1994 Johnson 15 hp problem - need help

Problem: no high speed, idle slow speed works fine. But when I throttle up it run normal for a short time then it just slows down. Its like it is starving for fuel or flooding out but it will stay running just at about 1/2 throttle even through the throttle is wide open. It does not mis fire.

What I have done so far:
1. New plugs
2. Carb rebuilt (float set according to repair manual)
3. Low speed idle set according to repair manual
4. Cleaned fuel pump screen
5. Dump fuel and remixed new fuel
6 stabilizer mixed in fuel

Could the gas tank be the problem? I took it out for a test run and when I got back to the dock for some reason I cracked the fuel tank cap and it let a lot of pressure escape. Could this be creating a vacuum at high speed.

Or could it possibly be some sort of electrical problem?

Please help, I'm at my wits end with this.
Jim

Could be a fuel pump going out. Try squeezing primer bulb while running wot. If that fixes it its the fuel pump.

Thank you for the response, I have come to the same conclusion and was planning on rebuilding the current one with a OEM rebuild kit. If this is not the fix it certainly won't hurt it any.

Could also be an air leak in a fuel line, if you haven't replaced them yet you might as well. Most auto parts stores will carry an "ethanol safe" fuel line, ethanol can break down the old ones if they're the originals.

I've experienced a bad primer bulb causing hard start and running conditions. I've also had a kinked fuel hose at the manifold and last but most unexpected was a bad vent on a BRAND NEW GAS TANK!

On some of the older models from the 70's and 80's, if you shifted before the outboard had throttled down or if the rpms were too high it's possible to throw off I believe the timing plate. I had it happen once and its also discussed in Leeroys ramblings. This could cause you to run at no more than half your rpms. You may have the next era of Johnson motors, but the problem sounds similar.

You might also be dropping a cylinder because of a bad coil. When you changed plugs did you check to make sure they were getting good spark?

Are you sure that it was pressure escaping from the tank or was it air going into the tank that you heard when you loosened the cap? It sure sounds like your tank is not venting. The next time you go out test the vent by leaving the cap loose when you are at full throttle if the engine stays at the proper speed then you know that the tank is not venting. I hope this helps you out.

dieselfixer said:

Are you sure that it was pressure escaping from the tank or was it air going into the tank that you heard when you loosened the cap? It sure sounds like your tank is not venting. The next time you go out test the vent by leaving the cap loose when you are at full throttle if the engine stays at the proper speed then you know that the tank is not venting. I hope this helps you out.

Click to expand...

[url=https://www.tinboats.net/forum/viewtopic.php?p=318000#p318000 said:

nomowork » Yesterday, 23:26[/url]"]I've experienced a bad primer bulb causing hard start and running conditions. I've also had a kinked fuel hose at the manifold and last but most unexpected was a bad vent on a BRAND NEW GAS TANK!

Click to expand...

That's what I'd look into. Systematically eliminate all variables in the complete fuel supply. Starting at the tank working to the carb. Inspect pick up hose inside the tank, vent cap, connections, primer bulb, ect. ect.

Thanks to everyone who posted. I have a lot of things to take a look at now all good suggestions which I will examine before I post an update around Tuesday of next week.

Could it be the cap on the OEM fuel tank not allowing it to breath? If so, can the cap be cleaned some how? All I see for replacement parts is a new cap at around $35.00 or the gasket. I'm not sure how the cap works can someone enlighten me on just how it breaths? Lastly, could this be resolved by cracking the cap while operating the boat?
Thanks -

When it does this does the squeeze bulb suck flat? If it does it could be the vent on the cap. I have never had one apart but I sure it could be done. Yes the cap can be run cracked open but it will leak gas and fumes

It has an OEM cap however I do not see a hole or opening on the top to let air in. On the underside I see a small hole. I have not stuck anything into the hole on the underside yet to see if it is clogged. I'm not sure how this thing breaths. Is the tank suppose to be under pressure when the boat is running and the cap is in place? It seems to me that it should not be under pressure and the fuel is pump out of the tank by the fuel pump. Do I understand this correctly?
Thanks -

I would say that you are correct. Most tanks are vented somehow and needs to draw air in as the gas is sucked out. Does your squeeze bulb suck flat when the motor stalls out? Does your tank have a guage in the cap? Try cracking the cap just a bit and try it, JUST REMEBER THAT GAS CAN LEAK OUT.Is it possible that someone changed the cap with one from another can not an OMC?

No, the cap says OMCright on it. The motor does not stall. It runs great at low idel but when I go to wide open throttle it operates like it should for about 50 - 75 yards then just loses power until its running atv1/2 throttle even through it is wide open. I have not had a chance to test it eith the tank cracked. I would like to un-clog the cap if it is clogged that is why I hoping someone on this site can tell me how it works. It's a plastic cap and plastic tank.
Thanks -

Two questions, is there a guage in the cap and does the cap have a vent you manually open in te center of the cap. On some caps without a guage there is a small hole in the cap and that is the vent

You may want to check the spark plugs. You may be running on only one cylinder. I had that happen when I had to have a helicoil replaced. I have heard that some of these 9.9/15's run surprisingly well on only one cylinder. This could account for a power loss.

First, let me say thank you to all the great people on this site with your help and suggestions I have fixed the problem. I rebuild the fuel pump and purchased a new vented gas cap. Not sure which or both of these fixed the problem but I did notice that the diafram i removed was consideably weaker than the one from OEM replacement kit. Anyway, after the fuel pump rebuild I took it out to the lake (in the rain) and ran it. It ran very well as a matter of fact it has never ran this good since I've owned it. So at the end of the day I did the following:
1. Installed a solenoid with a push button starter in the battery box - now the electric start is working.
2. Cleaned and rebuilt the carbarator. Set throttle to proper timing and adjusted the throttle cable to work properly.
3. Cleaned and rebuilt the fuel pump.
4. Installed new filter screen in the fuel pump.
5. New manually vented gas cap.
6. Drained and replaced the lube in the lower unit.

Now she is purring like a kitten. It's a nice little outboard. It will not get you across the lake real fast but it will get you there. Most important to me is that it is reliable. Since this is a fishing boat speed is not an issue.
Again, thanks to all for the help and support. Tin Boats website rocks!

Well, I thought the problem was fixed and I was wrong took it out fishing and was experiencing the same issue as before. I pulled the carburetor off today and removed the bowl to inspect the float and found small black specks inside the bowl big enough to mess with the jet on the float so I used some carburetor cleaner and sprayed the bowl and the jet around the carburetor to clean it out. I suspect it was coming from the fuel line so I replaced that with some new marine grade fuel line from the fuel pump to the carburator and put it all back together. Plan to take it out for a spin on Monday. Hope this finally will do it. I have taking this carburetor off so many times now I feel as though I could do it in my sleep

Took the boat out again today and it is experiencing the same problem. I'm not sure what to do or what could be wrong with it. Anybody have anymore ideas? I at my wits end with it. It runs great at slow speeds or up to 1/2 throttle but when I go wide open throttle it des not increase the speed at all. I need help Tin Boat people.