1976 75hp johnson top cylinder wont suck

75-JOHNSON

Seaman
Joined
Jul 2, 2009
Messages
55
all three pistons have 120 psi compression. top cylinder wont suck very well. put some plastic wrap over carb ports and middle and bottom suck the plastic right in when you turn it over but top barely does anything. switched around reed valves and still no difference. any ideas?
 

F_R

Supreme Mariner
Joined
Jul 7, 2006
Messages
28,195
Re: 1976 75hp johnson top cylinder wont suck

all three pistons have 120 psi compression. top cylinder wont suck very well. put some plastic wrap over carb ports and middle and bottom suck the plastic right in when you turn it over but top barely does anything. switched around reed valves and still no difference. any ideas?

Throttle linkage sync????

OR, there is more to compression than the compression readings. Compression is affected by rings and cylinder walls and is the pressure above the rings. Two-strokes also have compression below the rings. The piston skirt is a big factor in that. What I am suggesting is a blown piston that still has the rings intact.
 

75-JOHNSON

Seaman
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Jul 2, 2009
Messages
55
Re: 1976 75hp johnson top cylinder wont suck

is there any way to remove and examine the piston without splitting the crank case? is it possible to remove the connecting rod bolts through the carb side of the engine?
 

red boat

Petty Officer 2nd Class
Joined
May 4, 2005
Messages
141
Re: 1976 75hp johnson top cylinder wont suck

Do a compression test. If you dont have a compression guage you can get a loaner from most auto parts (o'reillys, autozone etc) I think you will find that top cylinder is low on compression. Oops just reread all cylinders have 120psi compression. On some motors you can get to the connecting rod bolts without splitting the case but you wont be able to get it back together correctly without splitting the case.
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
Re: 1976 75hp johnson top cylinder wont suck

The crankcase seal in the bearing block on the top of the crankcase might be bad. This would prevent the crankcase for the top cylinder from inhaling/exhaling. The cylinder compression will still be fine since it can still pull air back through the exhaust port.
 

75-JOHNSON

Seaman
Joined
Jul 2, 2009
Messages
55
Re: 1976 75hp johnson top cylinder wont suck

thanks guys, i took it apart and the bottom ring was broken, however it did not cause any scaring or anything which is good news.
 
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