60 hp Johnson 1985 2cyl - low top speed?

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Foxman

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Hey all, new guy to the forum here and looking for a little advice. I have a 1985 Sea Nymph 171CC that I purchased last year that I have been fixing up. The motor is a 1985 Johnson 60hp 2 cylinder. So far I have rebuilt carbs, replaced fuel pump (non VRO) and replaced the fuel filter. Also new impeller. Compression test shows 120 psi in. Both cylinders. It starts/runs/idles fine, doesn’t blow smoke. With a fully loaded boat and me (estimating 1200 - 1400 lbs) the top speed I can hit is 28 mph. This is with 17” pitch prop. I’ve seen people post hitting 30+ with 40 and 50hp outboards. I looked at the throttle cables and adjusted properly- looks like the carb butterfly is not going 90 degrees at WOT, but adjusting the cables made no difference. Wondering what I’m missing. Tried finding a 19” pitch prop. For the 3.75” gearcase but they are not available.
Any ideas on where else to look for more top end out of this rig?

Appreciate any advice, and happy to be part of the Tin boat club!
 
Foxman said:
Hey all, new guy to the forum here and looking for a little advice. I have a 1985 Sea Nymph 171CC that I purchased last year that I have been fixing up. The motor is a 1985 Johnson 60hp 2 cylinder. So far I have rebuilt carbs, replaced fuel pump (non VRO) and replaced the fuel filter. Also new impeller. Compression test shows 120 psi in. Both cylinders. It starts/runs/idles fine, doesn’t blow smoke. With a fully loaded boat and me (estimating 1200 - 1400 lbs) the top speed I can hit is 28 mph. This is with 17” pitch prop. I’ve seen people post hitting 30+ with 40 and 50hp outboards. I looked at the throttle cables and adjusted properly- looks like the carb butterfly is not going 90 degrees at WOT, but adjusting the cables made no difference. Wondering what I’m missing. Tried finding a 19” pitch prop. For the 3.75” gearcase but they are not available.
Any ideas on where else to look for more top end out of this rig?

Appreciate any advice, and happy to be part of the Tin boat club!
Click to expand...

What peak RPM does the motor achieve?
That is, with the throttle fully advanced, how fast does it turn?

-W
 
Good question. I don’t have a tach. Remote does have the 3 wire plug for a tach though. Maybe I can find more that simply plugs into it... will research.
Thanks!
 
You'll need a tach to see if you can get more out of it.

I don't think you are too far off as is, you are probably only looking at 30mph, give or take, that's what most 16-17 foot tin boats with a 60-75hp outboard will run. Aluminum boats just aren't made for speed, it costs a lot of money in tooling to manufacture a hull that will perform like glass, which is easily molded for the best performance.

I have a 17' deep v, it would do 30 with a 60 and 33 with the 75 it has now. My buddy has a 17' mod v with a 70, it will do 35.

Play with weight distribution too, if you have batteries, try moving them to the front, or any other heavy gear. It can make a huge difference.

I don't think anyone is doing much over 30 with a 40 or 50 in a fishing boat. Maybe with a small runabout type of deal.
 
Thanks for the advice! Will rig up a tach to check how close to 5000-5500 I am. I do have 2 27 series (trolling) batteries under the center console to help with balance. My other thought was to run with the cover off in flat water and manually open the butterflies to 90 degrees to see if it makes a difference. (With help from an assistant ofcorse)
It’s not a speed boat, I get that. Just wanting to make sure I’m getting all I can out of the ol’ Johnny.
 
I would think you will find that you are over propped at the moment.
That engine needs to turn 5500 at the bare minimum. The 2-cylinder 60hp was not a powerhouse to begin with compared to the 3-cylinder.
Get a tach hooked up so we can see some reliable numbers to begin with. Lets go from there. Even a TinyTach will do the job.
 
As others have said, you're not too far off with your setup. We have an Alumacraft Pro 17 that goes 32 mph with an old Chrysler Force 50 on it that's bored .020 over and has had the exhaust ports cleaned up, polished and relieved. But to get it over 30 I had to install a CMC PT jack plate so I can ventilate the prop and get it to 5,700 rpm. In the end, it wasn't worth it to gain 4 mph because we rarely run it at full throttle anyway.
 
You may want to check that the cam follower bushing is not worn or missing as this will not allow butterfly to fully open. M.W.
 

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