Intake Manifolds

[Pro-pulsion]

I've not really paid much attention to it until recently but why do all or almost all of the aftermarket intake manifolds for the SuperJet (62t cases) have the throttle cable mount on the opposite side of the OEM setup? I've read about, if I recall correctly, something called the left turn syndrome. I've never experienced it personally but I vaguely recall reading that some skis, BRP seems to be the only one that comes to mind, had a lean issue when the ski was in a hard left turn. So the answer seemed to be changing the orientation of the carbs but I thought that was for a different style or brand of carb...Mikuni BN, I-body, Keihin? As far as I have experienced it hasn't been an issue with the Super BN. So what gives? Why do the aftermarket manifolds usually have the throttle cable mount on the PTO side of the mani?

 

[Storbeck]

Even more trippy...oem yamaha 38's on a 701 are oriented the opposite of the oem 44's on a 760. No idea why, I've heard the turn thing too but also but not really sure what the deal is.

And on a triple they put them 90 degrees from either of the twins!

 

[Pro-pulsion]

It's definitely weird. I just don't understand why for the SJ, the aftermaket guys know the throttle cable is mounted on the mag side, why change it? It would be easier to accept if there was a good reason for it like copyright infringement? Since Mikuni makes the OEM intake manifolds for Yamaha, they must have had the design first? I dunno, just seems inappropriate lol

 

[Harbor]

Fuel line routing is cleaner with pumps on the outside and Diaphragm vent hole on the inside can’t get block by the Large exhaust outlet hose. Butterfly opening downwards on the outside bringing main jet fuel to outer most part of intake gives more time to atomize and is a smother rounder flow radius.... my 02

 

[Jr.]

Butterfly opening downwards on the outside bringing main jet fuel to outer most part of intake gives more time to atomize and is a smother rounder flow radius....

Bingo!

 

[Storbeck]

That doesn't explain why sometimes people deal with much more complex throttle linkages to do the mounting with shaft perpendicular to crankshaft, does it?


I'd be interested in what @MTRHEAD and @egbrig thought about it.

 

[Wetbuffalo]

What’s interesting is that I have seen some of the 1200 pro boats with the cards flipped back to the original super jet way.

 

[TheBuzzard]

Some years ago I saw the Japanese pro boat had the carbs on the old way so they clearly didnt think the way the throttle plates opened made any difference. It was the boat with the R&D dry pipe manifold with their carbon chamber. I like to think it makes a difference......

 

[MTRHEAD]

That doesn't explain why sometimes people deal with much more complex throttle linkages to do the mounting with shaft perpendicular to crankshaft, does it?


I'd be interested in what @MTRHEAD and @egbrig thought about it.

Don’t actually know. Never seen it the other way until I saw it on the Yamaha 701 38mm twin carb. Never tried Big carbs like the oem 38’s. All other skis including the single carb yamahas have cable in rear or pumps to the outside. when I ran Kawasak’s I always ran sidedrafts. I’m going to guess something to do with the 38 velocity and low speed emissions. notice the cast boss only in the Yamaha 38’s to direct air over the booster - there’s something going on with air flow in the oem 38’s in their application. Not on Mikunis sold over the counter

 

[Storbeck]

Don’t actually know. Never seen it the other way until I saw it on the Yamaha 701 38mm twin carb. Never tried Big carbs like the oem 38’s. All other skis including the single carb yamahas have cable in rear or pumps to the outside. when I ran Kawasak’s I always ran sidedrafts. I’m going to guess something to do with the 38 velocity and low speed emissions. notice the cast boss only in the Yamaha 38’s to direct air over the booster - there’s something going on with air flow in the oem 38’s in their application. Not on Mikunis sold over the counter

Which way do you orient the side draft carbs? I would guess with shafts vertical (?) It would be super convenient to have the diaphragm plate upwards so you could change jets easy so there has to be some reason that doesn't work because nothing is ever that easy.

 

[MTRHEAD]

Most if not all sidedrafts I ran were jets forward. Just like OEM Yamaha (800 and 1200) motors.

Jet changes are not fun when jets forward.

 

[bored&stroked]

Jet changes are not fun when jets forward.

I have yet to find jet changes fun on any carb in any vehicle ever made.

 

[Big Kahuna]

When I first had my cylinders ported on my first SJ, it took a while to get jetting correct. I did not mind doing them with the stock 38's. Me and a buddy had ours ported at same time. so went through the same issues, he had a dry pipe, I had a BPipe. We got to where we were doing them pretty quick..... Much easier than my BJ's on a speed plate actually!!!!!!! less hoses to deal with!!!!!!!

 

[Pro-pulsion]

I too have become rather efficient with hauling carbs off the ol sj, but I"m with ya BK on the extra hoses. I eventually figured out a way to position the carbs for jet changes by only getting them away from the mani with no hose removal. might be due to the length of the hoses but I'll take the quick change method any day over multiple hose removals and new tie straps every time.

 

[Pro-pulsion]

Another thought about manifolds just came back to mind, the crossover tract. I know a filled crossover makes tuning easier, but I also recall the only one I ever saw that was an aftermarket mani with a crossover tract was an old R&D mani I used to have. I contacted R&D about it asking if they ever did back to back testing on the difference between open crossovers and solid wondering which was better for the application - freeride/freestyle or racing - they said the open crossover yielded better holeshot response (meaning for us freeriders better bottom end) than the solid version, but the solid version was as we know easier to tune and better for racing applications. Makes me wonder why more aftermarket companies both now and then didn't have both options available. R&D seems to have been the only one but even they didn't keep production on them current up to their final days. One would think that if the goal for freeriders is maximum out of the water punch, the open crossover would be the choice.

 

[bored&stroked]

They know if everyone wants the crossover and it makes the ski hard to tune, everyone will blame their product for being a poor design and give them a bad reputation. Not worth it vs just making the no crossover that most people want anyways. Most people fill the crossover in the oem manifolds from what I read.

 

[Pro-pulsion]

Yeah that's good point, placing blame where it is actually not due. It's too bad though that the option isn't there but you're probably right on the spot with that assumption. I haven't filled the crossover on any of my manifolds, the only carbs I have been having a hard time tuning are my 46's. But that's just me because I'm really picky about how I want my ski to run so it's never precisely tuned until I'm really happy with it lol. I just recently dialed in a Blaster for a friend, it's running a Riva manifold with no crossover and it runs well, really well actually

 

[TheBuzzard]

My guru senor says boost bottles are the ticket, but size of bottle is critical. That crossover passage could be crap but that R&D manifold is just a copy of the stock 760 manifold with crossover, so maybe Yamaha knew something. A manifold is not going to make enough of a difference in performance for me to give a crap