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]

Massively versatile slang right there!

[nihil44]

I have been looking a little more closely at the filler cap. Under the centre cross piece (red arrow) I can probe a spring. This spring holds up a diaphragm which appears to have an inflated donut on its periphery (green arrow). I am going by probing as it is impossible to see and photograph these areas. This may be a membrane and not a donut but in either case the purpose of this donut or membrane seems to be to seal off the pin hole in the cap.

Why? Dunno. I have run the car with the cap on and have closed the pin hole with my finger at both idle and about 2000 rpm with no effect on the running of the engine in either case. So, I guess if it is not affecting performance then I will leave well alone and turn my attention to other things. Mystery unsolved