20 hp johnson problem

[knight hunter]

Today I put my 1976 Johnson 20 hp in a barrel of water to check it out B4 mounting it on my boat. it started good ( 2 pulls) It would run good on fast in neutral, but when I slowed it down to shift or slow, it would cough or stall. It almost seemed like it was getting a shot of air in the fuel line somewhere. I checked all the connections from the fuel tank to the carb, all were tight. Then I bought a new fuel line(from tank to motor). still have the same problem. Is it possible that I need a new tank( this one is original) or should I be looking towards the carb? fuel filter? Any idea's. I also mixed up fresh gas and added some Sea Foam B4 I started it. Thanks in advance for any replies. I also put in new plugs. my fuel tank is an OMC metal tank. When I close the tank and pump up the primer bulb, the tank is air tight, when i open the tank cap, there is a release of air pressure. By reading other posts, I get the impression that there is a valve in the cap. If that's the case, why is my tank air-tight?

 

[The-Machinist]

Re: 20 hp johnson problem

Are you sure that the vent in the center of the fillcap is screwed open? It could be vacume pressure you are hearing.
The dying issue can maybe be solved with a carb adjustment, If that doesn't solve it its probably time for a carb cleaning/rebuild.

 

[F_R]

Re: 20 hp johnson problem

Coughing or sneezing at idle is called a lean sneeze and is caused by running too lean. Before you go tearing into anything, give the low speed needle a tweak toward rich (counteclockwise). That is what it is there for.
(Most) OMC tanks are vented by a valve in the outlet housing where you connect the fuel hose. The two little pins that are pushed in when you connect the hose open the fuel outlet and open the air vent. The air vent is a one-way vent -- that means it lets air in but prevents air and fuel from being slopped out. That is why they can build pressure. In addition, due to the construction, the air vent valve requires a slight vacuum to pull it open. But the vacuum is less than the suction of the fuel pump so the pump can draw fuel out.
There is no user operated air vent in the gas cap on these models.

 

[tashasdaddy]

Re: 20 hp johnson problem

http://forums.iboats.com/showthread.php?t=167352