78 25hp Johnson has kicked my butt.

[edstanley]

Hello All,
I've got a 78 Johnson 25hp that has kicked my butt. I have replaced the fuel pump, soaked the carb in an ultrasonic bath, rebuilt the carb with a new Sierra kit. I've got plenty of fuel and good blue spark at the plugs. I can get it to run on full choke only. I put a new low speed needle in it but it doesn't seem to matter where I tune the low speed. The carb is CLEAN, so I feel reasonably sure its not a trash or old gas problem. I've replaced the 2 vacuum lines on the block. I have checked the compression and I have better than 130psi on both cylinders. I was going to give up and take it to my boat mechanic but he says he's 3 weeks behind and doesn't have time for something that old. Does anyone here have any wisdom they would like to share?

 

[boobie]

Re: 78 25hp Johnson has kicked my butt.

When you had the carb off did you check the high speed jet in the float bowl for being clean ??

 

[Sandor27]

Re: 78 25hp Johnson has kicked my butt.

I had a similar problem once. Did you make 100% sure the main pickup tube in the carb is 100% clear? Did you blow carb cleaner through it and see? Mine turned out to be some deteriorated cork plugging this orifice. Carb cleaner soak did not touch it. Had to use a small copper wore strand to push ti out from the front (jet access screw removed ).

Also need to check that you have the donut/oring shaped gasket around the pickup tube as well.

having to run on choke means you are not balanced air/fuel. So you are probably blocked in some passage in the carb.

I assume you are using fresh fuel?

 

[edstanley]

Re: 78 25hp Johnson has kicked my butt.

I cleaned the carb with carb cleaner twice before I finally put it in the bath. I can see clearly through the high speed jet and it looks spotless. The pickup tube looks clean and I have run a piece of copper wire through it. My gas is fresh and the tank is clean. I have replaced all fuel lines and the primer bulb is less than a year old. I'm getting plenty of fuel to the carb, just not through it unless the choke is at full choke. I do seem to be getting a ton of smoke and black exhaust residue in the trash can that I have the motor sitting in. I read somewhere, that bad reed valves may cause this. I don't know much about reed valves. How does a motor run with bad reed valves?

 

[boobie]

Re: 78 25hp Johnson has kicked my butt.

When you installed the float bowl did you have the gasket around the high speed nozzle ??

 

[edstanley]

Re: 78 25hp Johnson has kicked my butt.

I slipped it down over the pickup tube so the housing around the pickup tube on the bowl would seal against it when tightened up.