Re: Is a PCV valve necessary?
You'll find out why PCV Valves are used when the valve covers blow up through the hood, or the oil pan blows off the bottom.
A PCV Valve is two things -- a restriction to limit the amount of air pulled out of the crankcase to keep fumes from collecting in the crankcase, thus leaning the mixture -- it's a controlled vacuum leak, and it's a one-way valve to prevent explosions in a backfire.
Normally just sitting the valve is open. Normally hooked up properly the valve is still open - it's spring loaded with a light spring. The valve is always open unless a backfire occurs, then the backfire's pressure in the intake manifold -- air going backwards in the valve - shuts the valve off to prevent fire from getting in the crankcase. Simply blow through one gently, then harder to verify.
Sometimes engines are rebuilt when they aren't needed - not routing a PCV Valve properly is sometimes the cause.
The PCV SYSTEM is important. It creates a draft through the engine, sucking all the unburned gasses out. Fresh clean air MUST get in one side of the engine - that's usually a hose to inside the airfilter element, or has it's own filter to keep out the dust.
The other side is the PCV Valve line. It hooks to the intake manifold BELOW the carb - sometimes it's at the carb base, sometimes at a monifold Tee. Never above the carb or into the air cleaner. The more centralized the location the better. The other end is hooked into the crancase - usually through a valve cover grommet.
Be careful - where it hooks is suction - the hole in the valve cover must have a baffle inside to prevent oil from splashing in the hose. Many times that oil splash is diagnosed as blowby. Remember the rockers are throwing lots of small oil droplets off, those droplets can easily get sucked up in the hose.
The vacuum in the manifold draws air and fumes through that hose, through the open PCV Valve, then into the manifold, not into the carb.
A popular misconception is blowby richens it -- oil from the blowby does not burn - oil itself won't burn well enough to change the mixture at all, but it will foul plugs.
If the blowby is so bad it's got enough gasoline in it to be flammable, there's other major problems anyway. In those cases the PCV SYSTEM is doubly important to prevent an explosion!
Yes, the PCV System is a small vacuum leak, but it's constant, so the carb compensates for it.
BMW's and the like don't use a valve in the PCV line, but that hose is much smaller and doesn't carry the fire in a backfire as well. But when they do blow - and they do - BMW - or Mercedes etc., makes lots of money!
When you hear a PCV Valve making noise it's because the vacuum in the manifold (if it's really hooked to manifold vacuum) is fluttering/pulsating for some reason. It's a good indication of something else wrong, ie. valves, overlap, low compression, exhaust restriction etc.
Yes, 2 PCV Valves can be run in parallel. Just make sure there's baffles on both and a fresh clean air source.
90 lbs compression may not be all that bad -- as long as all cylinders are about the same. It could be the method used to take the compression -- engine warm, battery fully charged, all plugs out, throttle blocked open, accurate screw-in gauge, and exactly the same number of pulses on each cylinder - 3 to 4.
Any procedure other than that is fooling one's self.
Rebuilders love misdiagnosis!