90HP Mariner two-stroke 2001 not performing as it should? Speed is bad, gas is good

[GreedyMuffin]

Hi!

I recently tried my "new" boat and my "new" engine. Bought seperatly. A 2001 90HP mariner two-stroke, and a Alicraft 18 Norwind from the 1980.

The boats weight is only 500KG. The hull does not have a very strong V-shape, but is perfectly decent.

The 90HP 2-strokes from 1995-2017 is basically the same engine it seems like. Minor changes(?) and it seems to be a very strong 90HP, a very good engine indeed.

With 90HP under optimal situation, the 90HP would push the boat to around 35 knots, while I am struggling to reach 30.. People with 70 and 75HPs reach 30-31.

But on the other hand, my fuel usage is very low. After the two first hours of testing. Much calm and low RPMs, with some longer RPM runs, I only used 10L of my 25L tank (temporary).

I could easily sacrifice a few knots for a lower gas-consumption, but I am worried that my engine is trash?

The compression is okey I belive, 7.5 bar on each cylinder, so they are exactly the same. It was a dealer who tested for me before I bought the engine, and they said it was in good shape.

I am running a 13x19P Alu from Mercury at the moment. Not good RPMs, but not bad either. Around 5100-5200 with two people. I would like to try a 15x4 or a 17x3 prop soon as the boat is quite front-heavy.

Thank you! Hopefully you can help me fix the "issue" and hopefully, seventeen year old me do not have a busted engine.

Happy boating!

 

[flyingscott]

Is the throttle opening the carbs all the way?

 

[GreedyMuffin]

Hi!

I had to change gear wires from another boat (my old boat with 60 HP) due to the length. So I had to adjust the white screws. Will see tomorrow or later today.

I got two (Org) + Faria RPM-gauge. Could my gauge show a incorrect reading on the RPMs. or should I trust it? It came with the boat. So it have probably been with it all the time. Idles around 800 RPM.

Thank you for answering!

 

[flyingscott]

Your rpms are not bad 5500 is max for that motor. I would double check the tach. Are you checking the speed with a speedo or a GPS.

 

[GreedyMuffin]

GPS on a app on my smartphone (Samsung S6)

I can test with the other tach as well, but it is correct at idle at least.

 

[QBhoy]

Wee tiny bit down on compression but hey, there are a few variables when doing this....
If it helps any, I've just got rid of a wee project boat that had similar specs. Here is some comparable info.
It was a 16ft Fletcher (uk speedboat) with a 96' mercury 90hp, same engine I believe.
She had a black max 13x19" prop and ran about 5500 rpm (if tachometer was accurate) trimmed up and at about 44mph GPS running light

 

[GreedyMuffin]

Yeah. The compression worries me.. Should I be worried for long-term usage?

Does the way of testing affect the compression? The "low" compression really bothers me..

They are all the same, so I thought it would be okey.

Yeah, I am very familiar with the fletchers. Almost bough a 17 GTO a while back with a 50HP. Thought about selling the 50 and putting the 90 on it.

Just to have it said. On the first trips, the engine got the oil injection + 1.5/2% in the fuel since I was not certain if the OI worked, but now I do and I do not pre-mix anymore.

 

[GreedyMuffin]

Would something like a hefty dose of Seafoam help in my case?

 

[flyingscott]

Your compression is even on all 3 and it's not bad at 110. Try another gauge and look at your linkages. If your boat is front heavy that will slow you down. Is that boat fiberglass? If it is could it be waterlogged? What does it weigh?

 

[GreedyMuffin]

It is not waterlogged I believe. I've checked several places outside and inside. The fiber seems ti be hard and stiff. It is glassfiber. It is front-heavy without a engine installed, now with the engine installed, it seems fine.

The weight should be 500 KG + engine + fuel etc. 500KG dry-weight.

Yeah, I thought that the compression was not that bad? It was tested at a dealer-ship. I have a cheap Comp. tester.

Linkages? Do you mean inside the engine, and/or the gas/gear wires?

Thank you!

 

[flyingscott]

The throttle and timing linkages, make sure everything is moving correctly. Since you had to change cables make sure everything is operating correctly and all 3 carbs are opening as they should.

 

[GreedyMuffin]

Okey, I thought you meant that, that was why I wrote gas/gear wires. I will! Thank you all again for the help!

 

[GreedyMuffin]

The carbs are opening correctly. Could just barley move them a tiny bit. Nothing noticeable though.So I think we can clear them.

 

[merc850]

Have you tried trimming the engine out this would raise the bow.

 

[GreedyMuffin]

Yes. If I trim to much out, the engine's RPM goes up to 100-200, then I back it in a bit, and it deceases to the "normal". If I trim too much in, the bow goes down and water everywhere, poor handling etc.