Johnson 15 hp motor die's on full throttle

[arctic_dan]

Hey guys, Im new to the forum. I signed up to see if anyone can help me on my old school Johnson 15 hp outboard motor.

The problem: The motor will run and start fine but when I put it to full throttle it will die ad run erratic. I can pull the choke and it will get up and go like hell and run great but when I take the choke off the problem persists.

What Ive done: Ive taken it all apart and changed the pump gaskets,seals & diaphram. Cleaned the carb, changed gaskets/seals. From the gas tank all the way to the carb there is good flow when I pump the primer bulb.
I took it apart once again and realized that the airbox nipple that presses on the carb is worn right out and not sealing on the carburetor. Could this be the problem? At this point Im not sure what else to check and don't want to buy a new airbox if thats not the problem. Any help would be greatly appreciated!

 

[jbuote]

Just curious.. When you cleaned the carb, did you take out, clean and replace the high speed jet?
​Thinking that might still be clogged.

​Others will probably have other thoughts... I'm still pretty new, so keep that in mind as well.. LOL

 

[jimmbo]

Did you set the float correctly? Often missed when reassembling a carb is the gasket/seal that surrounds the nozzle where it passes through the carb body/fuel bowl

 

[arctic_dan]

First things first and thanks for the replies!. Yes I checked the main jet. took it out. and blew air threw it and was very clear and clean. Also blew air in all the air passages in the carb.
The floats I have also checked and adjusted. After putting the bowl back on I would blow air in the fuel intake nipple and would slowly rotate the carb to see if it opens and closes properly. Everything seems to be in excellent condition as far as that goes.
Like I mentioned in my first post If anyone knows if that air box nipple that fits in the carb what its purpose is and if it would do such a thing?
The motor has to be under load when it does this as well. If i just test it in my yard the motor will not act up. Only on the lake will it starve for fuel on top end.

 

[JJsays77]

I'm searching for the same answer! This problem is unsolvable I've found no one can figure this out and this motor has been around for many years :grumpy:

 

[oldboat1]

How many years? What motor do you have?

 

[arctic_dan]

I hear you jjthejetplane773, I've been trying to figure this out since last year with no luck. I still use it. Its just not very reliable. oldboat1, My motor is a 15 hp 1983 johnson motor. As I recall when I purchased it, it ran fine. When I took it out for the summer it was doing that. I wonder if i should put a bigger main jet? But when i think about it that wouldnt really fix my problem, It would just be a temporary fix until the prob got worse.

 

[oldboat1]

The choke knob goes in with the long slot pointed down, as I recall on my 9.9, and the choke linkage arm fits up through the knob stem -- bevelled edges in the opening, so the flap fully opens and closes when operating the knob. If it's in upside down, will guess the choke would not work properly -- probably will not fully open or fully close. Should give you some wonky operation.

That grommet is at the opening for the plastic slow speed carb adjustment knob, I think. It might be possible the carb would not draw properly if the grommet is messed up, but seems like a long shot.

guesses.... Don't think the main jet is an issue unless it's buggered up.

 

[arctic_dan]

The main jet is clean as can be. I guess my next step would be to buy a new airbox. Dang.

 

[oldboat1]

Boy, that seems like a stretch. Might try running without the airbox?

You are concentrating on the carb, but have you measured spark and done a compression test? Drop cylinder test?

 

[racerone]

Not an airbox problem !!----Run with a timing light and observe the flashing light when motor falters.--Run with the cowling off ?

 

[arctic_dan]

I have not checked the compression or spark plugs. If id put a timing light on it I would have to do that on water under load at WOT. May be hard to do. Why do you guys think its an electrical problem when the problem is fixed when I introduce the choke.? I guess ill start with new plugs and a compression test next and let you guys know.

 

[racerone]

That then screams of fuel problems.---Clean the carburetor.---Fuel pump mounted the correct way ?--Reed valves checked ?----Lower crankshaft seal problems ?-Bring it over I have about 15 / 20 of these motors for parts and it would be an easy fix.------Perhaps do some of the trouble shooting steps as suggested even if they do not make any sense to you.--Using the timing light is easy , you lay it on the floor of the boat or a seat and observe the light.-Simple trouble shooting seems to be a lost " art " in my opinion..

 

[scout-j-m]

Don't bother with the airbox. It makes no noticeable performance difference on or off.

 

[Crosbyman]

if electrcals are ok & not overheating due to poor cooling ....


""when I take the choke off the problem persists"" so it is starving


have u tried pumping the bulb when it is about to croak ... if it goes good it means it is starving on fuel ...

bad fuel pump ? restricted flow in hoses , ,,dirty carb ?

​ho.... you did open the vent ?!

 

[arctic_dan]

racerone to use the timing light i would need to check it when the problem appears and thats under load at wot. Checking at adle wont tell me anything. Ive have done much troubleshooting. Thats why Im here. To see what people with more experience can tell me. I work on snowmobile a lot but outboard motors I have never worked on. I thought about the crank seal but from what I've read everyone says carb and pump. Can an outboard motor suck in air from a crank seal? That's what im not sure about. I was thinking to try running it WOT and when the problem occurs to spray starter fluid or something combustable around the seal to see if it cures the problem due to a leaking seal. I also wasn't aware there were reed valves on this motor. but then again reed valves usually help bottom end and throttle response on a snowmobile that is.
scout-j-m Thank you for confirming this.
Crosbyman Last time i tried priming the bulb when it was doing this problem nothing happened. The bulb stayed relatively hard. When i take off the gas line that goes to the carb and pump the primer bulb there is no restriction. Im guessing that the carb bowl was full and closed by the needle. If the bowl if full theres no reason for it to bog down on WOT. It is venting yes i unscrew the gas cap to make sure.
Does anyone have diagram of the pump so I can go through it again and confirm?
Thanks for all the responses

 

[ondarvr]

With everything you've said so far, it points back to the carb, time to take it back off and make sure it's clean and the float is installed and adjusted correctly.

 

[Crosbyman]

sticky inlet needle ?? ... does it have a spring clip type to pull it open when the fuel is extracted by the carb vacum... maybe it should

​btw when did all these problems start anything special happened

http://www.leeroysramblings.com/Rebuilding_fuelpump.htm
http://www.leeroysramblings.com/rebuilding_1987_fuelpump.htm

​what is engine model and year
http://www.leeroysramblings.com/John...-fuel_pump.htm

 

[arctic_dan]

Thank you I will take my pump off and recheck this. Thank you. It only started doing this after a winter storage. Ive noticed these plugs on the carburetor in the link you posted. I was not sure what they were for. Maybe I will take them off and see if its clean behind there. Since I know what they are for now. By the way its a 1983 motor.

 

[Crosbyman]

the dime size plug on top covers the low speed drip chamber (3 very small holes you can see top side rear of the carb ) f and not likely to be your problem

that circuit feeds gas to the engine for iddling and at wot gas mostly comes from the H/S fuel jet

​you can read up on this stuff tsuff in the bible of oldies... your carb is not that different from older versions (same DNA )


btw do you or not havethe HS needle spring clip attached between the crab float and the HS needle


http://boatinfo.no/lib/johnson/manua...ohnson.html#/0