L Jet vacuum leak pursuit '74 1.8, How does this work? Options

[nihil44]

I have bragged to my carburettored friends about how the '74 1.8 L Jet starts off the key even after a lay off of a month or so. Lately it has required two cranks to start. I suspected a vacuum leak as I pulled a plug and the pluf looks like the mixture is lean but the exhaust tips are black



I made a smoke maker based on this YouTube site

THE BEST SMOKE TESTER YOU CAN BUILD!! / THE MR. FUSION MINI BUILD - YouTube

Applied smoke into the intake system and this is what resulted.



Over the years I have read about ensuring the oil filler cap has a good O ring and cap seal otherwise unmeasured air will enter the intake system downstream from the Air Flow Meter and cause a disturbance in the A/F ratio determined by the ECU.

There is a factory hole in the oil filler cap as revealed by the smoke test. I have 2 filler caps and they both have the small hole.

How does this work if the L Jet intake system is supposed to be closed?

I was expecting the smoke test to reveal a vacuum leak in the intake boot or elsewhere and that would be the 'aha' moment. Not so lucky. However I would like to confirm that the intake system is vacuum leak free.

Would appreciate some explanation from the collective brain trust.

A word of caution: If doing a smoke test, perform it outside or in a well ventilated garage otherwise it will set off the smoke detectors. How would I know this?

David

[Van B]

It was gnawing at you eh? Well that pin hole is there to vent excess crank case pressure. Have you been driving the car much?

[nihil44]

Van,
Haven't been driving the car much. Biblical rain and floods in our area at present so 914 stays inside.

I thought the pin hole vent would work the other way - ie crankcase pressure would keep it closed. Positive pressure would force the membrane over the pinhole to close it off. The spring would keep it closed and negative crankcase pressure would cause a pressure differential to force open the diaphragm and allow ingress of atmospheric air.

Probably southern hemisphere physics!

[Van B]

Van,
Haven't been driving the car much. Biblical rain and floods in our area at present so 914 stays inside.

I thought the pin hole vent would work the other way - ie crankcase pressure would keep it closed. Positive pressure would force the membrane over the pinhole to close it off. The spring would keep it closed and negative crankcase pressure would cause a pressure differential to force open the diaphragm and allow ingress of atmospheric air.

Probably southern hemisphere physics!

Perhaps, I made my assertion based on observation. I only feel air coming out of mine when the engine is cold. As the cylinders warm up, the air out of the oil cap stops. Thus, my conclusion that cold engine blow-by was creating positive crankcase pressure that needed to be vented.

Maybe this is something worth testing in the absence of an expert to explain? Should be easy enough to make a test rig.

Edit: I still say that before you start trying to diagnose potential problems, the car needs to be driven regularly to make sure your issues are not prolonged storage related issues. No car can survive storage.