Question on compression check

[standish]

I realize I sound like a complete idiot, but when I check compression I crank the engine as in, turn the key, right? Not by hand? I've never checked compression before and just want to be sure... I'm quite new to outboards.

Ground the plugs wires, remove plugs, insert compression gauge, give a couple quick (with the key) cranks?

This seemed like the right place since I have a 90hp 1990 force, but since it's not an engine specific question I apologize if this is misplaced.

 

[Bigprairie1]

Re: Question on compression check

I have the same motor/same year and here is what I did.
1st: Up at the dash pull/disconnet the cord on the deadmans switch. This will disable your ignition for the test.
2nd: Pull the plugs
3rd: depending on the type of compression tester you have (push in or screw in) insert the compression tester in the top plug hole. Crank no less than 2-3 seconds per cylinder with a good battery.
4th: Note the compression reading
5th: do the same on the remaining (2) holes.
Now: I do it all again just for general reference and entertainment purposes.
As a second test you can shoot a bit of 2 stroke oil in the cylinders and do a 'wet' test. You should find that the compression is a little higher.
Note: depending on the series of your motor in 1990 the top cylinder has approx. 10psi less compression than the bottom (2) cylinders. There is reference to this in the manual so you can check it out yourself. So don't be surprised if the top reading is slightly less.
Ideally the compression will all be (well?) over 100 psi. and within 10-15psi of one another. A great test would be 130-140psi top hole and 145-150 bottom cylinders.
Anyways....give it a whirl, so to speak.
BP

 

[Frank Acampora]

Re: Question on compression check

Remove all the plugs, ground the wires so you don't damage the ignition system, screw in the gauge and crank over the engine using the starting switch a minimum of 4 revolutions to allow compression to build. Throttle must be wide open--so be careful, don't have people or kids near it--the engine will be in gear and the prop will turn unless you block open the throttles by hand at the carbs.

OOPS! Engine will not crank in gear, however, you can jump the solenoid or as I said before, block open the throttles

 

[maxum247]

Re: Question on compression check

Your in the right place!

 

[Bigprairie1]

Re: Question on compression check

oh yeah, definitely follow Frank and Maxum's points....key!!! They know of what they speak!! (....or write more specifically).

BP

 

[standish]

Re: Question on compression check

Thanks you guys!

I'll give this a whirl tomorrow and be back with more questions after that I'm sure

 

[standish]

Re: Question on compression check

Well my compression came out rather poorly, looks like I'll need to have it rebuilt.
Top: 105
Middle: 55 (yikes!)
Bottom: 100

 

[maxum247]

Re: Question on compression check

Man! That's tough! What's the history of the motor and the reason for the compression check?

 

[standish]

Re: Question on compression check

I just bought the boat last month, 1990 bayliner capri 18ft. I bought it assuming the engine was bad, but hoping otherwise (owner said it was good but since I didn't have a chance to fire it up I just assumed it was worst-case).

Anyone use a send away rebuild service? I live in a smaller town with only one marine shop so I want to make sure it gets done properly.

 

[maxum247]

Re: Question on compression check

Well my compression came out rather poorly, looks like I'll need to have it rebuilt.
Top: 105
Middle: 55 (yikes!)
Bottom: 100

Something you can try!
Get a can of Quicksilver Power Tune Engine Cleaner from Mercury Dealer.
Treat engine following it's manufacturer's instructions.
Quicksilver Power Tune is designed to remove combustion chamber deposites and free stuck piston rings.
Retest the compression after the treatment. Good Luck!

 

[Bigprairie1]

Re: Question on compression check

Well my compression came out rather poorly, looks like I'll need to have it rebuilt.
Top: 105
Middle: 55 (yikes!)
Bottom: 100

These are not good numbers however are you dead sure you did the test properly?
ie: what kind of compression tester did use? (thread in?...was it in and sealing securely?)
did you spin the motor over enough?
You might want to borrow or rent a second compression tester and give it one more try to see if you get different/better readings. Try a press fit compression tester and get a buddy to help you. One cranks while the other keeps the tester firmly sealed over the hole.
If you haven't done a compression test lately it might be just a bit of incorrect technique or maybe an non-sealing gauge.
Anywhooo, give it some thought and be sure you did everything correctly with the right tools working correctly. I made this mistake once myself and was overjoyed to find out it was me...and not my motor that was in need.
BP

 

[standish]

Re: Question on compression check

I did it three times on each, and came out the same each time, marginal difference with wet test.

It's a screw in tester, brand new. It's whatever brand QuestAuto parts private labels.

I'll give it another go tomorrow, and see how it stands.

Offtopic, again, but does anyone know if the 18ft bayliner capri is rated for a higher horsepower engine? If I have to get it rebuilt, it might be worth spending a bit more for some extra kick, assuming the boat can handle it.