Problem:  Crankcase blow-by or oil consumption

If you view the crankcase as a sealed unit with a check valve -- "Breather Valve" -- that only lets air out and no air in you have the basis of a 4 cycle air cooled engine.   When the piston comes down pressure would build up in the crankcase. This pressure is released out the breather.  When the piston goes back up it creates a vacuum in the crankcase, because the breather won't let any air back in.  If there was no blow-by from the combustion process, when the piston came back to bottom it would get to a balance.  No vacuum or pressure.  Since there is a little leakage past the rings, gaps etc. when the piston gets completely down, there may be a slight pressure that will go out the breather.  Since we are no longer allowed to vent this out to the air, it is vented into the air cleaner and any oil vapors that might be there are burned.

These crankcases operate with a normal average vacuum of 10 to 22 inches of water, measured with a water manometer.  If there is a leak into the crankcase, like a dipstick seal etc, the vacuum will pull air into the crankcase and it will have the same effect as cylinder blow by.  When the piston comes down the extra air will be discharged through the breather. As larger amounts of air are discharged, they carry oil vapor and mist. This can cause the engine to use significant amounts of oil and Smoke a lot.  The same thing happens if the breather is not working as it lets air back in the crankcase.  It used to be common for the old 19 cube Briggs 8 hp aluminum block engines to really smoke and burn a lot of oil.  Many were fixed by just putting on a new crankcase breather.  As they blow oil through them, they often get "gooked up" and don't function well.

The newer V-twins normally run lower crankcase vacuum because the pistons go up and down at different times compared to the opposed twins and as a result of this less air is displaced in the crankcase. One of the first tests we run on an engine that acts tired it the crankcase vacuum test, this plus compression and a cylinder leak-down test can tell you all about the condition of your engine. 

We typically use a dipstick cap with a hose barb in it, or a cone shaped rubber plug with a hose barb, or a hose barb in a plastic oil fill cap. You can make a manometer with a valve, a piece of clear plastic tubing and a piece of wood to hold it.  Take like a 1 x 4 about 3 ft long.  Stand it up and fasten one end of a clear plastic tube near the edge on the top corner. Next bring the tubing down to near the bottom.  Put a few nails in a semi circle or nail a jar lid to the face of the board at the bottom.  Route the tubing down the face of the board and around the jar lid or nails at the bottom and back up the other side to the top of the board and fasten.  Next put a valve in this line that came back up as what is left of the hose is what will connect to the engine.  You should now have a board with a piece of (like 1/4") hose that is a U about 30 inches high.  Next fasten a ruler or yard stick in the center of the U.  With the board fastened vertically and above the engine, determine where you will tap the crankcase.  If the unit has a vacuum operated fuel pump you can "tee" into the pulse line.  With the valve open and the U vertical put water into the open end (the side you started with) until it is about 1/2 way up in the U.  Now note what the ruler measures (water level) and note the reading.

Turn off the valve and connect the hose to the engine. Start the engine and open it to 1/2 throttle. Slowly open the valve. The water should rise in the leg of the U going to the engine and should go down in the leg that is open to the air. Note the difference in readings.  If the static reading was 18inches when the U was static and now one leg reads 26 inches and the other leg reads 10" you have 16 inches of crankcase vacuum. Now open the engine wide open and see what you have. If it is below 12" you need to check for cylinder blow by, or a crankcase leak or bad breather, NOTE Very tired engines may actually have continuous pressure in the crankcase and be blowing out the breather all the time.

CAUTION: Have the valve closed when you start the engine. If you don't the first down-stroke of the piston WILL blow all of your water out of the manometer and on the ceiling.

We use a gauge calibrated in inches of water, but this tube setup is as accurate as it gets. Gravity don't lie.

©2002, by Edens Ltd^™   All rights reserved.