D300 running uneven at rpm over 2500

[Fredrik]

After winter storage my Suzuki 300 from 2007 runs very unevenly at higher rpm. Starts and runs smoothly at lower rpm. I suggest leakage in fuel hoses or similar. Will try to change. Any other suggestions? See video for how it looks/sounds
https://youtube.com/shorts/ywj7J2z1u8Q?feature=share

[Freddy jakob]

It is normal Have you tested the boat at speed?? When it is disengaged, the revs are blocked so that the engine does not destroy itself

[Fredrik]

It is normal Have you tested the boat at speed?? When it is disengaged, the revs are blocked so that the engine does not destroy itself

Yes I tried it at speed and got the same problem.

[Freddy jakob]

Then there is a possibility that you have a fault with the neutral switch.
Best is to use sds to check this. Sounds like the rev limiter kicks in at about 3000rpm

[Fredrik]

Ok so if the neutral switch is malfunctioning the engine think it’s in neutral and stall the engine like in the video even though I’m driving?

[Freddy jakob]

Yes, I mean to remember it that way. If Mooligther or redlowrey are reading this, they probably have a more correct answer. They are super skilled and top notch at solving suzuki problems

[Fredrik]

Yes, I mean to remember it that way. If Mooligther or redlowrey are reading this, they probably have a more correct answer. They are super skilled and top notch at solving suzuki problems

Thanks, I’ll try to clean and dry the connections this weekend. I think I’ll change the fuel lines as well in any case. They are old.

[Moonlighter]

Ok, when the engine is revved in neutral as per your video, there is a inbuilt protection system that cuts in at around 3000rpm to prevent the engine being unnecessarily over revved. Indeed, revving an engine like that in neutral is pretty well pointless…..

It cuts spark, restricts spark advance and fuel supply at 3000rpm and the end result is the engine runs like a race car does when its on the pit lane limiter. Badly.

That is exactly what is happening in your video and it is doing precisely what it should be doing.

There is a neutral throttle switch on the gear linkages on the engine that tells the ecu when the engine is in neutral or in gear and thus controls this protection system.

If your engine is doing the same thing when it is in gear and the boat is underway, and wont rev past about 3000rpm and has doughy acceleration up to that point, then that neutral switch may have failed or gotten stuck.

On an engine that age, it is probably simplest just to replace it as they arent expensive. Go to the “Buy Suzuki parts” link in the heavy BLUE line towards the top of this forum and use your engine details and you will be able to find the part number and expanded diagram that shows you where it is. Not a hard thing to replace yourself if you are handy around engines.

[Moonlighter]

Here you go. Part #7 on this link:

https://www.boats.net/catalog/suzuki...e-control-unit

Actual part $40
https://www.boats.net/product/suzuki...3e069059dc3380

[artdf175]

When ask if you have run it at speed?
that means boat in the water and running on the river/lake not on a hose in the driveway ..or tub
also one of the first things to look as this spark plugs when engines are seemingly missing

[Fredrik]

Ok, when the engine is revved in neutral as per your video, there is a inbuilt protection system that cuts in at around 3000rpm to prevent the engine being unnecessarily over revved. Indeed, revving an engine like that in neutral is pretty well pointless…..

It cuts spark, restricts spark advance and fuel supply at 3000rpm and the end result is the engine runs like a race car does when its on the pit lane limiter. Badly.

That is exactly what is happening in your video and it is doing precisely what it should be doing.

There is a neutral throttle switch on the gear linkages on the engine that tells the ecu when the engine is in neutral or in gear and thus controls this protection system.

If your engine is doing the same thing when it is in gear and the boat is underway, and wont rev past about 3000rpm and has doughy acceleration up to that point, then that neutral switch may have failed or gotten stuck.

On an engine that age, it is probably simplest just to replace it as they arent expensive. Go to the “Buy Suzuki parts” link in the heavy BLUE line towards the top of this forum and use your engine details and you will be able to find the part number and expanded diagram that shows you where it is. Not a hard thing to replace yourself if you are handy around engines.

Thank you for the advice. I have never tried to rev it like this before when not in gear so I was not aware of the neutral switch function . I splashed the boat after winter and took it out on a short test run. This is when it immediately started to act like this at plus 2500 rpm. No fault code and it starts and runs normal at lower rpm. It certainly sounds like the switch have gone bad over winter and I will order the part ASAP and change it out. Will
come back with results

[Fredrik]

When ask if you have run it at speed?
that means boat in the water and running on the river/lake not on a hose in the driveway ..or tub
also one of the first things to look as this spark plugs when engines are seemingly missing

Yes I have run it in the ocean in gear. The sparks plugs where changed last year and as it runs smooth and well all the way up to 2500 rpm they are not my first suspects. I have a set of new ones however so maybe I’ll change them if the neutral switch is not the error

[Moonlighter]

2500 rpm is not 3000rpm, unless your tacho is not reading accurately.

The neutral protection happens at 3000rpm, or very close to that.

If you can, get the engine connected up to the SDS diagnostic computer. That system can immediately show if the neutral switch is working correctly or if its something else. And it can test run other tests as well.

Anyway ….

Check all the plug leads to make sure they are connected correctly to the spark plugs. i would be inclined to change the plugs first since you already have them, and then water test it to see if the problem still persists before doing anything else or buying any other parts.

2500rpm may well be the speed at which the engine starts to get under load to push the boat onto the plane and under load is where a bad plug or lead will start to misfire.

Make sure to run it in gear and accelerate as normal to get the boat onto the plane and see how it goes then,
.

[Fredrik]

2500 rpm is not 3000rpm, unless your tacho is not reading accurately.

The neutral protection happens at 3000rpm, or very close to that.

If you can, get the engine connected up to the SDS diagnostic computer. That system can immediately show if the neutral switch is working correctly or if its something else. And it can test run other tests as well.

Anyway ….

Check all the plug leads to make sure they are connected correctly to the spark plugs. i would be inclined to change the plugs first since you already have them, and then water test it to see if the problem still persists before doing anything else or buying any other parts.

2500rpm may well be the speed at which the engine starts to get under load to push the boat onto the plane and under load is where a bad plug or lead will start to misfire.

Make sure to run it in gear and accelerate as normal to get the boat onto the plane and see how it goes then,
.

It’s starting after 2500 rpm close to 3000 so it sounds like the rev limiter. I’ll check the plugs tonight and bring the boat out to test. I read in another thread that you could manually close the switch by connect a dummy connector to the inlet below the fuel filter. Maybe I’ll try that as well to definitely make sure if the switch is faulty or not.

[Fredrik]

Update: I went to the boat this afternoon and cleaned the small fuel filter, checked the fuel lines (some kinks, needs replacement) and disconnected the contact for the neutral switch. It was not corroded (this was on the side that is not inside the clutch lever). Cleaned it and put everything together again. Lost a nr 10 wrench in the ocean…. Then I went out for a drive and at first it stumbled a little but after a minute or so it started to run as normal. Then I drove around for 15 minutes testing a bit and it ran fine. Hopefully it was the fuel and that cleaning etc fixed the problem, Now I have neutral switch as a spare. See video for how it ran https://youtube.com/shorts/b8-muZ8n20g