1959 10hp Johnson Seahorse: idles well, no top end

As, the title says, I have a '59 Seahorse 10hp. Seems to run very well, it idles smooth. I even have it idling so slow and low it sounds like it will stall -but it doesn't. And I can run it for 2-3 hours non-stop, but, I could've sworn I was closer to 27mph last season.

The only thing that has happened is at the very start of this season I overheated the engine due to a failed thermostat [i had the boat in the water not 10 minutes, idling, then WOT, then white smoke within minutes]. White smoke came out of the cowling and the engine stalled. I replaced thermostat and then did all of the below.

The boat it is on is a 10'4" Achilles soft-bottom inflatable with just me [180], 3-6 gallons of fuel, and 30 lbs of gear/safety equipment. I've tried;

different angles on the transom
different seating positions [with a tiller extender]
hydro-stabilizers [from W-lm-rt]
different fuel
carb teardown
fuel pump teardown
I've pulled the head and seen no visible water leakage from the copper ring [don't have a compression tester] then torqued the head bolts slightly more than what it took to take off. No difference in power output
I've reversed and ground [to "square-ish"] the clutch dog [so that it doesn't jump out of gear anymore]
I SeaFoamed it and got an extra 2mph [from 15 mph to 17mph] via higher RPMs.

Now again, she idles/runs just fine, she just doesn't go very fast anymore.

Now that I think about it -at WOT she sounds, "tinny" a little high pitched, 90% throttle sounds "better", "warmer" with a slight reduction in speed.

Also on rough water, when I am bouncing over waves, the engine drops RPMs for a second, then comes back up.

Any ideas from you gurus?

Thanks!

ill13

Re: 1959 10hp Johnson Seahorse: idles well, no top end

check spark. might have to pull the flywheel.

need both cylinders firing.

i would suggest a new water impeller change and gear oil to check for signs of water in it. just for the heck of it too.

Re: 1959 10hp Johnson Seahorse: idles well, no top end

Both cylinders are firing. I pulled each of the plug wires off while running. It hurts to do that, but it's quick!

It has new coils in it, so spark is nice and blue.

Impeller is brand new. Water pump is fine, I can touch the head while it's running.


thanks!

ill13

Re: 1959 10hp Johnson Seahorse: idles well, no top end

Has it ever ran fast since you put the stabilizers on?

Re: 1959 10hp Johnson Seahorse: idles well, no top end

Overheating a motor is never good - might have hurt the rings, losing some compression.

Re: 1959 10hp Johnson Seahorse: idles well, no top end

did you check the gear oil? there is some resistance there if water is in it. make a difference prob not.


i forgot to aska bout compression? did you check that after you got the overheat?


only other thing i can think of is either you to prime the motor see if that helps while running or your carb needles are misadjusted.

Re: 1959 10hp Johnson Seahorse: idles well, no top end

Something I read on another thread here, about priming the bulb. The post I read said the bulb should get firm after priming. and that the float/needle assy. should stop the fuel flow -and not dump fuel out of the overflow [or where ever it was escaping]. I think this may be part of the problem

Thanks for the ideas!

ill13

Re: 1959 10hp Johnson Seahorse: idles well, no top end

Nope, float/needle assy. seems to work fine. Lower unit oil a little sparkly, but that's to be expected given how rounded the clutch dog is.

I don't have a compression tester, but it's beginning to look like that's the area I have to go to.


ill13