SX240 variance in engine RPM

[blacksapphirez]

Is it just a cable adjustment or could it be due to the different impeller pitch? I might as well get another port side impeller and swap it in starboard

Port


Starboard

 

[swatski]

I would not get an aftermarket impeller, you would hate it. You may want to bend the port side impeller (out) on trailing edges, about 1.0 - 0.5 mm, to bring the RPMs down by a 100 RPM or so and get off of the rev limiter. You might gain another 1-2 mph, too.

I am assuming you are running the OEMs which work the best in those boats, IMO. Those are easy to bend, and bending out on the trailing edge is the easiest way.

But, what I really think is you need an L13 in that Port side pump!

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[blacksapphirez]

i was thinking of getting the port impeller on the starboard side to match the pitch maybe the starboard can rev up to 7800 then!

 

[swatski]

I think with these engines you want to be at about 7,700-7,750 RPM max, as at 7,800 you start licking the rev limiter. Most will say to aim at 150-175 RPM below the limiter. So, you are pretty darn close to where you would want to be on the starboard side, I think.

So, the way I understand it, in your boat swapping the starboard side with another port-pitched impeller would not be benficial, you would loose A LOT of RPM as the port is pitched more aggressively from factory in that model year, I think. Because those pumps do not counter-rotate, starboard pump loads better bringing the engine RPMs down (as it is receiving water at a better angle that mimics a left hand turn, basically). It is the opposite on the port side (which is receiving water at an angle that mimics a right hand turn resulting in higher RPMs and a closer cavitation threshold).

That is why I think the L13 on the port side is TOTALLY the way to go with those boats, especially the new (2015+) models with the same impeller pitch from factory. In theory, another L13 on the starboard side would then permit better tuning of both pumps together to get them to synch up. (And that is the theory I am currently testing! )

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[blacksapphirez]

I will wait for your result with the cones

 

[Britboater]

I've only recently noticed a differance with rpm wot, port being the higher. I use the boat mainly with family aboard and very rarely hold wot but did a last mile dash home the other day on glass flat water and it was there, however as soon as the revs drop the engines come back into line. Prop pitch explains this but I didn't know they are different, kinda strange.
I'm always keeping the revs synced but the starboard engine always creeps up over the port so more wot is obviously needed .

 

[Drift Away]

Is it just a cable adjustment or could it be due to the different impeller pitch? I might as well get another port side impeller and swap it in starboard

Port


Starboard

I have a 2016 SX240 and had the same issue until I went in for my 10 hour service and they adjusted the cables. Can't tell what year your boat is but you may just have a stretched cable or they just need adjusted.

 

[blacksapphirez]

I tried to adjust but there was no room for the throttle body to go any further without hitting the stop.

Your boat should have same pitched impeller on both engines which would make sense to have same RPM but mine is pitched differently from the factory which "may" explain the difference... and its only 150rpm...

 

[Drift Away]

I tried to adjust but there was no room for the throttle body to go any further without hitting the stop.

Your boat should have same pitched impeller on both engines which would make sense to have same RPM but mine is pitched differently from the factory which "may" explain the difference... and its only 150rpm...

Ohh i did not catch that part of the post. from what I understand that will absolutely change the RPM range.

 

[Cobra Jet Steering LLC]

From what I understand there are a lot of reasons that the two engines may not be exactly even and I doubt you can ever get them to match for very long so if it is within a few hundred R P M I would think you may just want to leave it alone.
The throttle body of one may be cleaner or dirtier than the other, the pump liner and impeller may be worn differently or have dings or even burn marks near the hub differently than the other, the intake grates or cleanout plugs may have slightly different drag levels, a spark plug in one may be a bit less effective than the other, a coil or ground wire may be slightly different or less effective. The internal operating temperatures may be slightly different or the water levels in the mufflers could be a bit uneven causing different levels of exhaust backpressure. Or perhaps the tacks are not calibrated exactly the same. The possibilities are practically endless, getting them real close to each other is an accomplishment in itself , so you can waste a lot of time playing with it but I doubt you will do a lot of good and it won't change the end results on top speed etc.

 

[swatski]

Your boat should have same pitched impeller on both engines which would make sense to have same RPM but mine is pitched differently from the factory which "may" explain the difference.

Not exactly. Here is a great write up from Group K that explains why having two different pitches actually helps keep the RPM the same (bold text for bottom-line):

From http://www.groupk.com/yjetboat.htm

"About Pump Loading
All technicians that build high performance PWCs quickly learn a few fundamental rules about PWC pump function. As you engage a jet-pump driven watercraft in to a high-speed (power-on) left hand turn, the engine/pump gets very heavily loaded, and engine rpms are usually “pulled down”. However, making a right hand turn at exactly the same speed results in no rpm loss at all … and often results in an rpm increase, often accompanied by mild cavitation.
The reason for this phenomenon is that the entry angle of water coming in towards the impeller has a big effect on how well the pump get “loaded”. In left hand turns, the water comes toward the prop at an angle that very effectively “loads” the pump with water, and allows for very minimal “cavitation”. A hard right hand turn has exactly the opposite effect because the water is coming in toward the prop in a way that is very “unfriendly” to pump loading. Anyone who has driven a single engine Exciter 135 will experience this phenomenon in a big way. Using aftermarket impellers with a better blade/hub design can greatly reduce the cavitation experienced under all conditions …including in right hand turns. But even the best designed prop cannot eliminate right-turn cavitation altogether.

Twin Engine Pump Loading
The phenomenon described above affects the twin motor jet boats in an entirely different way. The water intake surfaces on the bottom of twin motor hulls are on angled surfaces on each side of the hull. This “angled water entry” gives an effective entry angle that mimics the pump loading of turning the boat. This means that (while driving in a straight line) the drivers side pump (starboard) is receiving water at an angle that mimics a left hand turn (thus loading the engine rpms down). At the same time, the passenger side (port) impeller is receiving water at an angle that mimics a right hand turn (resulting in higher rpms and a closer cavitation threshold). This is why the left (port) motors of most twin-engine Yamaha jet-boats tend to run higher rpms than the right (starboard) engines.
Getting past the whole theoretical aspect, the functional result is what’s important.. Because of the phenomenon described above, the right side pump is generating more actual thrust than the left pump at any given rpm. This causes the steering wheel to constantly be “tugging” toward a left hand turn, and at the same time causes the left side engine to rev higher than the right engine in order to generate the same thrust."

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[swatski]

I've only recently noticed a differance with rpm wot, port being the higher. I use the boat mainly with family aboard and very rarely hold wot but did a last mile dash home the other day on glass flat water and it was there, however as soon as the revs drop the engines come back into line. Prop pitch explains this but I didn't know they are different, kinda strange.
I'm always keeping the revs synced but the starboard engine always creeps up over the port so more wot is obviously needed .

Most of the new (2015-current) 240s will spin the port side faster because Yamaha stopped using impellers with different pitches (probably cost saving). Above is an explanation why.

I would argue that any exception to those rules is due to damage and/or irregularities @Cobra Jet Steering LLC listed above.

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[Evildwarf]

As mentioned above its only 150 RPM difference....
your speedometer is showing 54-ish mph....
Man just sit back and enjoy!

 

[FloJet]

Ya only if I could get my AR to hit 54ish. Lucky SX owner!

 

[blacksapphirez]

56mph

 

[FloJet]

Show off!

But really. Why does the SX go so much faster than the AR?

 

[blacksapphirez]

No tower and colder air. it was about 60degrees.

Also have ribbon delete. that's the biggest factor

 

[FloJet]

Oh I see. Well I will be doing the ribbon delete soon. I have a tower and water usually in 80s during the summer here. But shoot if I could get 50-51 consistently, I would be very happy. I'm about 47-49 right now consistently. I was always wondering why I was able to hit 51 in April but not there after. I guess the water temperatures had lots to do with it.

What was your top speed before the ribbon delete?

 

[blacksapphirez]

don't quite remember... but i don't think I broke 50 during hottest days of summer and ribbon still in place...