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

[wonkipop]

here you go @nihil44 .

why your remark about negative pressure made me think.'

the VW pure crankcase vent works this way.
on the L jet 412s and buses.

the membrane and spring inside is a 2 way valve that can respond to either crankcase vacuum (neg crankcase pressure) or alternatively intake vacuum the same way.
restricts or even closes off fume chimney from crankcase.

i can't remember all the combos for engine crank vac v positive pressure in relation to intake vacuum, high low and almost none.
all has to do with engine at idle, or engine under hard revs or engine at high rev cruise etc.

the valve is designed to respond to all those conditions.

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now -- there is just the oil cap version on the 914 to see if there is indeed anything to that and its not just a straight up cap with seals to the oil filler tube.

but you were right with your hunch. crankcase vac when it happens is involved.
its not always positive pressure. and van is right. because sometimes it is positive pressure. but the pin hole aint a pressure relief valve. at least for the VW version which i can understand. but not ours yet.