Logo
FULL SITE MENU

Johnson 15HP no top end

D

David Lithgow

Guest
" I just bought an '76 Joh

" I just bought an '76 Johnson 15HP for a small duck skiff. It starts on the first or second pull and runs very smooth. The problem is that it won't push the skiff on to plane at WOT. I don't have a tach attached but I'd be surprised if it is getting to 3500 RPM. I guessed carb trouble and got a rebuild kit. After tearing it down I find no obvious problems in the carb. The high speed jet has a coating of varnish but both it and the riser tube are substantially open. I checked compression with the following results. 60-80-94 #1, 60-80-89 #2. Those are the readings with each pull of the starter rope. Is the compression too low? The points are clean and right on the .020 spec stamped on the housing. The prop is a 10x5 work boat prop which should have over-reved. I've got a 9x10 weedless but I'm not sure the problem is prop related. Any ideas or things to check out would be greatly appreciated. "
 
"David.... Check to see that t

"David.... Check to see that the throttle butterfly is just starting to open when the scribe mark on the cam is dead center with the carb roller, and also that the throttle butterfly is opened fully at the full throttle setting.

Spark plug recommendations within that year and horsepower rating can get somewhat confusing as a few changes were made in that area.

You should be using Champion UL81J plugs with the gap set to .030 .

Remove the s/plugs and check the spark. It should jump a 1/4" gap with a strong blue lightning like flame. Let us know what you find.

Joe
"
 
" Thanks Joe,

It really see

" Thanks Joe,

It really seemed like a fuel problem but now I'm not sure. I'll check that spark after work today.

I took the boat out Sunday afternoon and it ran better than it has. It was almost getting topped out but was surging a bit. I ran about 3 miles from the ramp then it sputtered and stalled and would not restart. I started the long paddle back and stopped a couple times to rest and see if I could get it fired again. I put in some fresh plugs and it started but would not run above idle. I idled about a mile then it quit again and it was armstrong power the rest of the way to the ramp. It's not overheating, maybe the coils are bad. Errrrrrrrrrgh "
 
" dave,
still sounds like a

" dave,
still sounds like a fuel problem, I would check the fuel filter, you may have a bugger in it. also check your high speed jet for top end. sometimes they gum up after sitting a while.

mark "
 
I would guess that the thrott

I would guess that the throttle meshing gears between the twist handle & the short shaft to the timing plate linkage has jumped a tooth or two. This will not allow you to twist the handle enough to get over 1/2 throttle.
 
Back
Top