Just to reinforce some things mentioned in this thread so far:

1) The problem is most definitely due to the fact that your PCV system has no fresh air intake. Pressure is building up in the crankcase.

2) My guess is that the crankcase pressure is pushing oil out of a valve cover gasket and onto the header.

3) The reason that things worked okay prior to 1968 is that pre-1968 blocks had a rear crankcase vent at the back, near the distributor. For most of the C2 years, this vent was used to take fresh, filtered air into the crankcase via a hose connected to the air cleaner base plate. Your block does not have this vent provision.

4) The quickest, easiest fix would be to duplicate the fresh air intake that GM used starting in 1968 (when the rear crankcase vent disappeared). That intake was via a hole in the valve cover, connected to the air cleaner base plate with a hose. This means that you can't use the unvented C2 valve covers that you now have. However, I think the vented aluminum valve covers that GM used on the Z28 (and possibly Corvettes) starting in 1968 are very handsome and have some of the general design cues as the C2 aluminum valve covers. All you would need to change are your valve covers and the air cleaner base plate.

5) If your long term goal is to retain the unvented C2 valve covers, doing so will require some careful planning and some minor machine work. I have seen photos of several different schemes posted here on CF. However, I suggest that your first step is to swap out the valve covers and air cleaner base so that you can eliminate this problem and focus on other tuning issues for the time being.

 

Here is your simple solution, and the one I used on my 64

https://www.corvetteforum.com/forums...1&d=1512525245


Unfortunately you can't see both sides in this photo, but if you look closely at the inboard side of the driver side cover, above the "C", you can see a barely visible baffled hole, which is the fresh air intake, the passenger side has the same hole machined in, with a grommet for the PVC valve.

These covers are specially made for this ventilation problem, and are available from PLM at https://www.yourcovers.com/valve_covers_11026.php

I ordered a set and they solved the problem of keeping an original look, when using a 350 crate motor. Hopefully they can help you out too, give them a look.

I can get better pictures of my setup if you would like to see more. Good luck with your project......

 

Sorry to be a while responding, but I've been a trip to the Gulf Coast and just got back barely making it through the Alabama snow storm--6" snow in Alabama is big deal for driving. Gearhead Joe, you are right. Icouldn't see what there was crankcase pressure build up when I've got the vent to the carburetor base, but I guess i'st too small to handle full throttle blow-by, but that was definitely the problem. I also didn't realize that the crankcase does develop vacuum and that there has to be a source of fresh air intake.

To confirm this I did a really low tech test; I took off the sealed oil filler pipe cap, and to keep oil from coming out of the pipe and getting on the engine, I put on old white gym sock over the pipe and wire tie and made a vent sock. Took the car out and replicated the 4,000 to 4,500 engine speeds several times and sustained 3,000 to 3500 cruising speed for about 15 miles. No oil was pushed out of the valve covers, a very little bit of oil showed up in the sock, but it looked like it was only from fumes, no real splash of oil.

I'm going to get a set of those vented valve covers that Oaker57 mentions in his post. Thanks for the lead.

I do have one questions I just can't figure out. It's clear the crankcase vent is needed, and I would have suspected it was due to pressure coming from cylinder blow-by. How does a crankcase develop vacuum and therefore need a fresh air intake?

 

I think there are two distinct conditions that can occur with the PCV system, and both conditions need to be accommodated by the PCV setup:

1) Low-rpm, low blow-by conditions where the rather modest volume of air pulled through the PCV valve is sufficient to pull some fresh air through the crankcase air inlet and collect both the fresh air and the blowby fumes.

2) High rpm, high blow-by conditions where the volume of the blow-by is more than the limited flow of the PCV valve can handle. Under these conditions, the fresh air intake reverts to being an exhaust port for the excess blow-by.

For either condition, there is not really a "vacuum" in the crankcase, because the fresh air intake is typically a moderately large tube that will allow air to flow in either direction. I think it is a mistake to view the crankcase as being at a "vacuum" of the same magnitude that an intake manifold can have.

Rather, the pressure in the crankcase is typically just slightly below atmospheric pressure under condition #1 above, and it is just slightly above atmospheric pressure under condition #2 above.

It's best to think of the pressure difference as similar to what might be generated by a desk fan operating on the lowest speed setting.

 

 

http://motormagic.info/vacuum.htm

 

http://motormagic.info/vacuum.htm

Measure the amount of vacuum in the crankcase. With the engine at normal operating temperature, block off the PCV breather tube or vent to the engine (usually the hose that runs from the air cleaner housing to the valve cover on the engine). Pull out the dipstick and connect a vacuum-pressure gauge to the dipstick tube. A typical PCV system should be pulling about 1 to 3 inches of vacuum in the crankcase at idle. If you see a significantly higher vacuum reading, the intake manifold gasket is probably leaking and pulling vacuum on the crankcase (replace the leaky intake manifold gasket). If you see no vacuum, or find a buildup of pressure in the crankcase, the PCV system is plugged or is not pulling enough air through the crankcase to get rid of the blowby vapors.

Copied from this.

http://www.aa1car.com/library/pcv.htm

 

Thanks folks. I'll run the vacuum tests for enlightenment and report the results.

 

Hi Oaker57,
I always liked the look of the early valve covers with the Corvette script but never got them because of the no holes=no ventilation problem.
Your link brought me to the solution, PML drills a hole in each side so I ordered a set in 'as-cast' with holes.
Thanks again for answering my question today and can you tell me which fel=pro gaskets you used as the auto parts list very many.
Pete.


PS: Hope the op fixed his problem.

 

PM Sent