Compression check CANT be right.....

[pro-crastinator]

1958 Johnson AD-12 7.5hp - craigslist rescue.
Powerhead pulled and impeller replaced with oem. needed it.
Checked torque on head - nothing loose. Cleaned out dust bunnies from carb area.
Spark looked good. Tightened transon clamps on iboats forum designed motor "holder" (heavy and stable)
Surrounded bottom end in a bucket of water, put fuel in mityvac vessel and pressurized it with hand pump, fuel line to carb.
Pulled cord - and on third pull, it ran. It ran nicely and smoothed out in short order after fuel mix jets were adjusted.
Was not impressed with water volume coming out of exhaust but head stayed "comfortably warm" - so ok.
SHut her down and pulled spark plugs, screwed in compression gauge. The kind with threads and an O ring to seal.
Readings were 35 and 40. No way.
Motor ran like a swiss watch ( a smokey swiss watch)

When the power head was off, I looked at pistons thru exh port and it looked cleaner and relitavely scratch free.
All rings were intact and floating (not siezed in piston groove)

What am I doing wrong?

 

[GA_Boater]

What am I doing wrong?

Worrying about compression when she runs well.

Run her some more and recheck compression. Your numbers are almost within 10% and that's the most important thing. My old Merc shouldn't run with the low, but even numbers I got before the the first start in years.

 

[GA_Boater]

What am I doing wrong?

Worrying about compression when she runs well.

Run her some more and recheck compression. Your numbers are almost within 10% and that's the most important thing. My old Merc shouldn't run with the low, but even numbers I got before the the first start in years.

 

[Ibl0wstuffup]

I would rent another gauge from the auto parts store and compare. Yours might be leaking.

 

[F_R]

I agree, it shouldn't even be running @ 35-40. Well, they are supposed to be reliable. The head gasket may be blown between the cylinders, but if it is, why is it running?.

 

[boobie]

Agree, try another gauge.

 

[toddschubert]

I had the same problem with the gauge from auto Zone. Read around 40 on two different occasions with 2 different motors. I went to a local johnson dealer and begged and pleaded to use the mech.'s gauge in the parking lot. It read fine around 120.

This gauge was in a red plastic case but I forget brand. Try to find a different model gauge.

 

[Vic.S]

1958 Johnson AD-12 7.5hp - craigslist rescue.

SHut her down and pulled spark plugs, screwed in compression gauge. The kind with threads and an O ring to seal.
Readings were 35 and 40. No way.


What am I doing wrong?

You dont say how many times you pulled the engine over on the starter rope. It needs to be done several times to build up the reading on the gauge.

See the item re compression testing on the FAQs board http://forums.iboats.com/forum/engi...d-questions-faq/401081-compression-test-by-jb

 

[dazk14]

The "good" gauges will have 2 shraeder valves. Base of screw in hose, gauge end.

If the lower, spark plug end leaks, or gets loose, you'll get low readings as you've described....

start by snugging up the valves.

 

[pro-crastinator]

Will report back after this weekend. Thanks for the comments everyone.