Compression Test

[HighTrim]

Sorry all if this has already been gone over but i cant find it in a previous thread. What is the best way to kill the motor while performing a compression test? Kill the fuel or spark? I have a 1976 Johnson Seahorse V6 200hp. Also is there a safe way to decarb with seafoam in my driveway or should i be on the water. And finally when i change the spark plugs i was told to add a drop of oil in the chamber b4 inserting the plug, this wont foul the plug in any way? Ty and happy boating

 

[BoatBuoy]

Re: Compression Test

It's not supposed to be running when you do a compression test. Take all the plugs out and check one cylinder at a time.

A drop of oil in the cylinder before installing the plug isn't going to hurt anything, but it won't help much either.

 

[HighTrim]

Re: Compression Test

im sorry i meant kill it from firing up....you're still suppose to turn it over 4 times to test it no?

 

[BoatBuoy]

Re: Compression Test

If you pull all the plugs out, how's it gonna fire? Pull the lanyard out of the safety switch.

 

[HighTrim]

Re: Compression Test

I dont believe i have a lanyard on this motor...?

 

[mpriley]

Re: Compression Test

On the controls there should be a safety feature that requires a tab of sorts to be placed to hold down a button. This button must remain depressed for the motor to work. There is usually a lanyard attached to this tab that you should have clipped to you while you operate the boat. That way if you are thrown from the boat while it is under power, the motor will shut off.

Best of luck,
Mike

 

[Marc III]

Re: Compression Test

If you have no landyard, you should ground the plug wires to the block. Having the ignition system fire not connected to anything could damage the system. Get them clear of the sparkplug openings too.

 

[HighTrim]

Re: Compression Test

ty marc,

my plan was to change one plug at a time, ground it to the motor with alligator clips, keeping it away from the plug hole and disconnect the fuel line so that it doesnt fire up while i check the compression, does this sound like a good plan or no?

 

[HighTrim]

Re: Compression Test

I just went out and inspected the throttle controls and surrounding dashboard like you said but couldnt see any type of safety shut off of any kind. Ive never owned such a big motor b4, only 9.9's with the clip right on the motor then around my wrist. Ive also never owned such an old motor, this one being a 1976, could it be that they never had these safety features then, im only a novice boater and im unsure. ty all for ur help, ur always a great bunch

 

[HighTrim]

Re: Compression Test

sorry does anyone else have this style of motor?

 

[ezeke]

Re: Compression Test

If possible, run the engine to warm it and distribute the oil equally.​

Take all of the plugs out.​

Get the spark plug wires away from the plug openings to prevent firing of the fuel in the cylinders.​

Ground the black and yellow wire at the power pack if you want to. I never have, but wiill yield to the safety advocates.​

Be as consistent as possible in your method and write the results down by cylinder for reference now and in the future. after you have done each cylinder, do the sequence again; you may be surprised at the results.​

BTW, It makes no difference whatever if the throttle is open or closed. Use a screw-in type tester and make sure that the threaded part is not longer than the spark plug.​

 

[emdsapmgr]

Re: Compression Test

The lanyard-equipped control box did not come out till 1982. The service manual says nothing about damaging the ignition system during a compression test. Specifically, it says, to bring the engine to normal operating temp. Remove all spark plugs. Install compression tester in one hole. Open throttle to wide open. Crank at least 4 revolutions. Repeat for next 5 holes. Compression should be within 15 lbs of each other. (most use the 10% rule.) If your engine has the original (uncut) 643/644 heads and the powerhead is in good condition, I would expect the compression to be in the 120 lbs. range.

 

[HighTrim]

Re: Compression Test

ty both for ur detailed answers, they helped me out alot, and thats why i love this site, happy boating

 

[HighTrim]

Re: Compression Test

OK Guys,

I did the compression test with a screw in tester and changed the plugs. I pretty much got 105 across the board, a couple of them were 100, does this sound ok? Secondly, while i was waiting for it to warm up before i pulled the plugs, i went to test the trim and tilt and noticed it didnt work since the wires were cut 3 feet down the line. Theres a white black and green wire i believe, where do i run these to? Ty again for all ur help!

 

[bigbob_FTW]

Re: Compression Test

your compression check seems fine. all cylinders within 5 psi is good.

 

[emdsapmgr]

Re: Compression Test

What are the part numbers of the heads on the engine-that has a bearing on what the compression numbers should be. The compression when new should have been over 120, so yours is low. I would run a can of Bombardier Engine Tuner through the engine, then repeat the compression test. If it does not pick up any, it will still run fine with 100 lbs.