1999 Johnson 50HP Crankcase Pressure

[niebe008]

I am wondering what the crankcase compression should be on this engine. I have searched all over including my Clymer manual and can't find it. My pressure goes from -1psig to +1psig. My engine will run to a certain extent if I mildly pulse my bulb, but eventually sputters to a stop. I believe my issue is fuel related, but can't determine if it is the fuel pump or the crankcase compression. Maybe even something else, but these two are main suspects. Any help or input is appreciated. Thanks

 

[Tim Frank]

Re: 1999 Johnson 50HP Crankcase Pressure

What is engine compression?

 

[niebe008]

Re: 1999 Johnson 50HP Crankcase Pressure

The engine compression on both cylinders is 120-130 psi.

 

[194269]

Re: 1999 Johnson 50HP Crankcase Pressure

When I worked at OMC and Mercury, we measured C/C pressures at different times of a engine development. It is not a high pressure and is the ratio of the piston at TDC and the piston a BDC. The range is 1.1:1 and 1.4:1, the higher the ratio the tighter the C/C. Production engines normaly run in the 1.15:1 and 1.2:1, you can't measure this because the measurement is done with a liquid. Any spelling mistakes are do to early AM, very little coffee and fat fingers.

 

[niebe008]

Re: 1999 Johnson 50HP Crankcase Pressure

Thanks for the info. Aren't there more variables that go into the c/c pressure than just the cylinder compression? Such as leaks in top and bottom seals, middle seal between c/c's and other sealing surfaces? Are you telling me there is no hard set c/c pressure difference which is needed to drive a). the fuel pump and b). intake of fuel air mixture at carbs. The clymer book only states it needs to be significant pressure difference? Whatever that means.

By the way, the c/c pressure was taken on the lower c/c on the limiter valve which goes to drive the air motor on the fuel pump.

 

[petryshyn]

Re: 1999 Johnson 50HP Crankcase Pressure

IMO if the engine performs fine when assisted with the squeeze ball, the crankcase integrity is not in question. Look to the OMS pump and/or its fuel supply....

 

[Tim Frank]

Re: 1999 Johnson 50HP Crankcase Pressure

Thanks for the info. Aren't there more variables that go into the c/c pressure than just the cylinder compression? Such as leaks in top and bottom seals, middle seal between c/c's and other sealing surfaces?

Of course there are more variables, such as RPM....but the first thing you should be troubleshooting is the fuel pump.

Are you telling me there is no hard set c/c pressure difference which is needed to drive a). the fuel pump and b). intake of fuel air mixture at carbs. The clymer book only states it needs to be significant pressure difference? Whatever that means.

It means that you need enough pressure oscillation to drive the fuel pump. It is not a precise thing, and as a diagnostic measure is not very meaningful.
Fuel pumps fail far more often than these motors fail to provide sufficient pressure differential.

Cross your fingers that it is the fuel pump, most issues that would cause a pressure problem will be major fixes.

 

[niebe008]

Re: 1999 Johnson 50HP Crankcase Pressure

Thanks for the help and advice. I will look at the fuel pump then. I will start by performing the tests outlined in the book to see if any of the components inside are bad. I will post again when I have results. Thanks again.