Compression Test

[lall]

What is a good compression number for 1987 Force 120HP? I am buying a 1987 21ft Bayliner Trophy with this engine on it? I tested it and the motor read 60 across the four cylinders. <br /><br />THanks

 

[wilde1j]

Re: Compression Test

What procedure did you use? Warm up engine 1st? Readings are on the low side. Generally, should see > 100 psig all around. Tell us how you got readings.

 

[lall]

Re: Compression Test

I took the boat for a test run for about 1/2 hour. When I came back to the slip, I tested the compression.<br /><br />Thanks

 

[Lugnut36]

Re: Compression Test

Did you turn it over by hand while checking the compression or did you use the starter?

 

[lall]

Re: Compression Test

I used the key to turn it over. I give it a quick turn.

 

[dmessy]

Re: Compression Test

Cycle each tested cylinder at least 3 times on a warm engine to get a proper read. Should be around #120 each. No more that 10 percent varience between cylinders.<br />PS- Make sure the sparkplugs are in the wire cap and they are grounded to the block when you turn it over. Also , turn your lanyard safty switch to off position. Failure to do this will fry your ignition system!!!

 

[lall]

Re: Compression Test

dmessy, where is the Lanyard safety switch located? Also the engine prop was in water when I did the test and the Spark Plug wires were Disconnected. Should I have the spark plug wires plugged into the other spark plugs as I am doing test on one of the cylinders?<br /><br />Thanks

 

[dmessy]

Re: Compression Test

It should be on the control s or very close by. It is the switch that you are suppose to connect to the operator in case you go overboard. When it gets pulled out , the switch will kill the motor.<br />Just disconnect 1 wire at a time, pull the plug, reconnect to that plug, and put the plug in such a position that the metal on it touches the engine housing block securely. Naturally , it will be sparking so watch out for fuel fumes and keep a fire extinquisher very handy.<br />I've checked compression at dockside but you run the risk of dropping your tools in the drink or worse yet, getting your leg smashed by the boat when a wave comes along. A better method would be at home using muffs or better yet a makeshift test tank. Same deal, warm it up and turn it over a few times on each cylinder you check.

 

[lall]

Re: Compression Test

Actually, it was not that difficult because the motor lifted pretty high. Another question is that, Can I test the compression with the motor in the tilt position (if that is the right word)?<br />Do I keep the other spark plug wires connected while I am testing one of the cylinder?<br /><br />Thanks for the all the help.

 

[rlouns]

Re: Compression Test

I think what dmessy is trying to say is that while cranking the engine for a compression test on this engine you need to have all of the spark plugs out and stuck into their spark plug wires and laying against a ground so that they will spark as you turn the engine over and therefore decreasing the possibility of damaging your ignition system.<br />I have not heard this before so I am not saying this is correct I am just clarifying his statement.

 

[wilde1j]

Re: Compression Test

Like Robert said, compression test w/ ALL plugs out. Let us know what results you get.