Originally Posted By Cab_Burge
Years ago I had a customer bring me his 1968 383 C.I. Charger that he had put a set of after market cast aluminum valve covers on with the stock type PVC and twist on oil filler cap with no vent. It would start and idle fine for a short while(1 1/2 to 2 1/2 minutes) and then slow down by several hundred RPM, the motor was sealed up tight enough that the PCV was making a vacuum in the motor after it suck the pressure out of the motor shruggy I put a breather cap on it and he was happy after that thumbs
A vacuum pump will do the same thing on a leak free motor work thumbs
The pumping action of the pistons and rods going up and down and ring blow by will cause crankcase pressure, especially at high RPM work shruggy

That's funny Cab , 20 or so years ago I had similar issues on a customers stock 308ci Holden , after 20 minutes the motor would stall . Took a little while to work that one out , vented oil filler cap solved it .

Tex

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