Johnson 50hp not going above 4000rpm

I have a 1971 Johnson 50E that I purchased a few months ago from craigslist.

Previous owner said it came off of a 1978 Starcraft 16' Fisherman.

I have it on a 1978 Starcraft Super Sport. Same hull.

He swore it would push that boat 36mph and that he'd clocked it using an app on his phone.

My fishfinder has GPS speed on it, and the max I've seen is 25mph, but with the motor trimmed way up. Since the motor has manual trim, this gives me an absolutely horrible hole shot. It took 200 yards or more to get up to speed and hung around 12mph for what felt like forever before speeding up.

It came with a 17 pitch 13.25" prop.

I could tell by ear that the motor wasn't going very high in the RPM range. The plate on it says 5500 rpm is the max.

I put a 15x13.5" prop on it to try to increase RPMs. Got a much better hole shot, but still topped out at 24mph after experimenting with various trim positions.

Today I put the 17x13.25 prop back on to try a few different trim settings with it. I also bought a tiny tach and installed it so I could actually see my RPMs.

With the motor in the 2nd from the bottom trim position, I'm maxing out at 21.5mph and 4000 rpm. This trim position seems to give me the best hole shot to top speed ratio.

It sounds like its running well at 4000rpms, but obviously I'd like to see that additional 1500 rpms and some more speed.

I'm running fresh 50 to 1 gas with TCW-3 oil. I installed a new fuel filter on it.



My cavitation plate (with a doel fin) is roughly flush with the bottom of the boat.

Any thoughts on what could be causing the low RPM at WOT?

I measured from the spark plug wire to ground on each cylinder and it was around 1950 ohms. I understand that this measures the "secondary" on the ignition coil.

Measured from orange wire in power pack to ground wire coming out of coil and got 4 ohms and then it dropped to 0 after the tester was on it for a second or two, for both coils. Do I need to disconnect these from the engine to test?

I've also replaced all of the fuel hoses under the cowling with new hose and new clamps. My last tank of gas I ran through with 2 ounces per gallon of sea foam.

IMHO there’s no way that 50hp pushed that hull to that speed as the maximum theoretical speed of that motor wearing a 17” pitch prop is 33mph. And no boat ever achieves the theoretical speed.

Sounds like the 17” pitch prop is fine, but from what I’m hearing, rather than chasing things, I think you’ll better served in the long run to have someone check it out.

DaleH said:

IMHO there’s no way that 50hp pushed that hull to that speed as the maximum theoretical speed of that motor wearing a 17” pitch prop is 33mph. And no boat ever achieves the theoretical speed.

Sounds like the 17” pitch prop is fine, but from what I’m hearing, rather than chasing things, I think you’ll better served in the long run to have someone check it out.

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Ok, I had a feeling he was full of BS, because I talked to a guy at the boat ramp last week who had a very similar boat but it was a sea nymph and he had an 85 horse motor and he said it maxed around 35.

However, still doesn't explain the RPMs maxing at 4k.

I think I'll try putting the 15 pitch back on and seeing what the RPMs do, I don't want to lug the engine with too much pitch. It seemed like it performed a lot better with the 15 pitch prop.

What motor did you have on your boat before and how did it perform?
Did you use a doilfin before, is your boat heavy?


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Zum said:

What motor did you have on your boat before and how did it perform?
Did you use a doilfin before, is your boat heavy?


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I bought the boat without a motor, this is the first motor I've put on it.

Boat and trailer weigh 1600lbs, I had to get it weighed to get the trailer tagged.

I installed the doel fin because the bow came WAY up on acceleration and the fin really helped with that.

lugoismad said:

Zum said:

What motor did you have on your boat before and how did it perform?
Did you use a doilfin before, is your boat heavy?


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I bought the boat without a motor, this is the first motor I've put on it.

Boat and trailer weigh 1600lbs, I had to get it weighed to get the trailer tagged.

I installed the doel fin because the bow came WAY up on acceleration and the fin really helped with that.

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If the bow is coming way up that is an indication you have your motor trim setting too high.

Did a quick search and Nada says the boat should weigh approx. 650lbs.
Couple other searches and I think it's rated for a 85 or 90hp.
There's lots of info on these boats, some right here on tinboats.(actually 1 guy had the same motor)
I'm not a fan of a doilfin but some love them.... i wouldn't think the doilfin would cause a 1500 rpm drop but if it drags never know. Is your trailer heavy?..you said 1600lbs together...

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Zum said:

Did a quick search and Nada says the boat should weigh approx. 650lbs.
Couple other searches and I think it's rated for a 85 or 90hp.
There's lots of info on these boats, some right here on tinboats.(actually 1 guy had the same motor)
I'm not a fan of a doilfin but some love them.... i wouldn't think the doilfin would cause a 1500 rpm drop but if it drags never know. Is your trailer heavy?..you said 1600lbs together...

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Its a steel trailer, so I wouldn't be surprised it weights 900-1000lbs. I looked down in my bilge and the foam doesn't appear to be soaked or anything.

Make sure your tach is set for a twin cylinder and not 3. The 17" would be right if you are running in the low 30's if not put the 15 on. Sounds to me like weight may be the problem. My 48hp (basically the same motor as you have) is a on super light rig and can turn a 17" to 33 mph and a 19" to 34 on a cold day when solo. Add summer heat wife and dog and its 30 mph at 5400. That's with the 19. I need an 18 to be perfect. You can also scream that motor to 6500. On a heavy boat I'd prop it to pull over 6000 on a cold day and at least 5500 in the summer. They make gobs of power at high rpm and can run forever. Weight, weight distribution and trim setting is how you get good numbers. It takes practice but most on here can make a 25 mph boat run 28-29. Most can also add fat chicks, batteries, trolling motors, casting decks, bait wells and China bass bling and make a 28 mph boat run 21 just as easy.

I didn't notice if anyone suggested checking the throttle linkage to make sure it going all the open. Could be cable adjustment is off, or a problem in the engine linkage.

John

I have a 48hp spl johnson on almost the same size hull and weight. So VERY similar to what you have and I get 30 mph tops with 2 people and fishing gear.
If mine I would check the bowls and high speed jets.
Tim