1984 omc 75 hp compression

[JoshKeller]

J75ecrd


Hey guys, just picked up a cheap 1984 johnson 75/40 jet off of a totaled boat. Motor starts and idles well in the test tank. Definitely running on all 3 cylinders, as tested by removing plug wires one at a time while running. My question, is that the compression is 140, 135, 115. Removing exhaust cover, and looking into bore shows little if any carbon. Would you examine why the bottom cylinder is lower, and look at rebuilding, or just run it as is? Thanks guys

[EMDSAPMGR]

Most techs on this board will insist that any prospective new engine owner take a compression reading before purchase. A sensible tactic. Any time the compression varies more than 10% between cyls, the powerhead is suspect. Purchasing this with those compression numbers mean you are ready to withstand the possible expense of an overhaul. While the low cyl may still contribute to the running of the engine, the engine won't have the same zip as it did when new. Hard to tell really why that compression is low from your description. I'd tear into it to see just what is going on with that cyl and the carb that feeds it. If you continue to run it, at some point in the future you could have a failure that could make the block and rotating assy useless for an overhaul.

[JoshKeller]

I bought it knowing full well it COULD need a rebuild, and negotiated the price accordingly ($500 for the whole thing), knowing full well i can make a profit on just the jet pump. What could cause the bottom to be 20 lbs lower than the top two? Every other motor ive had that has had a ring issue, drops the compression down at least by half.

[delawarerick]

Sticking ring bad head gasket how long did the motor sit before testing does it idle rough?

[JoshKeller]

6 months sitting time. It starts easily and idles perfect in the test barrel. Id love to take it to the river and get a wot rpm reading, but it is frozen over.

[delawarerick]

I would decarb it before I ran it hard I got one to raise up 14 lbs with decarb.

[Lil' Blue Rude]

Sounds like a good excuse for a 56 cube powerhead.:thumbsup:

[JoshKeller]

Wish i could find one!!

[JoshKeller]

Also, unrelated, the air box is cracked and has a hole in it - is it ok to run these motors without the air silencer cover? I wouldnt think that would cause it to run lean, as it still has the cowling on it restricting air flow.

[EMDSAPMGR]

You were smart to account for potential rebuilding in your purchase price. You could have a cracked ring, but that's not common on those 3 cyls. I'd pull the carb apart that feeds the low compresson cyl. Possible one of the jets is restricted. If fuel to that cyl runs lean-over time that can cause the cyl to run hot and eventually can affect the compression as the ring weakens. Fixing the carb restriction won't help the compression problem, but would explain some things. I'm also a fan of using the Bombardier Engine Tuner to clean/improve the compression slightly. A second application of it is not a bad thing.

[sharpeye Mike]

Could be sucking just enough water from the exhaust chest or base gasket to wash out the bottom cylinder.

[JoshKeller]

Alright, tearing into the motor and i think ive found the issue. I found a piece of reed in the exhaust housing. This motor has been apart, and recently. Whoever did the rebuild didnt meas with the head - its chunked out pretty good. I would think this is the issue with the lower compression. I used a caliper and verified each cylinder is coming at 2.98" bore, so i dont believe it was ever messed with - just a piston thrown in it. The bore looks in great shape. New head, new gasket and see what happens?

http://i772.photobucket.com/albums/yy5/joshkeller8402/Mobile%20Uploads/20160131_110925_zpsdxxncfyc.jpg (http://s772.photobucket.com/user/joshkeller8402/media/Mobile%20Uploads/20160131_110925_zpsdxxncfyc.jpg.html)

http://i772.photobucket.com/albums/yy5/joshkeller8402/Mobile%20Uploads/20160131_110930_zpsp08sraie.jpg (http://s772.photobucket.com/user/joshkeller8402/media/Mobile%20Uploads/20160131_110930_zpsp08sraie.jpg.html)

[flabum1017]

I would get an inside micrometer and check as much of the bore as you can with the piston in it......look for it being egg shaped. It's possible that cylinder ran lean and melted the piston and lean = high cylinder temps. I would also pull the carbs and make sure they are clean.........

[JoshKeller]

Would the head likely be the cause of the lower compression? Cylinder seems to be 2.98 over the entire hole.

[flabum1017]

Did you use a micrometer down inside where the piston rides? Using a caliber will only measure the very top of the hole where the rings don't touch. Am out of round hole will cause low compression. If the hole is straight and round, then a new head and let the new cylinder break in and then re check the compression.

[flabum1017]

Oh, and finding a piece of reed in the motor isn't good, another piece could have passed through and damaged the piston ring..... maybe removing that piston would be a good idea.

[JoshKeller]

Yes, i used a micrometer. Just wondering if i should just go ahead and split the case and rering and hone it while its mostly apart

[flabum1017]

yeah, it's not much more work to do that...... just keep the bearings with their respective pistons..... and go ahead and get a set of fiber reeds from Chris Carson

[Lil' Blue Rude]

You didn't buy back your old motor did you? lol http://www.screamandfly.com/showthread.php?298512-1984-johnson-75-hp-low-compression

[JoshKeller]

Nope. Unless its changed hands again before i got it and was put on another boat, and became a long transom jet lol. There is a bunch of those 3 cylinder jets around here. Sadly, its the same exact issue with this one... is this a common problem in this model, or am i snake bitten?

[moparbarn]

Josh, IF you might want a second opinion, or any help.....I'm in Winchester, Va. You're probably within 1/2 hr. of me. Been a mechanic all my life, ASE master tech for the last 20 some years. Been around boats since 1957, went to OMC school in 1969, so this stuff isn't new to me. Retired, all I have is time.

[JoshKeller]

Awesome thanks! Got a number i can reqch you to discuss?

[moparbarn]

PM inbound, Josh

[JoshKeller]

Alright guys, installed the new head, and turned the powerhead over on the bench using jumper cables. Should be a great running motor now?

Top..
http://i772.photobucket.com/albums/yy5/joshkeller8402/Mobile%20Uploads/20160204_205052_zpssf7pm4qs.jpg (http://s772.photobucket.com/user/joshkeller8402/media/Mobile%20Uploads/20160204_205052_zpssf7pm4qs.jpg.html)

Middle
http://i772.photobucket.com/albums/yy5/joshkeller8402/Mobile%20Uploads/20160204_205017_zpsnf3axkir.jpg (http://s772.photobucket.com/user/joshkeller8402/media/Mobile%20Uploads/20160204_205017_zpsnf3axkir.jpg.html)

Bottom

http://i772.photobucket.com/albums/yy5/joshkeller8402/Mobile%20Uploads/20160204_205052-1_zpsg30m56fo.jpg (http://s772.photobucket.com/user/joshkeller8402/media/Mobile%20Uploads/20160204_205052-1_zpsg30m56fo.jpg.html)

[FMP]

Good and even, what is the book psi for it? You could add 20psi and run premium.

[Lehrtech83]

A telescoping gauge will work better for what your measuring. And a leak down test may give more answers as far as the head goes