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Old 10-01-2013, 09:55 PM   #21
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Originally Posted by 1turbofocus View Post
I dont agree , I was always taught and most boks say the same is you turn the engine 4 revolutions for a 4 cycle and 2 revolutions for a 2 cycle to do a compression test , cranking the engine over and over and over in my opinion isnt the proper way to test compression , cranking till the pressure stops does not give you an accurate compression psi


In theory, both ways should create the same result. If there is a difference (based on cranking time/revolutions) it can be attributed to obstructions in the intake/exhaust.

But in the same regard, the cranking speed will make the same type of difference. Any harmonic tuning of the intake/exhaust will change the speed of the incoming/exiting air. Which in turn changes the potential cylinder filling and pressure seen on a gauge.
To relate this to something tangible and understandable, look at a dyno graph and specifically the Torque plot. For the most part, it's a direct reflection of Brake Mean Effective Pressure (which is what really matters and is derived from compression ratio).

Basically, #3 is low.
But that doesn't mean something is mechanically wrong. With higher miles and unknown maintenance, I'd suck some Seafoam through the intake to try and clean any gunk off the backside of the valves. Yes the Gasoline spraying over them should keep it clean, but it just doesn't.
I've also had luck with leaving a cylinder filled with transmission fluid or Kerosene for a day or 2. It the rings are stuck it will help loosen them and they will be able to grab better. Working it back and forth a bit by hand also helps. Just don't go hog wild filling a cylinder.

A "leak-down" test will help diagnose if it's a mechanical issue. If it's a valve you'll hear it leak through the intake/exhaust. If it's rings you'll hear it come out the PCV/breather system.
If it's the rings you may be able to loosen them by rocking the piston @ BDC.

And this may sound really odd and hack but, it may actually help to run a hotter spark plug in the cylinder with low compression. That may help burn the fuel more completely and the average fuel used will stabilize as will power output. Though playing with spark plug gaps can yield similar results.
But this is a case where you have to know how to play with spark plugs and read them rather well.
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