Suzuki DF 140 not reaching max rpm

[phlipnstik]

I recently purchased a new boat that has a DF 140. A little back story, I’ve been running bass boats for 20 years. I’ve had a few Mercury Optimax’s and a couple Yamaha SHOs. I switched from a 21’ glass boat to a 18’ flat bottom aluminum boat with a jet. I just finished the break in period and noticed that I am only getting 5100 max RPM. Suzuki says I should be seeing 6000 rpm. I checked the throttle cable and it’s hitting the stop when the throttle is pushed all the way down. I wasn’t sure if it was a new motor thing and it will reach 6000 after being completely broken in. It does have a jet pump but everything I read is the RPM is not affected by a pump vs a prop. Suzuki Experts let me know how to correct this issue. I did creat an account on the Suzuki Outboard Forum but I can’t figure out how to creat a post on there. Thanks Tony

[JoePA]

The Jet pumps are designed to run in the outboards intended power range, 5000 - 6000 RPM and I believe with you running at 5100 that is correct. It may come up some as the engine breaks in but probably not much.

I did a little more research and it looks like you are right where you need to be at for WOT with a pump. Some of the folks that posted number were only seeing 4900RPM at WOT. What kind of speed are you getting out of the boat? How is the holeshot? Most of the time the boats aren't set up properly and the engine is mounted too low. Now that has zero effect on the engine RPM but it will slow the bot down and cause plenty of spray.

[phlipnstik]

Hole Shot is great, I wasn't using GPS for speed but I will the next time out. The speedo was reading around 32 mph. Its an 18x60 Flat bottom. It was lightly loaded, I have my three lithium batteries mounted up front to keep it running shallower. It did have 20 gallons of fuel and full livewells. The foot is mounted about .5 - 1 below the bottom, I dont get any spray from the motor dragging. I'm not to concerned with speed, I knew it wasn't going to be a speed demon. I assumed it would run around 35-38 mph if it was at 6000 rpms, but if I'm good at 5100 then I am happy with that. I just wanted to make sure everything was running like it should.

[JoePA]

I have a 1860 tunnel and my 90/65 pushes me about 27 to 29 depending on conditions. Yours sounds like its setup right and your speed seem to be pretty good for your boat.

[phlipnstik]

Great! Thanks for the feedback

[billnorman1]

In my humble opinion your motor could use a smaller pump to allow it to reach maximum power. I don't own a jet never did. Maybe you could investigate whether the drives can be blueprinted or modified to cause less drag. Is there a takeoff port to check the pressure inside the pump during operation? What horsepower RANGE is the pump designed for?

[Moonlighter]

In my humble opinion your motor could use a smaller pump to allow it to reach maximum power. I don't own a jet never did. Maybe you could investigate whether the drives can be blueprinted or modified to cause less drag. Is there a takeoff port to check the pressure inside the pump during operation? What horsepower RANGE is the pump designed for?

I tend to agree with Bill. Regardless of whether the engine is driving a prop or a jet impeller, these engines will perform best when they are operated in their optimum rev range. And 5100 is not optimum.

You want to target 6000rpm. At 5100 the engine is lugging, whether its driving a prop or a jet impeller.

I know zero about these jet units ….. is it possible to change the impeller pitch etc to allow the engine to rev closer into its optimum range? Seems like that is something that should be able to be done??

[phlipnstik]

I think the impeller only come in one pitch and the pump is designed by Outboard Jets and this is the only model pump that fits the DF 140. Maybe a stainless impeller would help a little. I’ll reach out to outboard jets and see if they have any input.

[phlipnstik]

Well, I emailed back and forth with Luke at outboard jets and I was told that 5100 rpms is the correct RPMs for the Suzuki DF140 with their XL jet pump.

[billnorman1]

Well it might be the right RPM for their pump. I think there is more to learn here, I don't suppose it will hurt your motor. BTW, most two strokes and other motor manufacturers do not have a 6200 limit on their motors.

It has been an interesting post, I doubt I'll ever own a jet pump motor.

[Moonlighter]

Well it might be the right RPM for their pump. I think there is more to learn here, I don't suppose it will hurt your motor. BTW, most two strokes and other motor manufacturers do not have a 6200 limit on their motors.

It has been an interesting post, I doubt I'll ever own a jet pump motor.

I’m with you Bill - its hard not to reach the conclusion that their pump is designed for outboards in the ~140 hp range, but doesn’t account for the different gearing of the Suzuki’s.

It would be very interesting to ask them if the particular model Philip has, is the same model with the same impeller pitch as for a Merc 135, Evinrude 135 or Yamaha 130 etc. If the answer was yes then we would know that its set up to match THEIR recommended rpm sweet spot.

[JoePA]

No jet pump, that I know of, is built to run at or near the top of the RPM range. Every pump is designed to run within 5k-6k RPM band. As far as the gearing with the Suzuki, its minimal at the offset to have any significant change at the pump as far as RPM wise.

[Tim Kelly]

Isn't the reduction gearing in the lower unit, so would have no bearing on a jet LU?

[JoePA]

Isn't the reduction gearing in the lower unit, so would have no bearing on a jet LU?

Yes but I think the offset crankshaft setup may be like 1:1.20 ratio.

[phlipnstik]

From my understanding the jet pump (XL) they use on the Suzuki 140 four stroke is the same size that they use on the 200 and 250 hp Mercury. I really just wanted to know if that was the max I would see. It’s no speed demon, running about 32 mph and that’s plenty fast enough when tuning in 6” of water.