Other 760 Yamaha with dual 38's

[waxhead]

Ya, but what about OEM 44's vs. aftermarket Mikuni 44's? That's where the real difference is... Am I wrong?

I only run stock 760 carbs because they’re affordable, and I haven’t noticed any difference in power compared to aftermarket 44s. If you running 44s your on a budget so these fit the situation perfectly . However, going up to 46s or 48s is a different story.

 

[kevbo]

Ya, but what about OEM 44's vs. aftermarket Mikuni 44's? That's where the real difference is... Am I wrong?

Stock 760 44s are Mikuni SBNs. They have a longer 1.60 butterfly and only 3 pilot holes. Compared to a 1.20 plate and 4 pilot holes for aftermarket SBN44s. I have had no trouble making part throttle plenty rich using OEM 760 carbs and manifold with the crossover open. Some guys say 760 carbs are no good but they were also selling new carbs.

 

[fastgtfairlane]

I can comment on the oem 760 vs a/m 44 mikunis. I have ran both on the same engine, in the same ski. The oem 760s I ran larger jetting by one size to get the tune right than with the a/m mikunis. I also did not drill out the return restrictor on the 760 carbs as I did on the a/m carbs. The a/m carbs were a little easier to tune especially on the low circuit. Fuel consumption is probably about the same.

Biggest difference I've noticed with 38s over 44s is mildly better bottom that's negotiable but the 38s definitely sign off much sooner where my 44s keep pulling.

 

[Mobile Chernobyl]

I'll throw my oem 44s back on this week so I can get as close to a back to back as I can. I swear they would pull outta the hole ever so slightly better than my 38s, i.e. launching outta a trough between wake waves on short notice or pulling out of a 360 spin with authority. I also wanna see if the top end is that much more dramatic.

 

[fastgtfairlane]

Is your engine ported? What pipe are you running? Obviously porting and the pipe will allow the larger carbs to show bigger benefit than a stock cylinder.

 

[WFO Speedracer]

I'll throw my oem 44s back on this week so I can get as close to a back to back as I can. I swear they would pull outta the hole ever so slightly better than my 38s, i.e. launching outta a trough between wake waves on short notice or pulling out of a 360 spin with authority. I also wanna see if the top end is that much more dramatic.

I am also going to say that there are external factors such as porting , pipes , compression , different ignition systems and pump load that determine whether it will pull off the bottom better with 38's or 44's but a back to back test would tell us what pulls better on your ski.

 

[waxhead]

There are so many factors that influence performance, and I’m definitely interested to hear what you find. However, I would never consider 38s over a bigger carb on a Superjet. If jetted correctly, 38s consistently perform the lowest compared to other carbs. My carb preferences are, in order: 48 Novi, 46 Mikuni, and 760 OEM.

 

[vcolf]

There are so many factors that influence performance, and I’m definitely interested to hear what you find. However, I would never consider 38s over a bigger carb on a Superjet. If jetted correctly, 38s consistently perform the lowest compared to other carbs. My carb preferences are, in order: 48 Novi, 46 Mikuni, and 760 OEM.

I agree. I had to learn the hard way. Put modified 38’s on my SJ and it was nothing like my other ski with 44’s. My buddy has a stock 760 bpipe with the OEM 44’s, that thing hits harder than any variation of 38’s I have tried.


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[kevbo]

I know people like to obsess over the size of their carb. Performance wise you would be better off boring your nozzle and bending your impeller to match the engine. Once that balance is found, 44s pull better than 38s, and are still easy to tune.

 

[SUPERJET-113]

I had OEM dual 44's on a DASA 800 that every other conceivable mod possible, and when I got rid of those stockers and put BlackJack 44's on it, it was a night and day difference in all around performance. The OEM's did fine and worked well, but the BrapJack 44's were a whole other world..

 

[Mobile Chernobyl]

I'll throw my oem 44s back on this week so I can get as close to a back to back as I can. I swear they would pull outta the hole ever so slightly better than my 38s, i.e. launching outta a trough between wake waves on short notice or pulling out of a 360 spin with authority. I also wanna see if the top end is that much more dramatic.

Back to back results... no surprises really.

Bottom end is the same (i.e. Just fast enough to barely balance, like 1mph, standing up and punching the throttle).
Top end WOT for 5+ seconds is the same (I'm able to hit 47mph on both 38s and 44s)

Mid range tho... 44s take the cake (and eat it too in fuel consumption). 1/8 throttle moving 5+mph then punching it is better. Mid range pipe feel is unbelievably better - its like I went from 5k+ feet of elevation back down to havasu in power levels. Riding out of a spin or stab is so much easier - in that the ski just braaps and boogies right out of the situation.
-I'm not sure why the last one is but it just feels like the 44s are violently sucking water into the intake grate and producing thrust when the 38s feel more like they're cavitating - which on the surface makes absolutely no sense to me.

Perhaps Kevbo can lay down some logic i.e. impeller pitch effect. I'm on a 10/16 hooker / 12 vane mag pump setup I repitched myself from a 9/15.

 

[kevbo]

It takes an overly rich mixture at idle and pilot for best response from low rpms. We have no accelerator pump so having a rich mixture is needed because it temporarily goes lean when you quickly pull the throttle open. Stepping up your pilots one size and opening the low screw a little will likely make response better. It will also be blubbering more and using more gas when cruising slow. This situation gets worse with larger carbs installed on the same engine. A pair of 46s will work but require an even richer part throttle mixture. It sounds like your pump load is good since the small carbs go the same top speed. This means your peak rpm is over the pipes max torque. This is where you want to be for good power response, and it helps prevent melting a piston. As for impellers, the most difficult thing to achieve is no cavitation under high power at slow hull speeds. There is some art to blade design but generally flatter blade angles and larger pump diameter always work better below 10 mph.