I've never been a fan of eliminating the PCV system, but the Mikuni manifold (like most L-series Mikuni/Weber dual sidedraft manifolds) provides no provision for a PCV. Love the carb setup from a performance standpoint, but would rather not vent the crankcase to the atmosphere. It's smelly, not good for the atmosphere, and not good for the engine either. PCV removes harmful blowby vapors from the crankcase that would otherwise condense and contaminate the oil. Downside of PCV is that the blowby vapors somewhat dilute the intake charge with oily residue that can reduce the effective octane and contribute to buildup on the intake valves and combustion chamber. In a healthy engine the positives should far outweigh the negatives, though. Since I would put my engine in the healthy category with a reasonable compression ratio, I decided to try and implement a PCV system.
There's three plugged ports available in my Mikuni manifold. Two small ports, one between runners 1/2 and one between runners 3/4, and a larger one in runner 4. I presume the latter is for a vacuum assist power brake canister if so equipped.
In the stock L16 intake manifold, the PCV valve threads into the intake manifold under the stock downdraft carb and is exposed to vacuum from each intake runner, which is probably ideal since all four cylinders would share in burning the crankcase gases. One could connect the PCV to a junction joining the two small ports in the Mikuni intake, and that may ultimately be the way to go, but I decided to first try using the larger port in the #4 runner since I found a fitting with the correct threads from a stock L16 intake manifold that I had on hand, and it was much easier to plumb. Probably not ideal since most of the crankcase gases will be burned by the #4 cylinder (#3 will also get some), but I'm hoping the benefits will outweigh this drawback. If the #4 sparkplug ends up looking ugly, I'll consider running the PCV to all four cylinders, but for now, this is how I'm doing it.
Here's the block breather I was running. The hose was cut down from a stock 510 PCV hose a long time ago.
The metal block PCV tube is ~1" OD and the stock PCV hose reduces to ~ 5/8" to fit the PCV valve. The stock 510 hose is NLA, but a 240Z hose is still available and has a similar 90 degree bend with the same reduction in ID.
Here's the manifold port in the #4 intake runner that I will use (slot head plug). You can also see the smaller 3/4 port below it in the photo (allen head plug). The #4 port in the Mikuni manifold is BSP thread like the ports in the stock 510 manifold. I presume that the smaller ports are also BSP, not NPT, though I haven't verified this.
Here's the fitting I cannibalized from the L16 intake manifold. I'll have to cap the smaller nipple for now. I'll consider removing it if everything works as planned.
I also bought a new PCV valve.
Here's the PCV hose with valve that I cobbled together from the 240Z hose which I sectioned. I used the 90 degree reducing section plus a 4" straight section. I had the metal elbow leftover from the SR20 conversion on my other '72, so I used that with some nylon reinforced tubing I also had left over from that conversion. After I shot this photo and installed everything I realized that the PCV valve was backwards (!), so that has since been corrected.
This shot shows the hose setup as installed at the block PCV pipe end.
View from above, kinda blurry but you can see where the tube comes out from under the rear of carb heat shield and air cleaner relative to the firewall.
The fitting installed into #4 intake port with hose attached and small nipple capped off.
I ran the car in the driveway and it seems to run just fine, so I don't think there's any vacuum leaks. I did notice that the hose to the intake manifold has a "pulse". This weekend I plan to go for an extended test drive to see how it runs and get a plug reading. I think ultimately the PCV should be tied into all four intake runners, but it will be interesting to see how well it works off the #4 port.