This will be a long one, sorry...
My 170 is still on its original build with 150k miles and is not far from needing a rebuild or replacement. I strongly suspect that the 4th rings are leaking as supported by oil fouling the plug and reduced compression on that cylinder. Given that...
I converted my '63 170 in a totally home built fashion but based on the OEM CA idea. I plugged my draft tube port with a home built vulcanized rubber plug (this was before I discovered this forum and I didn't know how well this would work, so this plug was meant to be temporary, I've had no problems, so it's still there 2yrs later.) Replaced the stock valve cover with an aftermarket unit with both front and rear openings. Used a twist-in, non-vented oil cap in the front port. Found a matching sized grommet to fit a stock '69 Mustang 351w PCV valve (had it laying around) and the rear cover port. I did try, with each of the following plans, using a vented oil cap. In every instance it promoted dipstick blow-by by reducing the intake's draw on the crankcase.
At first I plumbed the PCV valve to the air cleaner just outside of the filter. I found that the system wasn't flowing enough as the crankcase was venting through the dipstick tube, evidenced by oil percolating around the stick handle and soaking the side of the block. I removed the PCV valve and reconnected the line to the cover with an open port 3/8" hose to 1/2" NPT plastic plumbing fitting. The system flowed fine with no blow-by through the dipstick tube. However, at sustained low engine speeds (long stop lights or traffic back-ups), oil residue would build up in the air cleaner at the carb opening and then would "wash-over" when I opened the throttle heavily resulting in embarrassing blue plumes. Sustained cruising rpm's seemed to keep the oiling at an unnoticeable amount, a quick stop wouldn't plume as bad.
Later, I replumbed to the intake tract below the carb. My engine has two 1/4"npt ports : one in the carb spacer (2sp AT was plumbed to here, MT conversion left the port available) and the second is actually in the intake log directly across from the head and just barely aft of the 3rd intake port.
I tried the carb spacer port first with great success using the plumbing fitting. The blue plumes went away, spark plug fouling reduced drastically (formerly #4 would foul out in about 2 weeks, now about 3 months), idle speed almost doubled (reduced crankcase pressure I'm sure), throttle response was better, and mpg's increased by about 2-3.
After about 6 months, I revisited the PCV valve installation. Performance overall reduced and the dipstick tube began venting again. So, I tried plumbing directly to the intake log. The intake could draw strong enough with the PCV valve without the dipstick venting. However, the engine idled rough despite all efforts and the 3rd plug would oil soak and foul after extended idle speeds. I rationalized that since the port was almost directly across from the 3rd intake port it was drawing heaviest on the PCV and least on the carb, as opposed to when plumbed to the spacer the PCV charge had a chance to mix with the intake charge before being sucked into the log where it could be more evenly dispersed among all cylinders, or at least as best as the log will allow.
So, I reverted to the open PCV line plumbed to the carb spacer. This has yielded me the best performance and least adverse affects. I haven't tried the proper stock PCV valve with the 1/4"npt threads yet. Though I did try laying my spare valve on its side (as I suspect the proper stock valve should install) with direction of flow in mind. I found that it restricted flow enough to again cause the dipstick to vent.
When I rebuild the engine, I will revisit using the stock PCV valve in its intended position. Until then, this appears to be my best option considering the state of the engine.
