatch hair one way or the other

Thought high speed screw only affects it at or around wide open. So at 3/8th throttle that's only on the jet. Correct me if I'm wrong

 

[Jr.]

and for the knuckleheads that list their jetting as having 127.5 jets in there is a joke! you either wasted money on 2 half sizes that weren't necessary or thats just what you had laying around. you could have put a 125 or a 130 in there and turned the screw a red snatch hair one way or the other

I have tuned more carbs in my career than I hate to admit, However, I respectfully disagree with your statement!

 

[waxhead]

@egbrig @Jr. @waxhead

Any guesses on what 1 turn of the tuning screw is equal to in jet sizes? I tried getting this information from Mikuni and mikuni distributors and no one knew.. Also talking about the corse threaded screws.. I have one carb that is so fine threaded I can turn the screw out 8-9 turns before it comes out...

I really dont work like that, But i do try and have my jetting in such a way that my adjuster is about 1- 1.5 turns out. this allows me some fine adjustment if needed with out having to change a jet. When my tuning is done I then go one lower in the high and one higher in the low to see if i can get more torque, just as a test

 

[egbrig]

Buzzard, I agree with much of what you said but fine tuned sometimes does land on a half jet. Yes, you can get very, very close with a full step but sometimes it just comes in a little better at a half step. I must admit though I usually tune to a full step but guys like Phil and Paul get that little finer tune. Also I don't mess with oem carbs much anymore and I really forgot about the thread pitch difference on the screws, thanks for the reminder. But the biggest thing you hit on was, yes, you will have to take the carbs off a couple of times to get it right.
I've been fortunate and years back got Ed to give me one on one lessons on carb tuning. Working around with Phil keeps it current but I don't get to tune much or need to I've got Phil, in fact I've always been lucky to have world class tuners around me. I have ideas and they've brought it out.

 

[egbrig]

Oh, AM carbs are the Mikuni's we buy from Hot Products and OEM's made by Mikuni and Kiehn come on the ski's, although they can all look very close. To keep it simple I just don't bother with oem's any more. I believe you should be able to call the guy who built the engine and get jetting spec's and not have to figure it out for yourself, it is part of the reason you buy what you do or should be. So I tune with 46's, Dasa, Full Spec and Novi. I really enjoy talking about how carbs work and all that can effect it -and it is alot.
Truth is, it's not simple, there is no one answer simple rule. Listen to guys who know and the best time is when your at the water and the guy can prove his thoughts cause his boat runs that good. Then you have to work on it yourself. These are not oem boats.

 

[DAG]

Easyest way to tell AM and OEM apart is AM carbs are sliver and have 4 verticsl holes at the butterfly. OEM carbs are black and have 3 vertical holes.

 

[Kaylee.S]

Easyest way to tell AM and OEM apart is AM carbs are sliver and have 4 verticsl holes at the butterfly. OEM carbs are black and have 3 vertical holes.

Most of the time yes, but the AM Mikunis can be had in black or silver. Big difference between the oem yamaha and AM SBN's tho.

 

[bored&stroked]

Easyest way to tell AM and OEM apart is AM carbs are sliver and have 4 verticsl holes at the butterfly. OEM carbs are black and have 3 vertical holes.

Can someone show me a pic of what you mean.... my carb is silver, has no choke, but I don't see any holes in the butterfly at all.

 

[Pro-pulsion]

I believe he means at the butterfly where it meets the inner venturi also called the minimum air intake line. When the butterfly is at rest there is the slight air space at the bottom of it, when you open it up you will see a series of small holes called progression ports. As the butterfly opens, it (the butterfly) in a manner of speaking uncovers these holes or more appropriately allows larger volumes and velocities of air to pass over them, in which they become more effective through the progression of the fuel delivery process. Some carbs have three in a straight line, some have one in front, two beside each other and a fourth at the back. It depends on what they (either Mikuni or the manufacturer of the machine) had in mind for the carb's intended purpose.

 

[Pro-pulsion]

Here ya go, a Yamaha OEM SBN44 with 4 progression ports.

 

[bored&stroked]

Here ya go, a Yamaha OEM SBN44 with 4 progression ports.

whoa those are tiny, I must have missed them on mine. Many thanks!

 

[egbrig]

This is not me figuring this out, this was passed on to me from Ed Brazina. There are between half dozen different 44's out. The size, number and location of holes
vary. Don't think you are going to eyeball a .015-.020" hole, as well as it's physical location or proximity to adjacent holes. As much as it may seem at times, I'm not trying to be a Dik but trying to pass on info that I have learned so others don't make bad decisions based on bad info. So if you are not good (and honest with yourself) at tuning to
you really want to fight something that you could pass by if you'd just spent a little more $$ and bought it right the first time.