1971 Merc 135 hp compression test

[rtpassini]

Its been many years since I've been on here. Just picked up a new boat and have some questions (gotta start remembering how all this stuff works again. haha)

As far as I know, its been sitting minimum a year. Before I start throwing money into it, I figured I would do a compression test. I've seen some conflicting info on how to do this.

1) pull all the spark plug wires
2)remove plug and crank motor
3) repeat?

Im assuming its important to make sure you have a good charged batter also.

And since its been sitting, should I spray some fogger in first?

Is that all there is to it? I saw stuff about removing all the plugs then testing. I've also seen you need to start it up and run it (not an option at this point)

 

[Bosunsmate]

Re: 1971 Merc 135 hp compression test

Ahoy there.
Pull the plugs and ground them well away from the sparkplug holes, or better still disconnect the ignition somewhere.
Warm is better but cold will give you an idea if a cylinder or more is way different from another cylinder.
Not sure about putting in fogging oil, i spray in some two stroke oil mix both in the cylinders and the carb if i know its being sitting for a while before i start any test you want those crank bearings nice and oiled

 

[Bosunsmate]

Re: 1971 Merc 135 hp compression test

In that age motor you wont be able to just pull out the lanyard line, so instead you could use a +ve from the battery onto the small terminal +ve on the starter solenoid with the key in the off position. Only crank for about 6revs max

 

[Lone Duck]

Re: 1971 Merc 135 hp compression test

Here is what I do. Assuming the motor is on your boat. I'd pull all the spark plugs , Tilt the motor up, pour an ounce of mix oil in each cyl. Then turn engine over a few times by hand. Then I pour a little fuel 1 ounce or less in each cyl. Turn engine over by hand. Then put the plugs in loosely without the wires on . Tilt the motor down and hit the starter for a few seconds . Then I pull the plugs and spin it with the starter again. Leave all plugs out and do your compression test on each cyl. Of course each guy that comes on after this will have a different way. And mine will be wrong. Just make sure that your plug wires are away from the hole when you are clearing the cyl of the fuel oil mix. Or jump the solenoid without turning on ignition.

 

[rtpassini]

Re: 1971 Merc 135 hp compression test

wouldnt fogging oil be the same as putting oil mix in?

also, theres some faint marker numbers written on the motor. it looks like compression numbers. 130-135. we'll see what the numbers show.

what mixture does this motor take. any recommended oil/octane?

 

[Bosunsmate]

Re: 1971 Merc 135 hp compression test

wouldnt fogging oil be the same as putting oil mix in?

also, theres some faint marker numbers written on the motor. it looks like compression numbers. 130-135. we'll see what the numbers show.

what mixture does this motor take. any recommended oil/octane?

It might be i have just never come across fogging oil before as i can boat pretty much year around

 

[Maxz695]

Re: 1971 Merc 135 hp compression test

OK I,m no master at triginamitry? But pull all the plugs. Get a spray bottle with fuel mix an insert into the cyinder. with a full chrged battery insert the plugs and keep them all within 1/4 of a ground screw plate plug hole etc.. (Use ground wires if you feel it nessecary due to leaking fuel). The engine will not start without the pluge installed. Turn the engine over several times free spin to get the oil mix to get into the ring groove. Now Take your comp gage and insert into cyl #1 take a reading. Repeat #2 and #3. If the readings are lower than you expect insert one plug at at time to turn it over and force the fuel mix into the ring groves. Then retest. If you have a stuck ring try using some sea foam and spray directly into the cylinder and insert the plug to get it into the rings let it sit and soak the retry testing. If no luck after tired of waiting rip it apart. JMO

 

[Lone Duck]

Re: 1971 Merc 135 hp compression test

wouldnt fogging oil be the same as putting oil mix in?

also, theres some faint marker numbers written on the motor. it looks like compression numbers. 130-135. we'll see what the numbers show.

what mixture does this motor take. any recommended oil/octane?

I put the oil & oil mix in to make absolutely sure that all crank and rod bearings have lubrication before you spin it over with starter. The rings will also be lubricated. your mix is 50:1

 

[rtpassini]

Re: 1971 Merc 135 hp compression test

sounds great. Thanks for the help guys.

Any oil recommendation? should I use 93 octane? (it really doesnt matter to me paying a bit more to help prevent issues)

Will it make a decent mess when I crank it over without the plugs in and the gas mix in there?

 

[Lone Duck]

Re: 1971 Merc 135 hp compression test

sounds great. Thanks for the help guys.

Any oil recommendation? should I use 93 octane? (it really doesnt matter to me paying a bit more to help prevent issues)

Will it make a decent mess when I crank it over without the plugs in and the gas mix in there?

A little, thats why you have the engine in the down position when cranking,it gets blown away from engine. OH! don't look in hole when cranking. Heh Heh!!

 

[Faztbullet]

Re: 1971 Merc 135 hp compression test

Before you get to carried away check for spark first as the ignition system on that motor aint cheap...

 

[rtpassini]

Re: 1971 Merc 135 hp compression test

will do.
Any specific way I should do this? Im assuming I'll need to get the oil and gas mixture in there so its not cranking dry while checking for spark correct?

 

[Lone Duck]

Re: 1971 Merc 135 hp compression test

will do.
Any specific way I should do this? Im assuming I'll need to get the oil and gas mixture in there so its not cranking dry while checking for spark correct?

YEP! lube it whichever way you choose before you spin with starter, ( takes 10 to 20 min) then do all your spark, compression etc checks .