Posted by Chriscase on Monday, January 22, 2007 at 16:03:42 :
In Reply to: Re: Back fire into air filter? posted by David Sherman on Monday, January 22, 2007 at 00:25:31 :
A Positive Crankcase Ventilation valve. It opens during heavy engine use, closes at idle. So it Positively makes sure the Crankcase is Vented.
It incidently shuts under backfire. The other end of the newer systems (post road draft tube) is a hose to the air filter. This allows the exces gases to flow into the engine in the reverse direction when the PCV is closed. Therein is the flame hazard. Depending on the location of hose inlet to the air filter, you can get a backfire up through the carb, around the 'flame guard' aspect of the filter, and into the crank via the vent hose. If other conditions are perfectly wrong, Bye-Bye oil pan.
Your vent system should have a couple of flame guards'- one, the PCV valve, the other the filter element- paper ones have a simple screen inside the folded, oil soaked paper. Oil baths are made of steel mesh, so are an inherent flame guard. The chevy truck I mentioned earlier had a vrass screen in the vent hose fitting at the air filter- on the wrong side, inside of the filter. Using an add-on fitting that went on the outer side of the housing fixed his problem.