1986 johnson 90 hp seahorse..going to 5krpm full throttle ok ?

[johnson426]

Re: 1986 johnson 90 hp seahorse..going to 5krpm full throttle ok ?

Ok guys I found out my prop size is 13/75x15 and motor is trimmed all the way down(I have manual tilt) I tried 2 people on the boat or 4 people, but the rpm will go to 5200rpm. The only difference I see is with more people it takes longer for it to plain. let me know if anyone can help! thanks

 

[ezeke]

Re: 1986 johnson 90 hp seahorse..going to 5krpm full throttle ok ?

On most boats, trimming the motor all of the way down will place the A/V plate at a downward angle away from the boat. This will lift the stern and force the bow down, so that the boat plows.

Since you have only manual tilt, it would be preferable to set your thrust rod in such manner that will place the A/V plate on a plane very close to level with the planing surface of the boat. In this manner the motor will push the boat in a straight line, rather than lifting it or forcing it down.

 

[AEROCOOK]

Re: 1986 johnson 90 hp seahorse..going to 5krpm full throttle ok ?

Ok guys I found out my prop size is 13/75x15 and motor is trimmed all the way down(I have manual tilt) I tried 2 people on the boat or 4 people, but the rpm will go to 5200rpm. The only difference I see is with more people it takes longer for it to plain. let me know if anyone can help! thanks

A 15 pitch prop should allow that 90hp to rev way past 5200 unless the boat is really heavy or there are other issues with the motor it's self. Not sure what an 18 foot Manatee weighs but is it possibly waterlogged? To begin with, I think you should trim it out 1 hole at a time to see if the RPMS come up. Also, have you confirmed that the butterflys on your carbs are perfectly horizontal when the throttle lever it at full?

 

[johnson426]

Re: 1986 johnson 90 hp seahorse..going to 5krpm full throttle ok ?

When I rev it in neutral, it will go to like 6000 but under load it wount go past 5200. Can someone explain waterlogged ? Im not sure exactly how much it weights either, I tried to look in the paper work but no signs of weight. The motor seems to run strong and I did check the butterfly for the throttle, they open up all the way!

 

[ezeke]

Re: 1986 johnson 90 hp seahorse..going to 5krpm full throttle ok ?

It's not a good practice to run the motor higher than 1500 RPM with no load - could cause a run-away.

Waterlogged refers to the floatation material in the boat's hull. As the cell break down over time, they are able to hold water which can only be removed by removing the material. The saturated cells add weight to the boat which slows down the boat and disturbs the design balance.

 

[johnson426]

Re: 1986 johnson 90 hp seahorse..going to 5krpm full throttle ok ?

ok, I moved the tab up one up from all the way down and took it out and it still goes up to 5200rpm..Seems like trimming the motor or the load is making any difference..should I change the prop ?

 

[johnson426]

Re: 1986 johnson 90 hp seahorse..going to 5krpm full throttle ok ?

I was talking to couple of my buddys and they think my motor is under powered for the boat since on the badge of the boat it says maximun of 140hp and usally a boat thats rated for that much should have came with a johnson 120..is that correct ? Can I hurt the motor from having a under powered motor and only revving to 5000 rpm ? thanks for the help!

 

[ezeke]

Re: 1986 johnson 90 hp seahorse..going to 5krpm full throttle ok ?

First raising the motor is not the same as trimming the motor . When you raise it you have to move it up one set of holes on the transom bracket.

I ran manual tilts for over 30 years; The thrust rods were never closer to the transom than the second set of holes.

Generally, anything around 70% of the rated horsepower is adequate. You need to lower the pitch on the prop until you reach the target RPM so that you don't lug the engine; all you will lose is some top end (which is a lot cheaper than an engine).

 

[johnson426]

Re: 1986 johnson 90 hp seahorse..going to 5krpm full throttle ok ?

okk I see.my outboard is mounted on the boat on the second hole from the top. My motor is trimmed to the second hole from being closest to the transom. Should I trim it up one more tab or it is good ? Should I move the motor up or down ? or should I leave it the way it is ? thanks

 

[ezeke]

Re: 1986 johnson 90 hp seahorse..going to 5krpm full throttle ok ?

It depends on the angle of the transom relative to the planing surface on the bottom of the boat, but experiment at 3 holes out. If you get too much bow rise, put it back to 2.

If you put a stright edge on the bottom of the boat and extend the line past your A/V plate, you can see what the angle of the plate will be relative to the planing surface. It should be tilted very slightly upward in the direction of the boat.

 

[kenmyfam]

Re: 1986 johnson 90 hp seahorse..going to 5krpm full throttle ok ?

okk I see.my outboard is mounted on the boat on the second hole from the top. My motor is trimmed to the second hole from being closest to the transom. Should I trim it up one more tab or it is good ? Should I move the motor up or down ? or should I leave it the way it is ? thanks

How about a photo ????
Picture can be worth a thousand words

 

[johnson426]

Re: 1986 johnson 90 hp seahorse..going to 5krpm full throttle ok ?

ok thanks for all the help guys, im gonna try to take a picture tomorrow to show a better picture of what im talking about

 

[Chinewalker]

Re: 1986 johnson 90 hp seahorse..going to 5krpm full throttle ok ?

And, no, your boat should not be underpowered. I have a 90 Johnson on my 18-foot Starcraft and it flat out boogies - running about 35-37 mph. I have a 15-inch prop on mine and peak around 5700-5800.

 

[johnson426]

Re: 1986 johnson 90 hp seahorse..going to 5krpm full throttle ok ?

ok guys, I been going thru my paper work of my boat and I found out my boat weights about 2,000 pounds and its a 18,2 ft mantee cuddy cabin boat with a 1986 90hp johnson with a 13/75x15 prop and manual tilt. Im pretty sure thats not the same size motor it came with and the badge on the boat says max 140hp, so is this motor too small for my boat ? Only reason I ask that is because it only goes up to 5200 max on full throttle and thats where I would have to leave it, in order to plain.If I try to back it off couple hundred rpms to release some stress off the motor, the boat will start to stand up.If someone can help me out that would be great because I dont want to ruin the outboard because the boat is too big and reving it to its max. thanks!

 

[lmuss53]

Re: 1986 johnson 90 hp seahorse..going to 5krpm full throttle ok ?

It's a big boat for the motor and it's probably about right at the minimum HP for the boat, but if you get it running in the recommended range you won't hurt it. I would drop to about an 11 pitch prop if you can find one. You may have to get someone to repitch one for you. I would also drop the trim rod one hole closer to the transom, at least.

The Turning Point prop selector doesn't list Mantee Cuddy's but I used an 18 foot, 2000 pound boat with an 8 foot beam and 40 gallons of fuel with 300 extra pounds of gear and it suggests a 13. Giving the results your are getting now I would still try to find an 11 to try.

I would not continue to run this motor at WOT at 5000 rpm especially if it is struggling to get there and feels bogged down running there. You are asking for trouble doing that. A long time ago I took an 85 Johnny off a 17 foot deck boat and hung it on a 24 foot pontoon. Not knowing what the lugging effect would do I ran the boat at WOT at about 4500 to 5G's for 20 minutes or so and blew it up, just like that.

 

[Chinewalker]

Re: 1986 johnson 90 hp seahorse..going to 5krpm full throttle ok ?

Okay, now knowing the weight and that it's a cuddy, that changes things a bit. I would definitely go with a lower pitch prop. At 5200 you're almost there. Try a 13 and see where that puts you.

 

[kenmyfam]

Re: 1986 johnson 90 hp seahorse..going to 5krpm full throttle ok ?

Agree, downpitch.

 

[johnson426]

Re: 1986 johnson 90 hp seahorse..going to 5krpm full throttle ok ?

thanks for all the help guys but im new into boats, I just need to know which one of the number is the pitch..the first number ?

 

[ezeke]

Re: 1986 johnson 90 hp seahorse..going to 5krpm full throttle ok ?

The first number is the diameter, the second is the pitch.

 

[johnson426]

Re: 1986 johnson 90 hp seahorse..going to 5krpm full throttle ok ?

Ok thanks guys and just to make sure, so im looking for a 13/75x11 correct ? anywhere I can buy cheap and could exchange if i need a different size ? also how can i tell if my prop is damged ? thanks