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    I am well along building the Howell V2 and wondered if anyone could tell me how the PCV check valve is supposed to work per Jerry's design. I have the parts made but it is not obvious to me how they function.
    Wish I could help you but I don't have that set of drawings. Most PVC valves open when the pressure in the crankcase goes positive and relieve the pressure. When the pressure drops or goes negitive the valve will close and trap the negitive pressure in the case.

    Positive pressure is bad in that it tends to push oil out everywhere it can. It can even force the rings off the cylinder walls and blow gaskets out if it gets too high. Negitive pressure isn't as bad and can help seal the rings by vacuuming them to the cylinder walls. The hot rodders are putting in low tension rings for less friction and using vacuum pumps to keep the crankcase negitive to seal the rings.
    • CommentAuthorsundog
    • CommentTimeMar 17th 2013
    You could probably get away with a diaphragm type check valve, or a ball-and-spring check valve if necessary. having seen a bunch of diaphragm check valves, keep in mind it's necessary to anchor the diaphragm so it can't "blow" up into the valve body.

    A good example of PCV needs is the difference between a flat and an opposed engine.

    A flat engine is a 180 degree V twin. The pistons move in the same direction, so the crankcase volume remains nearly constant. Opposed engines have the pistons moving in two different directions at once, which pulses the crankcase pressure. VW engines are flat engines, whereas subaru or BMW motorcycles are opposed engines. Flat engines are cheaper and easier to design, because the pistons share a common journal, whereas an opposed engine needs separate journals per pair of pistons.

    The "Gotcha" is that any engine in which the pistons share a common journal, but the pistons aren't 180 degrees opposed, the crankcase pressure varies with crankshaft rotation. In which case you need a PCV valve, or at the very least a breather with a diffuser. 2 stroke engines absolutely rely on crankcase pressure to supply oil-fuel mixture to the engine, and in most modern engines, provide pressure pulses to "pump" the fuel through the carb. This eliminates the need for a fuel pump, and helps feed an engine that can't get enough fuel by gravity feed alone. Case in point, a 250cc 2-stroke engine at 12000 RPM requires more fuel than can be supplied by gravity pressure alone. A tiny orifice allows crankcase pressure to "push" on the fuel reservoir to provide positive fuel pressure to the metering needle. A bypass allows "extra" fuel to flow back to the gas tank. That's why most weed eaters and other small 2 stroke engines have two fuel lines. Although those carbs utilize crankcase pressure to allow the carb to run in any position, even with the gas tank below the carb. A super-simple solution to a complex problem.

    Unfortunately, I don't have plans for the V twin, although I'd really like to obtain a set. However, money isn't as much of an issue as free time. Between work and home, I barely have two 'Round-Tuits to rub together.