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]

well it's a good thing those caps are NLA...
I suppose I better start searching for one of those too. I know for sure mine vents on occasion when the engine is cold.

[wonkipop]

well it's a good thing those caps are NLA...
I suppose I better start searching for one of those too. I know for sure mine vents on occasion when the engine is cold.

you can't get the bus version of it either.

and if you look through that samba thread its pretty clear that at this point in time 99% of those bus ones are kaput. but the engines are still running.

........not much you can do about it?

except maybe rig up a new pcv valve set up external to the existing one and somehow gut the cap and seal the pinhole there?

the cap pcv looks to be a delicate thing to balance crankcase pressure to intake vacuum so its a draw very closely matched to whatever blow by there is.
since there is no other way inlet air is drawn in to the system unlike the d jet cars with their vented heads? you would always want a slight draw from the crankcase i think,
just to help the oil return down the pushrod tubes. but it all must be very slight as you don't want any significant unmetered volumes of air - although i would have to think hard about that and give myself a headache as that air has in a round-about way gone through the afm already. so its kind of a balance of losses?

it does look like its designed to fail wide open - unless you got unlucky and somehow remnant diaphram blocked the channels to the engine intake line.

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i could be wrong but i think i have seen images where potentially they kept on using that central diaphram part for later golfs and passats in the second half of 70s. probably NLA for those too. but a possibility would be to see if they can be gotten hold of.
then a disassembly operation done where you insert that cap piece with diaphram into our oil cap assembly if they can fit together. its really only the diaphram bonded to upper cap that is needed.

i would only be willing to do it with another second hand 914 cap and see if its possible to take it apart without destroying it. and if so is there a diaphram part you can then fit into it and then i guess glue it all back together with the spring positioned in it. with care it must be a possibility that it can be taken apart the same way it was assembled in the first place.