seemingly weak running '55 15 hp

[Philip_G]

posted in the prop forum too, because I think it might also be a prop issue, but I'll seek advice here too if I may.

'55 15hp I took possession of a year ago, I replaced all the lower unit seals, the impeller, and both coils and plugs, I left the plug wires alone.

Motor had sat for awhile but ran OK, a month or two later I rebuilt the carb.

Motor runs fine at full throttle, the throttle disc spins right up as far as it can so I don't see a way to adjust it for any more. I've set the high needle as best I can.


Problem is I'm only getting 10mph and about 3800rpm pushing a 14' tinny at 5800'MSL. Sounds strong, runs smooth, seems to run on both pistons, compression was in the high 70's before it ran, hopefully higher now that it has some time on it.

I also notice at trolling speed it seems like one cylinder is dropping out, it'll purr along beautifully and then it'll sort of cut out, run like crap, then a few seconds later or maybe after a throttle blip run normal again. No sneezing or anything, tweaking the low needle doesn't seem to help.

I get good spark on a spark tester, well over 1/4". Like I said the plugs were new last year and look fine. I'm running 24:1 mixture with pennzoil synthetic. I haven't brought myself to replacing those pesky plug wires, that's next I guess.

 

[jay_merrill]

Re: seemingly weak running '55 15 hp

I like to use an inductive timing light on a running motor, when I suspect intermittant ignition problems. You'll need a 12V battery in the boat to run the light, but this is an easy and effective test. Just tape the trigger on the light closed and lay it somewhere that allows you to glance at the light, while driving the boat. If the ignition is "breaking up," you'll notice it on the light - instead of a steady state flicker, you'll see the pauses in the flashes.

 

[lindy46]

Re: seemingly weak running '55 15 hp

When you rebuilt the carb, did you remove the welch plug on top of the carb and clean those orifices inside? That is the idle circuit - could be plugged if adjusting the needle is making no difference. Also, have you done a link and sync? The throttle butterfly should just start opening when the rubber follower hits the mark on the cam, and should be wide open when the throttle is fully advanced.

 

[Philip_G]

Re: seemingly weak running '55 15 hp

I like to use an inductive timing light on a running motor, when I suspect intermittant ignition problems. You'll need a 12V battery in the boat to run the light, but this is an easy and effective test. Just tape the trigger on the light closed and lay it somewhere that allows you to glance at the light, while driving the boat. If the ignition is "breaking up," you'll notice it on the light - instead of a steady state flicker, you'll see the pauses in the flashes.

I'll give it a shot, I think I have a light around here.

When you rebuilt the carb, did you remove the welch plug on top of the carb and clean those orifices inside? That is the idle circuit - could be plugged if adjusting the needle is making no difference. Also, have you done a link and sync? The throttle butterfly should just start opening when the rubber follower hits the mark on the cam, and should be wide open when the throttle is fully advanced.

Yes I did, but I can pull that plug I guess and try again. I can probably blast some cleaner through it.
No, I guess I'm not sure what you're asking. You mean the throttle cam wheel that opens the throttle?

 

[lindy46]

Re: seemingly weak running '55 15 hp

Yes

 

[Philip_G]

Re: seemingly weak running '55 15 hp

Hrm, thanks I'll give it a peek here in a few, how do you adjust the contact point though? Can you just bend the arm a little?

 

[lindy46]

Re: seemingly weak running '55 15 hp

Hrm, thanks I'll give it a peek here in a few, how do you adjust the contact point though? Can you just bend the arm a little?

Two bolts on underside of cam - loosen and move it in and out.

 

[Philip_G]

Re: seemingly weak running '55 15 hp

Two bolts on underside of cam - loosen and move it in and out.

hrm, I'll look around some more. It's maybe 1/4" off- beyond the mark. Thanks for the advice!

 

[Philip_G]

Re: seemingly weak running '55 15 hp

here's a pic of the plugs

 

[lindy46]

Re: seemingly weak running '55 15 hp

If adjusting the cam doesn't do it, I've also seen the link rod between the roller arm and the carb throttle arm set up wrong. there are two holes in each arm where the rod can connect. I seem to recall that the carb arm hole closest in is the right hookup point. Can't remember which one for the roller arm. But you can play with that and see which combination causes the butterfly to just start to open when it hits the cam mark, and causes the butterfly to be wide open at full throttle. I think this problem arises when someone removes the carb to clean/rebuild it, then forgets which hole the linkage rod went in.

 

[robert graham]

Re: seemingly weak running '55 15 hp

Lots of things it could be, but maybe check/clean carbon deposits from exhast ports and/or exhaust system. On an old motor that ran all those years on stright 30WT oil at some rich oil/gas mixture, carbon could be causing motor not to breath. Maybe carbon on piston crowns, rings, etc. I have a old 3HP outboard that gets carbon right in the exhaust port, but easy to scrape/clean out every year or so. Just a thought. Good Luck!

 

[Philip_G]

Re: seemingly weak running '55 15 hp

If adjusting the cam doesn't do it, I've also seen the link rod between the roller arm and the carb throttle arm set up wrong. there are two holes in each arm where the rod can connect. I seem to recall that the carb arm hole closest in is the right hookup point. Can't remember which one for the roller arm. But you can play with that and see which combination causes the butterfly to just start to open when it hits the cam mark, and causes the butterfly to be wide open at full throttle. I think this problem arises when someone removes the carb to clean/rebuild it, then forgets which hole the linkage rod went in.

I know exactly what you mean, I looked at those arms and it seemed to me moving to the further holes out would reduce my travel, I believe you're right it's in the closer of the two.

 

[Philip_G]

Re: seemingly weak running '55 15 hp

Lots of things it could be, but maybe check/clean carbon deposits from exhast ports and/or exhaust system. On an old motor that ran all those years on stright 30WT oil at some rich oil/gas mixture, carbon could be causing motor not to breath. Maybe carbon on piston crowns, rings, etc. I have a old 3HP outboard that gets carbon right in the exhaust port, but easy to scrape/clean out every year or so. Just a thought. Good Luck!

It could be, I don't even know how to get at the exhaust with the aquasonic crap on it though