force 50 high speed miss

[savdxgrace]

I just aquired a '85 pontoon boat with a '85 Force 50hp. I had to replace the head gasket to get it running. It also got a new t-stat and brand new spark plugs. It seemed to run pretty good at the house with the water hose hooked up. The first time I put it in the water, it ran good for about 10 minutes or so and then seemed to develop a miss at higher RPM's. I stopped, checked the compression again after the gasket change, looked at the plugs and let it cool for awhile. When I went out again, after a few minutes the miss came back. It is not a constant dead miss, something seems to be breaking down when hot. The motor never died nor did it leave me stranded, just was not at full power. Once in awhile, it would pick up and run great for just a minute or two and then return to the underpowered condition. Where do I start???

 

[olsenk514]

Re: force 50 high speed miss

sounds like a possible fuel delivery problem. I had a similar problem with mine and I replaced the check valves in the fuel pump and got a new tank, line, and primer bulb. The motor was trying to draw more fuel when it was revving higher and the check valves were restricting the fuel because they were gummed up. The problem went away completely.

Also, just making sure but is the vent on your tank opened up? A closed vent will allow the engine to run until it cant pull anymore vacuum on the tank.

 

[moparman]

Re: force 50 high speed miss

And while your replacing the check valves, replace the pump diaphragm also. If this boat has been sitting awhile, you also need to replace the impeller if it hasn't been done in the past couple years.It sounds like a fuel delivery issue,but you also need to look at the plugs to make sure your not getting water back on them if the gasket it blowing again. Was the head flat, Did you torque it in the right order and correct torque value?Just some things to consider

 

[savdxgrace]

Re: force 50 high speed miss

When I first tried to crank the engine, it did not fire at all. I checked the compression and had 0 on the top and 110 on the bottom. I removed the head and found the blown gasket. I also found in taking the head off that there was no t-stat. I replaced the gasket, t-stat and t-stat grommet. I removed the lower unit and checked the impeller. I had purchased one but did not see any reason to comdemn the old one and put it back in. The engine fired up as soon as I applied the choke and it seemed to run well with the hose hooked up. I am getting good water flow from the engine.

As far as the fuel system goes, I did remove the carb bowl and clean it out before starting the engine. I only found what appeared to be oil in the bowl. I also used a fuel tank that has been empty and dry, stored in the garage for a couple of years. I flushed each hose and component of the fuel system with clean mixed fuel all the way to the carb before ever cranking or firing the engine. I took the cover off the fuel pump and checked the screen, it was okay. I did not secifically inspect the check valves. The vent on the tank was open.

The head did not show signs of warpage to any extent, although I did not put a straight edge on it. I did torque the head according to the manual in the proper order to the proper spec. I rechecked it after I had run the engine that afternoon and checked it one more time the next day. The plugs did not show signs of any water. I also rechecked the compression and it is still 115 on both cylinders, the same as right after the head gasket change.

How do you "spec" the coils? This engine has what I think are called "fat" coils, there are two of them.