Boat motor problem

[turkeyfootnc]

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]

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]

What year is the motor?

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

[Jumping Cholla Jigs]

It sure sounds like a fuel problem. Clogged filter, pin hole sucking air or bad pump diaphram.

[Swanie]

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.

[thudpucker]

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.

[turkeyfootnc]

Matt,
The motor is a 2000 model

[Barnacle Bill]

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]

Matt,
The motor is a 2000 model

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

[turkeyfootnc]

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.