The solution is not to take the oil cap off when the motor is running.
Do you do this to all your other cars?

No.. it's not a habit....but all I know is that it idles much better with the PVC
disconnected....It's not my imagination.....An another thing I could never
get the carbies balanced......bet it's no problem now.
And I'll take a bet there is something wrong with the PCV.

Take the plate off the PCV, if the rubber diaphragm is hard, toss it and replace. Don't toss the valve thingy underneath it...

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Had a quick look at that yesterday but was running out of time..
It seemed quite soft......but on the weekend, I'll examine it more closely.
To get me by so I can breath, I ran a hose from the vent down under the body......just like the good old days.

As it turns out the cap is vented (I thought there would be a hole in the top)
I cleaned the vent out and all is okay...so it seems like the crankcase ventilation system is marginal. So it might be that the canister
coming out of the chest cover is a bit choked.

This is correct. The vacuum hose (and the PCV) causes a negative preasure inside the engine to draw fumes into the inlet system instead of them venting to the atmosphere.

When you open the oil cap, or take the vacuum hose out of the engine, the resistance of having to suck the air into the engine is removed - so more air is then allowed to be drawn up the vacuum hose. Because these vacuum hoses are usually connected to the inlet manifold after the carburetor it makes the fuel mix a LOT leaner.

If you are going to remove or block the engine venting via the vacuum hose and PCV then get the engine re-tuned to compensate - and add breathers to let any gas buildup out of the engine.

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