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Old 03 May 2015, 19:09   #11
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Country: UK - England
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Boat name: Nimrod II
Make: Aerotec 380
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Engine: Suzuki DF20 EFI
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I have the same 15hp engine and have previously owned the identical Mariner. These have been used on various SIBs from 3m to 3.6m... air, alloy and wood floor models at different times.

Some random thoughts...

I too don't think the RPM maxing out at 7000 rpm is true... sounds a little lower to me... more like 5500rpm when you are showing 7000. Also when you have hit an idle that looks higher on the tach than I'd expect.

Inflatables are imperfect beasts even when fully inflated (air floors more so obviously but yours looks like a hard floor) so they can suffer weird "slipping" feelings at max power. I've experienced this many times and it depends on loading, how the boat is trimmed, the wave state... even prop type and condition.

To me yours doesn't sound like a mechanical slipping of the gearbox as it's not that bad.

What's your boat loading balance like? To me with you, fuel tank, fixed transom wheels & battery etc in the stern it looked like it might be a bit bow high?

What weight are you and your passenger?

Are the transom wheels dragging in the water at all on the plane?

I think you are saying if it drops off the plane sometimes it doesn't easily get on the plane again. That can happen even with a 15hp if you guys are heavy, with a full fuel load plus battery etc... particularly if you are loaded too much towards the stern.

Do get used to the telltale variation with this model due to the thermostat. It was designed to run with a stat so leave it in.

The cutting out looks like a fuel starvation or inlet air leak issue. I'd be wondering if there could be any blockages in the fuel line or fuel filter. Also if the fuel pump diaphragm was OK and then of course is the carb dirty inside blocking the jets at idle.

Also these engines will stall like that if the idle mixture is too weak. See this thread...

2 stroke "sneezes" then cuts out

Also note this thread where I had a slow running/stalling issue with my Mercury 15. It was a hard to see hose that had split on the carb...

Mariner 15 2-stroke spitting fuel near carb.
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Old 03 May 2015, 19:16   #12
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I mentioned this sort of "slipping" at max power/speed can be the prop type or condition.

For example I had this slipping feeling with my Mariner 15 when I fitted it with a new correct pitch Solas aftermarket prop. Swapped it for a genuine Mariner one and the problem was cured.
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Old 03 May 2015, 19:36   #13
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Kestrel I've just looked at a few of your previous posts to see if I could see what model your inflatable was and see it's a Zodiac 350 alloy floor weighing 150lbs. Just as a matter of interest I had a Honwave 3.5 that weighed about that and that was much more sluggish getting onto the plane (when loaded with two "not slim" adults plus a teen) than an air floor model of a similar length that weighed 50-60lb less.

But more importantly I noticed you say this of your motor...

....funny story about that engine I bought the other day. Turns out it had been on a water mouse originally, and then the fellow who owned it had it run dry (I'm assuming he meant dry of oil with clean gas) by his kids. Whole thing seized up, and the shop he sold it to spent about a grand completely rebuilding it since the outside looked spotless.


I do hope the running issue is a simple fuel thing but I have to say the above information would ring alarm bells with me when the motor was proving unreliable from your first use of it. Not that uncommon for folks to do what they call a full repair after a major overheat yet the outboard never runs totally properly again.
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Old 03 May 2015, 19:37   #14
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Quote:
Originally Posted by Fenlander View Post
I mentioned this sort of "slipping" at max power/speed can be the prop type or condition.

For example I had this slipping feeling with my Mariner 15 when I fitted it with a new correct pitch Solas aftermarket prop. Swapped it for a genuine Mariner one and the problem was cured.
A quick question if you don't mind, what pitch prop do you think I should be running?
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Old 03 May 2015, 19:48   #15
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Standard will be fine as it's all I've used and has been perfect... think that would be a 9.5"... I'll check it out.

Or perhaps 9" is standard??
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Old 03 May 2015, 19:53   #16
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I can't completely rule out anything, but I personally doubt it's fuel tank or line related. It's a new tank, and new line. All standard stuff, nothing hodge podged together, it's all the proper Mercury connectors.

The guy who came over to look at my motor revved it up while keeping the gear shaft from engaging and putting it into drive, without the drive train engaged the thing revved up like a champ. My strong inclination is that it's drivetrain related.
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Old 03 May 2015, 20:02   #17
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Just looked... both my current Mercury 15 and the previous Mariner 15 used the standard prop which was 9" pitch.

Re the no load revving out of gear... it doesn't work how you are thinking. The water puts a load on the prop and therefore the engine so an outboard that is trying to push a boat that won't get on the plane properly will bog down well below it's maximum revs. Even misbehaving engines will rev to max in a no-load situation... it's the behaviour under load at lower revs in a real life on the water situation that's crucial to an outboard performing well.
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Old 03 May 2015, 20:04   #18
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First Time out with the Zodiac, Major Problems with the Engine...

I once had some issues with a brand new engine ,
1st one of the spark plugs had a broken ceramic insulator tip that looked ok gave a good spark it wasn't until a very close inspection after a change to the spare set of plugs that we noticed the insulter tip broken, it would start fine & run up to about 1/2 throttle then the power would fall , as soon as the throttle was backed off it ran fine again & would start with no problems .

Later we also looked at the float bowl on the carb that was full of particles of white sealant that had come from the overzealous person at the factory assembling the fuel pipe fittings on the tank that had passed through the primary filter .
Just a thought ?
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Old 03 May 2015, 20:09   #19
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Kestrel on your video around 2.30-2.40 were you altering the throttle to cause that drop and rise again in the revs?
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Old 03 May 2015, 20:48   #20
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Quote:
Originally Posted by Fenlander View Post
Just looked... both my current Mercury 15 and the previous Mariner 15 used the standard prop which was 9" pitch.

Re the no load revving out of gear... it doesn't work how you are thinking. The water puts a load on the prop and therefore the engine so an outboard that is trying to push a boat that won't get on the plane properly will bog down well below it's maximum revs. Even misbehaving engines will rev to max in a no-load situation... it's the behaviour under load at lower revs in a real life on the water situation that's crucial to an outboard performing well.
True. I didn't think of it that way. Truth be told I'm not sure at all what is causing the problem, I don't think my tank or line is defective but I'll bring the whole kit anyway.

To answer your question about the video, at 2:33 I brought it back down and advanced it back to full at 2:40. You can see the engine rev up on its own back to full thrust at 2:47. Throttle position didn't change during 2:40 to 3:00. You can see it change RPM spontaneously at the very start of the video, too. Anything above 5000 indicated is at WOT, though the actual RPM varies.

Is the starter motor supposed to whir and spin like a top as shown in the video? Sometimes after a failed restart it'd give me that (very disconcerting) noise of spinning long after having been disengaged.
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