I have just rebuilt the engine in my 67 (327/350) and after breaking it in and doing all of the adjustments, I was checking all of the hose connections, etc. During this process I pinched the PVC hose (with my fingers) that runs from the oil filler neck to the carb base. The engine immediately slowed down. Does anyone know if this is normal? I don't understand everything about PCV systems but this does not seem right. It did not matter if the filler cap was on or off, it did the same thing in either case. I don't think that this is the source of my high pitched whistle but I want to make sure that I have not done something wrong. Am I looking for a problem that does not exist? Any words of wisdom?
-
-
Re: 67 PCV System
Craig, PCV-positive cranscase ventilation. Speaks for it's self. All the valve is a check valve that is pulled open by the vacuum from your intake (engine) connection. It pulls the vapors from the crankcase to the intake and is burned off via the intake to combustion chambers.
You should as prev/mnts clean these valves with carb cleaner or equal and make sure they are opening and closing upon vacuum.(keeps them clean ungummed)
My engine will speed up when I set my valves if I don't tape off the suction side of the PCV valve (valve pulled out of valve cover)when I'm setting the valves on that particular valve cover side. Once I close off the suction side the valve it is no longer able to function as it has nothing (vapors) passing through it. But if I leave the valve exposed to the atmosphere w/out taping the suction side of the valve off my idle goes up and makes it impossible to set my valves hot even with the idle set as low as possible.
Pinching off the line while attached shouldn't make the idle slow down unless you have an intake or atmospheric leak in the line from the PCV valve. maybe I'm missing something here and can use some input.
Again my PCV valve is in my valve cover and doesn't effect my idle unless I remove it from the valve cover.I also have a BB but the PCV operations should be the same.
grr#33570 -
Re: 67 PCV System
I'm not sure of the exact architecture of your '67 L-79 PCV system, but most of this vintage continually draw fresh air into the crankcase, through the metering/anti-backflow valve and then into the inlet manifold. Inlet manifold vacuum bears on the downstream side of the valve, and a certain percentage of inlet air is drawn through the PCV system at all engine speed and load conditions.
Therefore, if you pinch off a line at idle you will likely see a decrease in speed.
Check that your PCV lines are properly routed and that the valve is free. (Shake it, it should rattle). If the valve is all metal you can clean it in carb clearner. It's a pretty simple system. If the valve is free and the lines are routed correctly, if should be functioning normally.
DukeComment
-
Duke You're One Of The
few who understand how PCV systems work. What you said above is correct. A small portion of the gaseous input to the engine is the fresh air/blowby mix in the PCV system. Choking off that mix should indeed bring a well built engine down a few rpms at idle.
Dale.Comment
-
I should also point out...
that this type of PCV system achitecture is common to sixties and seventies carbureted engines, but does not necessarily apply to modern EFI engines.
Since a backfire cannot propogate upstream of the injectors there is no need for an flow reversal check valve on a modern engine. With a carburetor some sort of anti-backflow valve or flame arrestor is required to keep an inlet backfire from propogating into the crankcase and causing an explosion. Blowby gas is a blend of both unburned mixture and exhaust products, and under certain conditions, may be combustable. Diluting it with fresh air helps make it less combustable by essentially leaning it out.
Many modern EFI engines just vent the crankcase to the air cleaner or throttle body. A throttle body connection usually includes internal passages to both upstream and downstream of the throttle valve. The passage below the throttle valve is restricted to limit flow, and the upstream port provides a crankcase vent passage for high throttle openings with a few inches of water depression.
Also, some midyear Corvettes do not have a PCV valve. Sixty-four models come to mind. They use a flame arrstor built into the PCV plumbing and use the slight depression downstream of the air filter element, not manifold vacuum, to draw fresh air into the crankcase to purge blowby and then into the engine as part of the combustion air.
In '65 or '66 they abandoned this system and went back to the earlier system using manifold vacuum and a valve. I've always wondered why. Could it be that some of the '64 configurations caused a few crankcase explosions? Another consideration could be dirt. By allowing blowby gas to flow through the carburetor the contaminants would likely tend to clog up the air bleeds quicker than pure filtered air, which could foul up the fuel metering and require more frequent carb overhauls.
DukeComment
-
Re: 67 PCV System
Thanks to all that responded, it sounds like my system is operating correctlyComment
Comment