compression 57 3 hp Johnson

[scubachuck]

Top cy 40 psi
Bottom cy 60 psi
took head off bore looks good
took exhaust plate off rings don’t look stuck

[garry-in-michigan]

If the head is not warped, I would say some one has been using one hundred to one oil mix in it . . .

• This reply was modified 4 years, 6 months ago by garry-in-michigan.
• This reply was modified 4 years, 6 months ago by garry-in-michigan.

[scubachuck]

ok i’ll check the head and no to the 100 to one

[crosbyman]

any problems at this stage…???.

why not just run it for a while and throw in some Mercury power tune (follow directions)

then retest compression after a few hours …who knows maybe #s will come up enough

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[frankr]

40 psi, if accurate, is seriously sick.

[billw]

60 is more like it, at least run-able. Why not try a new head gasket? Maybe the old one was leaking. If not, then she has to come apart, which isn’t a huge deal.

Long live American manufacturing!

[fleetwin]

Don’t be afraid to “deglaze” the cylinder walls using some fine emery cloth, this might help the rings seal and bring up the compression….These engines don’t have much compression, so 60 PSI isn’t that far out of the expected range. These engines have such small combustion chambers, compression readings tend to be inaccurate. You have the head off, no signs of scuffing/scoring, no signs of stuck rings/scuffed pistons with the exhaust cover off….Be sure the head/block surfaces are flat before installing the new head gasket…

[Mumbles]

Maybe try spinning it over with a drill at a steady cranking speed and see what the numbers are.

[crosbyman]

start simple and cheap…. run it with decarb and fresh gas/oil for an hour or two and report back ….the compression #s

Joining AOMCI has priviledges 🙂

[aquasonic]

With 40 PSI compression in one of the cylinders, I’m not sure if the motor will even run. My first thought is a blown cylinder head gasket. Is there any sign of a breach in the old gasket? Is there any sign of overheating such as burnt/discolored paint on the cylinder head.

A new head gasket is fairly inexpensive for this model, and easy to replace.. If you decide to replace it, then there is a very easy lapping procedure that will give a fresh, new, and flat mating surface. I make it a habit to always lap the cylinder head when installing a new gasket. Every one that I have ever lapped is slightly warped in a concave manner in the direction of the block. I’m hoping this is your solution.

Let us know what you decide to do.

[Author]

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