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Compression readings

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whelaniii

Regular Contributor
I have a 1996 v4 90hp Johnson

I have a 1996 v4 90hp Johnson can some one tell me what compresion generally runs for engine when new ?? also I see there is a different reading from port and starboard why is this??I am reading 95 psi for both cyl.on port side and 105 on other side
 
"John.... You're right in

"John.... You're right in the ballpark with that compression. The difference in the readings between banks? The factory reps say it's just the nature of the beast, something to do with the crankshaft design is the answer I've always received from them (dating back to about 1986 or so)."
 
Reverse the cranckshaft rotati

Reverse the cranckshaft rotation and it switches sides.
However if one of the new 60 degree it should have been slightly less difference.
 
As far as i am concerned on a

As far as i am concerned on a two stroke the compression test determines the following. The condition of the piston and rings and the cylinder wall above the exhaust ports. Nothing else enters the picture.
 
There is in principle no diffe

There is in principle no difference in what You may read out from a 2-stroke or 4-stroke.
Leaking valves in a 4-stroke is just replaced with leaking leaf-valves in a 2-stroke. But in both cases what one is after is mainly the ballance between the cylinders. What might cause a difference and how to 'read' it is another thing. What might be 'new' to some is that there in V-design 2-strokes might be a difference in cylinder bank reading due to crank-shaft rotation.
 
Leaking reed valves / leaf val

Leaking reed valves / leaf valves have nothing to do with cylinder compression!
 
So as the gas is escaping from

So as the gas is escaping from a cylinder as it is being compressed thru bad valves your saying that wouldn't affect your reading?????
 
If you look closely at a two s

If you look closely at a two stroke such as we talk about on this board you will observe the following. When the piston is going up in the cylinder( compression ) the reed valve MUST BE OPEN ( in the crankcase ) to let a fresh charge into the crankcase!!! So it means that leaking reeds have nothing to do with cylinder compression!
 
"You are a bit wrong again. Wh

"You are a bit wrong again. When the piston is going down, the sealing reed-valves will force all fuel/air mix from crank-case to piston top, if open- a lot will be sent out through carbs again - less air to be compressed at piston top - giving less compression reading. When piston then goes up again - reeds will open and balance out the vacum created in cranck case instead of sucking from combustion chamber (until intake port is closed). This 'positve' pressure is needed in the cylinder both to fill new air/fuel mix and to push out remaining exhaust. The effect on the reading will be less than a defective valve in a 4-stroke, but still part of the ballance difference between cylinder readings."
 
I used to play around with 2 s

I used to play around with 2 stroke motocycle engine and what I remeber is that when piston is a TDC it closes off both intake and exhaust ports( same as closing vales on four stroke) and that is when you would get compresion reading I think Not a student is right to say reed vales do not play a roll in compresion. I guess i will read up on reed vales from what I no about them they play a part if engine back fires wll read more on reed vales
 
"When doing a compression test

"When doing a compression test the reed valves do NOTHING at all.You could probably remove the reed valves or the back half of the engine for that matter, and still do a compression test on the cylinder."
 
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