Why does the engine seem to stall when you remove the oil filler cap?

[sujitroy]

Sujit

[barkerwilliams]

Carburettor creates a small vacuum in the crankcase to extract combustion fumes. The carbs are balanced anticipating this small, restricted gas flow. When you take the oil filler cap off you allow a large air flow through the crankcase to the carbs which upsets the mixture. The same carb vacuum also “powers” the distributor advance which depending on the revs on the engine may affect the ignition timing.


Alan.

[wilf]

It is a sign that the crankcase ventilation system is in good order.

If the idle wasn't disturbed when taking the oil filler cap off, it would mean there was an air leak in that system that someone had disguised by (incorrectly) retuning the carbs.

[sujitroy]

So I'm assuming that the vacuum source from the carbs comes from the engine breather pipe which connects to the RH valve cover. So I I remove that pipe it will do the same thing as removing the oil filler cap.

[thebadleys]

so i'm assuming that the vacuum source from the carbs comes from the engine breather pipe which connects to the rh valve cover. So i i remove that pipe it will do the same thing as removing the oil filler cap.

yup !!!

[Peter707R]

My vacuum to the distributor came from a brass plug between the carbs in the adapter - the vent from the cam cover went via a “t” adapter into the side of the Strombergs.

[DJT]

My vacuum to the distributor came from a brass plug between the carbs in the adapter - the vent from the cam cover went via a “t” adapter into the side of the Strombergs.

That’s the way it should be.

[CJG]

Strange when i remove my oil filler cap the engine speeds up slightly and on replacing, it slows down again ?.