1994 Mercury 200 hp compression check

[rfav21]

I'm looking at a boat with a 1994 Mercury Black Max 200 HP outboard. Can someone tell me how to do a proper compression test. Also what pressures are considered to be good numbers and what is the acceptable difference in psi from cylinder to cylinder. I've got a compression tester but want to know the proper procedure for this test. All help is appreciated.

 

[saumon]

Re: 1994 Mercury 200 hp compression check

Remove all plugs, flip the kill swith, open the throttle, screw the gauge in hole #1 and crank 3-4 sec. Repeat for all holes.

Anything between 100 and 140 is good but remember that actual reading is mostly gauge-dependant. The main thing you'll look for is consistency across the board, like no more than 10% variation.

 

[rfav21]

Re: 1994 Mercury 200 hp compression check

I'm not sure what you mean by kill switch, is that the same as a lanyard switch? I've also got a Johnson 150 and that has the lanyard, is Merc the same? Also should I warm the engine first? This engine was last run 3 years ago, winterized and not used since. Thanks.

 

[saumon]

Re: 1994 Mercury 200 hp compression check

I'm not sure what you mean by kill switch, is that the same as a lanyard switch? I've also got a Johnson 150 and that has the lanyard, is Merc the same? Also should I warm the engine first? This engine was last run 3 years ago, winterized and not used since. Thanks.

Yes, kill switch is the same as lanyard swith.

It's better to test them warm but it can also be done cold. You should, at least, put some oil in the cylinder before cranking it. Worth readind this:

http://forums.iboats.com/showthread.php?t=158086

 

[rfav21]

Re: 1994 Mercury 200 hp compression check

Saumon, thanks for the help. I did the compression check just as you siggested. All of the readings were taken with the engine cold. All of the readings on the 6 cylinders came in at 120 psi, plus or minus 2-3 psi. Do these look like acceptable numbers? And if the engine was warmed up would these numbers increase or decrease? Again, thanks for the help.
rfav21

 

[saumon]

Re: 1994 Mercury 200 hp compression check

Saumon, thanks for the help. I did the compression check just as you siggested. All of the readings were taken with the engine cold. All of the readings on the 6 cylinders came in at 120 psi, plus or minus 2-3 psi. Do these look like acceptable numbers? And if the engine was warmed up would these numbers increase or decrease? Again, thanks for the help.
rfav21

Yes, they were great. And it will increase when hot.

 

[rfav21]

Re: 1994 Mercury 200 hp compression check

Thanks for the help Saumon, much appreciated.