View Full Version : Boat motor problem

07-27-2006, 08:45 PM
I am having a problem with my Johnson 50hp motor. It idles fine, but when I get going around 10MPH the motor starts running like it has a fouled plug. It isn't a plug because I've changed them twice. I took it to a shop and they cleaned the carbs and said it was ready. I took it to the lake and had the same problem. I dropped the boat off and they worked on the carbs again and it is still not running right. While the boat has been at the shop, I have had 3 people tell me of having the same problem. They have told me their problem was a pinhole in the fuel line or pump and the motor sucked in too much air. I changed the fuel line and the pump and I'm still having the same problem. Does anybody have any suggestions?

Thanks for your help!

Mike Epperson
07-27-2006, 09:00 PM
First off, I am not a mechanic. But since you have tried most things on the fuel side, maybe it is electrical. Just my 2 cents.

Matt Smith
07-27-2006, 09:13 PM
What year is the motor?

Does it feel like you are losing a cylinder, is the motor surging, or what?

Jumping Cholla Jigs
07-27-2006, 09:51 PM
It sure sounds like a fuel problem. Clogged filter, pin hole sucking air or bad pump diaphram.

07-27-2006, 09:53 PM
I had a problem like that. I could not get full power. Found out one the the spark plug wires had a pin point hole in it.
Might be worth your time to check and see.

07-27-2006, 10:18 PM
That's about the speed where the points start bouncing when you have a Worn Upper main bearing. That's on an older engine with Points.
If you have one of the newer engines with the Solid state ignition, it could be the same thing because the Trigger coil might be sufferning from too much of a gap at times.

You'll need a buddy to run the boat, and the use of a timing light to figure this one out.
What you'll see (if I'm right) is a bouncing or staggering firing pulse.
The Light will come on in different places.
It would help if you could mark the spot on the flywheel with a bright chalk, where you expect the light to shine.
Then mark 10 Degrees before and after.

Then run the engine up to the staggering RPM. Watch the light.
If it's firing in the same place when it does fire, you have a Fuel problem.
But if the firing is randomly backwards and forwards of the correct timing pulst mark, you have the worn Bearing problem.

There could be other problems as well. I sure hope I'm wrong, for your sake.

07-28-2006, 06:39 AM
The motor is a 2000 model

Barnacle Bill
07-28-2006, 07:03 AM
Did you change the primer bulb? They are the first to go. Next, I would suspect a pinhole in the fuel pump diaphram like someone else mentioned.

Matt Smith
07-28-2006, 07:05 AM
The motor is a 2000 model

What about the other question? Losing a cylinder isn't usually a fuel issue.

07-28-2006, 08:57 AM
When you first take of, it runs great but after a few minutes, it starts running rough.

Hope this helps

Yes I did change the primer bulb. Yesterday I put a brand new fuel line and bulb on.

Barnacle Bill
07-28-2006, 09:10 AM
Get one of those cheap in-line spark checkers from your local auto parts store and check for spark when its happening. Electronics can break down after getting warmed up.

07-28-2006, 10:42 AM
Mine done the same thing but I found out the jet had worked itself out on the side of the carb. Ran fine at low speed but ran like it was only runnning on one cyc. after giving her the gas. There is a jet on the side of both carbs you can see from the side of the carbs.

speck chaser
07-28-2006, 11:17 AM
check the vent on the fuel tank

07-28-2006, 11:29 AM
This may sound ridiculous, but I've done it before. Make sure that the arrow on the primer bulb is pointed in the right direction. Also, does the primer bulb get good and tight? If the hose is on backwards, the bulb will just collapse.

07-28-2006, 12:09 PM
I would go with either a diagrapm problem, a primer bulb. But it could be a coil pack or a winding. But like IBNFSHN said first I would check the primer bulb and then go from there. When you find out what the problem was let me know. I'm a heavy equipment mechanic and like to hear what the problems were on engines. Just incase I run into that problem in the. Plus I want to be a boat mechanic when I retire from the military. They make great money.

Matt Smith
07-28-2006, 01:18 PM
I'm thinking it's a electrical problem. If your motor had coils, that would be the first thing I'd check. Since it doesn't, then I don't know. It's probably time to take it to a different mechanic.

07-28-2006, 02:12 PM
Next trip out when starting to give it gas. Hit the choke and see if that make it jump up on plane.

07-28-2006, 03:32 PM
sounds like a coil problem to me the engine will run good till the coil(s) heat up then they start breaking down had the same problem with mine changed the coils and it was fixed

07-28-2006, 04:12 PM
i had a 60hp merc that did the same thing....after endless trips to thje shop i gave up and bought a rebuilt 70hp merc.....it did the same thing.....on a full tank it ran good but anhy less than full it choked out....replaced bulb and lines....no help....ended up replacing the tank and now it runs great again....it was a small stress crack in the pickup tube inside the tank.........it's a shame i spent 900 bucks on the 70 when the 60 was a cheap fix

07-28-2006, 07:02 PM
Here is where you will find your answer.
This guy is fantastic!

Do a search at the top of the page or sign up and ask the Doc.


07-31-2006, 07:38 PM
The first thing I would do is a compression check.When the motor stalls out if it runs rough and jerks alot it is probally electrical.If it still runs smooth chances are it is a fuel problem.If you have a built in tank check the anti siphon valve in it. I hope this helps ya out....

08-01-2006, 10:33 AM
To eliminate fuel as the problem, I would take along a squirt bottle filled with the correct gas/oil mix. Run the engine with the cowl and carb breather off. When the engine starts running rough, squirt some of the mix into all carbs and see if this improves the running. If it does, you probably have a stuck needle valve or some other carb problem. If it doesn't help, it is electrical.

08-01-2006, 11:34 AM
coil is bad when it gits hot it will break down . check for cracks in coil . or go to this site.

08-01-2006, 03:16 PM
iboats.com also has a pretty extensive outboard repair forum.

08-02-2006, 07:50 PM
Thanks guys. I'm starting to think it's an electrical problem, probably in the coil. Since you have suggested this, it reminds me of a couple of years ago I had a similar problem in my old truck. When the coil got hot, it quit. This happened to me twice and after the second time I changed the coil and took the old one to be tested and come to find out the old coil was bad. I have not had this type of problem in the truck since and I'm starting to think this is my problem in the boat. I'm supposed to take a mechanic with me on the lake this Saturday to let him know what my problem is so hopefully he can get on it and get the motor fixed.

Thanks for all of your suggestions!

Matt Smith
08-03-2006, 06:35 AM
Turkeyfootnc, your motor doesn't have a coil as far as I can tell by looking at the schematics.

08-05-2006, 03:03 PM
I think the problem is solved. I had a mechanic drive my boat today and he noticed when the motor ran rough the tachometer stayed at 2000 RPM's. He assumed the rev limiter was acting up and looked in the motor and later figured out the sensor for the motor running hot and/or low oil was acting up. He disconnected the wire leading to the sensor and the motor ran great. He hooked the wire back up and the motor ran better than it had in a long time. I believe the motor is fixed! Thank you all for your suggestions.