compression ?

putback

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I have an old 1976 4 HP Merc thats been sitting for 15+ yrs.. It has 70 lbs compression on #1 and 65 lbs on # 2 after soaking the cyls with WD 40. . Thats within the acceptable % of differance in the manual. Trouble is I can't find what it should be. Seems low to me. Any advice?
 

gm280

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I agree with Mi dickdown. Get it running and maybe even use Sea Foam and see if those numbers come up...
 

Texasmark

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You are low but maybe not considering what you said. However, once you get up into a reasonable range the big guns pay more attention to uniformity of the readings between cylinders than the actual value. Actual value can have variables including operator expertise, gauge errors, engine temp, stuck rings and such where the differential pressure eliminates a lot of the guess work. 10-15 psig max on my 90 and 120 or above is ok so said my maint manual. Sea Foam is available at WM and most auto stores for about $7 a pint can. Go to the archives and read up on the "decarb" process which not only decarbons a carboned up engine, it also loosens up stuck rings which can improve your compression bottom line.

Mark
 

gm280

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I agree with Mi dickdown. Get it running and maybe even use Sea Foam and see if those numbers come up...

OMG Please forgive me for butchering your name. I honestly just seen what I did 'til just now. Too later to even try to edit it and fix it... Please accept my apology! :facepalm: :stupid:
 

racerone

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What is the serial # of this 2 cylinder 4 hp motor made in 1976 ??????----A picture perhaps.
 

putback

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gm 280. never even noticed it 'till your response. Had Murphy & his law with me all day at work. Came home in a foul mood. Reread the post, got a chuckle & a new attitude. Thanks to you and duckdown the weekend looks much better! I also appreciate the mechanical advice. Thanks, Leigh
 

racerone

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That could be the 2 cylinder motor known as the " gnat " and not considered one of Mercury's best models.
 

Texasmark

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What is the serial # of this 2 cylinder 4 hp motor made in 1976 ??????----A picture perhaps.
Good question. I had a red band of that approximate era and it was a single 4.5 as I recall; may have been a 4. I didn't even think about how many cylinders he said it had....was following the question.
 

putback

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What is the serial # of this 2 cylinder 4 hp motor made in 1976 ??????----A picture perhaps.
Can't get a pic to post, "over size", Serial # is 9118446, made in Belgium. If it is a Gnat and they have a history of being unreliable give it to me straight. I won't waste my time, I have little tolerance for shoddy equipment.
 

bruceb58

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We had one of those engines. Junk! Don't spend too much time on it.
 

Texasmark

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The engine I had was sound mechanically and asthetically. I just got spoiled on OMC alternate twins in that hp range years ago and a single just won't get it for me.
 
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