Re: How to do a compression test
ok, I'm one of those guys that asks the dumb questions. So why is it that you don't need to open the throttle??
OK one last time. A two stroke is different than a four stroke. The confusion comes because a four stroke needs an open throttle because the air is sucked into the cylinders. On a two stroke it is blown into the cylinders. Now read on before you jump at me.
On a two stroke engine, when the piston is on it's upstroke, air is drawn into the crankcase (thru the carburetor). On the downstroke, the air in the crankcase is compressed. When the piston reaches near the bottom of the stroke, it uncovers the intake and exhaust ports in the side of the cylinder. When that occurs, the compressed air in the crankcase blows through the intake ports, into the cylinder, and out the exhaust ports. When it is running, that flushing action is what clears the exhaust out of the cylinder and fills it with fresh fuel/air for the next stroke. True, there is a certain amount of back pressure from the exhaust when it is running.
But we are talking about checking compression at cranking speed, not running. When the piston is at the bottom of the stroke, the above flushing has occured. The air pressure within the cylinder is the same as atmospheric pressure outside. It makes absolutely no difference if the throttle is open or not, it is atmospheric pressure. As the piston continues on thru the compression stroke, first the ports are closed, then compression takes place. It is compressing from atmospheric to whatever the final result will be. That is what you are measuring--the difference between atmospheric and the final number.
One last, important thing: A two-stroke cannot run unless the above actions are taking place. It MUST draw in air to be compressed in the crankcase. It is true that the throttle restricts that air, but it does not block it off completely. If you look at the throttle plate, it has holes in it to admit some air, even if it is closed. That "some air" gets compressed in the crankcase and is blown into the cylinders, through the ports. Unlike a four-stroke where there would be a partial vacuum on the intake cycle, the two stroke never sees a vacuum in the cylinder. Again, it cannot run unless it has that compressed air in the crankcase. Even the "some air" from a closed throttle gets compressed in the crankcase and blown into the cylinder and out the exhaust, resulting in atmospheric pressure at that moment in time (cranking speed).
Hope this clears it up once and for all, but it won't.
