Basic trouble shooting

[thunder61golf]

Hi everyone, I have been running this little 16' aluminum boat for a few weeks now. It has been running pretty strong since i have had it, but i recently started really opening it up (more than 1/2 throttle). The issue is that it has only really gotten on plane once, took a few seconds (about 10) then i got it up to 28 MPH at 5200RPM, and dropped it to 24MPH 4800RPM. This was with fishing gear cooler, 10gal of gas, and just me in the boat. The very next weekend, i got out with same conditions, but add my son (100lbs) and the boat tops out at 14 and does not hit plane. In both cases the trim was set to the same location, same fuel mixture, same weather and conditions. I tried having him sit up front but that did not help. I brought it home after putting around fishing all day and checked it all out Carbs, plugs, plug wires, lower oil, fuel, fuel filter, exhaust port in lower is free of carbon build up. No new noises, or know issues that would have caused it to drop top speed.

Took it out again this morning got it up to about 20 mph to check and see if i spun my prop (both marks were lined up same as i left them). boat ran top speed of 20 with just me no gear 5000 RPMS. again no noises.

I am thinking it might be either an ignition problem (plug wires) or maybe a prop issue. Either way i have done a bit or trouble shooting, and i know that this boat should be getting up at least past 20mph no problem. tonight i am checking compression on all cylinders, and spark to all them, might buy a new set of plug wires.

SPECS for boat & motor:
older 16' side console mod v
1989 Mercury 50hp outboard (classic)
prop - 11.1"x13" solas
3/8 marine decking front and back
live well
under decking foam is dry (not water logged)
bimini top
new plugs, lower gear oil, recent carb cleaning on both carbs, new fuel filter.

any experts out there have any first hand experience with this let me know what you think i should be able to run with my set up, and or good troubleshooting advice i am all ears.

 

[skipper123]

You should be flying with the 50 on the back. The compression test may tell the story.

 

[Florida_Native]

I'm thinking its a issue with the prop. I have a 16' aluminum Jon boat and it's a wider one with a 25hp and I hit 30mph by myself. Actually had 5 people last weekend and still was able to plane.

If it starts bogging down past half throttle I could see it being and issue with the fuel pump if you have already checked the carbs and what not.


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[thunder61golf]

Compression on all 4 cylinders checked out(from top to bottom of power head): 94,93,92,92.

Next on the list will be fuel delivery. I was also thinking it might be due to weight distribution?


Does anyone else have a 16' aluminum with marine grade plywood decking and carpet, running a 40hp or 50 hp 2stroke?
What is a reasonable speed to achieve with this motor that seems to be in good condition?

 

[Pappy]

Why is this posted in two separate forums?

 

[thunder61golf]

Why is this posted in two separate forums?

It is both a motor question and a basic boat question. If it can't be you can take it down. Thanks

 

[FishinLite]

I have a '81 16' Lund with side console, floor, ... 60hp Merc.
With two adults and gear it pops up on plane and tops out about 33mph (gps).
Since you mentioned plug wires, I am thinking it is not running smoothly at high rpm.
My guess is that the high speed side of your stator is shot.
This is a common problem with older Mercs.

 

[thunder61golf]

Tried a larger pitch size prop, no luck
Lowered trim, moved motor closer to transom. No luck.
Switched fuel lines, boat ran 28mph wot with a bunch of bow raise, brought trim back to third hole"where it use to be" bow raise was minimal wot 33mph!

Thank you everyone

 

[mbweimar]

Two things:

First, that prop diameter seems a little small. I run a 40 ETEC with the factory prop. It's 13.5x15. Maybe try a larger diameter.

Second, get under the boat with a long straight edge. Hold it parallel with the keel, and make sure the hull is straight near the transom. A boat that sits on a trailer with bunks that don't extend past the transom, will develop a "hook" at the end. This can affect speed, and can also be difficult to fix. Here's an example.

 

[hsdtech]

Compression on all 4 cylinders checked out(from top to bottom of power head): 94,93,92,92.

Those numbers are low. Compression should be around 125.

 

[thunder61golf]

Runs an average 32 wot now. That's plenty fast for getting from honey hole to honey hole!
Thanks though.

 

[DaleH]

Runs an average 32 wot now. That's plenty fast for getting from honey hole to honey hole!

Sounds like you're in the ballpark, but also try this ... (Prop Pitch x RPMs) / (Gear Ratio x Constant of 1056) = Theoretical Speed

FWIW I had a similar hull with mid-90s vintage Merc 40hp (propped correctly) and she'd turn screws for 30mph. I swapped out the OB and put a 60hp on it (wearing a load prop) and she'll hit 40mph without tripping the rev limiter.

What is the rated maximum RPMs of that motor? I'd pitch the prop so you turned no less than 200 RPMs off.

 

[thunder61golf]

5500 I'm turning 5 flat now. I might try what said, but I am easily disrtracted when I hit water, my mind set is on fishing.

 

[wmk0002]

Compression on all 4 cylinders checked out(from top to bottom of power head): 94,93,92,92.

Those numbers are low. Compression should be around 125.

They aren't terrible. Anytime they all have only a small variance between them means your rings are wearing evenly like they should. The low numbers just mean it has a lot of hours and probably isn't putting out peak power. Still could probably use new rings sooner than later but only after the real issue is found.