The Stovebolt Site Forums The Shop Area Engine Shop 261 PCV valve

Now that my 261 has been in service a couple of weeks there are some quirks showing up. One is that the idle isn't stable. I wondered if the factory PCV valve (metal valve that screws into the vent tower and a copper line to the manifold) was at fault so I dismantled it and cleaned it and made sure that the parts were in the right order, spring, piston, body, etc. per the factory manual.There's even a spec for spring tension. No change. If coming down a hill into town with a stop at the bottom it tends to stall after the high vacuum descent. Other times rolling to a flat stop all is normal.

So I took my vented filler cap off and put my hand over the hole and felt a strong suction. Hmmm... like manifold vacuum suction. So after thinking about this for a few days I put the Uni-Syn is use for syncing carbs on the valve cover hole with the engine idling. The PCV system draws as much air as one of the carbs. Can this be right? I substituted a non-vented filler cap in the valve cover expecting the idle speed to drop. It did not. So I'll drive it around for a few days and see what changes, it anything.

Where is air being introduced into the crankcase? There should be a line from the "clean" side of the air filter connected to the valve cover by a nipple near the driver's side rear of the cover. For the PV to work properly, there needs to be a "closed loop" ventilation from the air cleaner to the valve cover, to the crankcase, with air and any blow by or vapors drawn into the intake through the PV valve.
Jerry

Air has been coming in through the oiled gauze filter in the breather cap. I haven't rigged up a hose to the air cleaner yet. I didn't get the truck air cleaner from the wrecking yard when I got the motor. I'm sort of reluctant to cut into an original oil bath air cleaner.

I've worked on many engines that ran a vacuum in the crankcase. They would stall if you opened the system, Saabs and BMWs of the 70s for example. They'd come into the shop with a bad idle and it often turned out to be the o-ring on the dipstick tube.

Try ported vacuum.
How much spark advance at idle? Tweak the distributor position very slightly and see if RPM picks up.

At idle there should only be a slight amount of vacuum at the filler cap (or hose going to air cleaner).
If you have the original type valve, unscrew from the crankcase side (oil mist collector tube) and check.
A wild guess, normal would be 1 to 3 in Hg vac. (engine at idle).

Hope I get this right:
Stronger engine vacuum (idle, engine slow speed) to pull against spring/plunger (overcomes spring tension) to reduce flow.
Weaker engine vacuum (engine high speed) to pull against spring/plunger to increase flow.

So I used the non-vented filler cap which simulated a sealed crankcase system to experiment. With regards to idle quality it made no difference. I've also squirted carb cleaner at all the possible points of vacuum leak and haven't found any. It also sometimes idles at 800 when partially warmed up and sometimes at 500. The weather is warmish (60) and hi\umidity around 50%. Could I be seeing carb icing under these conditions? Once warmed up it's a steady 800RPM, 600 with the A/C on. Maybe I'm overthinking this and I should just drive it.

I'm tempted to leave it sealed with the PCV pulling a vacuum. That should preclude most if not all oil leaks. I've done this to my old British motorcycles and it has stopped the persistent oil leaks from the crankcase and head. Everything else still leaks (sigh).

Apparently my logic is going in a different direction than yours.
The PCV was originally an RPO intended for big trucks working in a dusty environment (unclean intake air).
A PCV is not required to have a normal idling or running 261 engine.

I would try this:

Remove PCV valve and leave the oil mist collector tube open to atmosphere.
Remove the vacuum line from the PCV and block the line to seal off vacuum.
Remove the filler cap, run the engine and check idle.
If the idle is still problematic then you can start looking elsewhere.