Johnson 6hp Seahorse - low RPM at wide open throttle

[Jonnytutu]

Hi folks,

1st time post and my boat lingo is not too hip, so be patient.
I have a 6R 73s 6hp Johnson Seahorse (1973) that I bought a week ago, motor had not run in the last 3yrs. Gave it a carb clean, changed the spark plugs and adjusted the low speed nob/thing. Also changed the leg oil, checked the fuel pump, seems to work well.

Problem - the engine can only just get my boat (8ft inflatable rigid hull) up on a plane, takes far too long to plane, seems like almost 30-45 seconds. It also sounds like the rpm's aren't going high enough when the throttle is wide open......I guess overall it feels like the engine is not performing like it should. Prop is slightly dented but doesn't seem

So, what do I need to look at in the engine?

I have tried pumping the gas bulb thing when running - doesn't do anything.

Not sure what to do next......

oh and also I have checked the highspeed needle valve that is set in the float in the carb, looks clean.

Thanks for any help and/or ideas....

 

[djboat100]

Re: Johnson 6hp Seahorse - low RPM at wide open throttle

How heavy is your boat (including yourself and gear), and how are u distributing your weight? A 6hp will only plane the lightest of boats, and only with a lot of the weight forward.

 

[jrs_diesel]

Re: Johnson 6hp Seahorse - low RPM at wide open throttle

I have planed my 9 ft inflatable with a 3.5 HP, wasn't very fast but it did it nonetheless. My 9 ft boat has an inflatable keel. Your 6 should not have much of a problem pushing an 8 ft boat. Here's a few things to check:

Motor height - Make sure the anti ventilation plate (the flat plate just above the prop) is even with the keel of your boat. If it's too low, you are dragging more of your motor through the water than you need to. This should not be too bad of a problem. You'll lose a little speed, and splash the back of the boat and motor with spray. The only way to fix this is to built a transom lift.

Motor angle - Try different trim angles. Tilt the motor up and adjust the trim angle with the adjuster that sits between the transom clamps. There should be four trim angles you can use on the 6 HP. There is a small rod poking through the clamp on the left hand (looking at the motor from the front) side of the clamp that you lift up on and move into another slot. When I had a 6 on my 9ft boat, trim angle was critical to planing quickly. If I had it too far in (low) if took a lot longer to plane.

 

[kbait]

Re: Johnson 6hp Seahorse - low RPM at wide open throttle

Pull one plug wire at a time when at high idle. It should run on one cylinder.. just slow down when one plug wire is pulled. If it kills immediately when you pull one of the wires, then the still-connected wire's cylinder isn't firing and you can investigate further.

 

[Jonnytutu]

Re: Johnson 6hp Seahorse - low RPM at wide open throttle

Hey Folks,

I have tried the pulling the plugs at high idle and it still runs fine so the cylinders seem to be good. But as for the angle of the engine....I have not tried that and I am kicking myself, a no-brainer, thanks for that heads up, will try tonight.

 

[Jonnytutu]

Re: Johnson 6hp Seahorse - low RPM at wide open throttle

Well, that did make a difference, but still slow to plane. Moving it up one helped. Playing with the motor angle - if i angle the motor then the thrust angle starts to change. won't the hurt performance at the same time? Maybe i need to raise the transom...

 

[Vic.S]

Re: Johnson 6hp Seahorse - low RPM at wide open throttle

Maybe i need to raise the transom...

Take care if you do this. These older engines were designed to run with the prop more deeply immersed than modern ones. Really a hang over from the days when boats were wooden and had keels.
The water pump is located in the top of the gearcase assembly, well above the level of the AV plate. If you raise the transom height too much you may starve the engine of cooling water.

There was a very similar question on here only a few days ago.

Check that the armature plate is moving to its full extent and that the throttle valve in the carb is fully open when the tiller twist grip is turned to the max throttle position.
The two pats of the throttle shaft in the tiller can become incorrectly meshed so also worth a check perhaps

 

[Tim Frank]

Re: Johnson 6hp Seahorse - low RPM at wide open throttle

How heavy is your boat (including yourself and gear), and how are u distributing your weight? A 6hp will only plane the lightest of boats, and only with a lot of the weight forward.

^^^^ I guess we aren't going to get that answer.

I am not sure what "slightly dented prop" means, but it doesn't take much on a small prop to throw it off.
Expectations sometjmes trump reality.
Have you ever had a 6 hp on that boat that gave you the performance that you are hoping for?

 

[Jonnytutu]

Re: Johnson 6hp Seahorse - low RPM at wide open throttle

Thanks for the help folks....

Ya sorry re-weight of the boat, it is about 70lbs, me 160, gas 20, so ya the boat would have just under 200lbs in it. As for distributing the weight I put everything up front including myself to try and get over the bow wave to plane easier. As for the prop, i am not really well versed in the world of props, but it does have some dings, maybe i should file them down. And no I've never had any experience with inflatables and a 6ho, this is my first go with this engine and boat....maybe i should borrow a different engine to make some kind of comparison.

as for checking the throttle valve - i did, no more room to play unfortunately, i was hoping that would be the cure. And yes, in regards to raising the engine i don't want to starve the engine of cooling water, hmmm, I will continue to play with the angle.

One other question, the engine has a two blade prop on it, would a 3 blade prop produce more power?

 

[djboat100]

Re: Johnson 6hp Seahorse - low RPM at wide open throttle

Should plane just fine with that little weight. The standard prop should be fine, I wouldn't mess with it--it's designed to push a boat about the size of yours. Just make sure that the previous owner didn't install a different prop.. although that's unlikely as the problem is the reverse as one would expect--if they installed a prop to push a larger boat, it would be overreving on your ultra light setup.