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]

ok @nihil44

i had a good look at this over lunch.

i noticed the cap has a little hole in the side where the lateral springs join.

this does not vent straight through into the very interior of the cap (ie the crankcase chimney - instead it opens into a hollow body. the bottom section of the cap assembly is a hollow body tube.

i reckon we have a pcv valve exactly like the ones on the other VW type 4 L jets.
its incorporated into the filler cap itself and a grip ring is added to that to assist getting the cap off.

i drew a cross section of how i think it works more or less.

it cannot be taken apart easily, probably destroy the cap. and be be buggered if i am going to do that as i know you cannot get one of these in australia nor probably find one at a junk yard to cut open and do an autopsy on.





i can't remember where i read that there is no pcv valve on a 1.8.
maybe i was imagining it.
but if i did read that somewhere i reckon it is wrong.
there is one? its built into the top of the filler cap?

the weird spring

i've stared at that spring for years trying to figure out why its there.
some sort of grip on the filler tube?
shrugged my shoulders at it.

it hit me today.
its the flame arrestor for a backfire.


more useless research provoked by your smoke machine teapot simulation?

[nihil44]

@wonkipop

Man that is some dissertation on the filler cap! Thanks for the cross section diagram. That is the way I thought it would be and I agree that I am not going sacrifice a cap just for dissection purposes or purchase one from USA. Hardly any 914 parts available in Aus.
I received another bunch of parts today from 914rubber via Fedex and commented at at the time that the amount I have spent on trans Pacific carriage of parts could probably purchase a couple of business class fares to Europe

It all makes sense. I too wondered about the function of the spring inside the cap and the flame arrestor explanation also makes sense.

Nice work!

David

[wonkipop]

@wonkipop

Man that is some dissertation on the filler cap! Thanks for the cross section diagram. That is the way I thought it would be and I agree that I am not going sacrifice a cap just for dissection purposes or purchase one from USA. Hardly any 914 parts available in Aus.
I received another bunch of parts today from 914rubber via Fedex and commented at at the time that the amount I have spent on trans Pacific carriage of parts could probably purchase a couple of business class fares to Europe

It all makes sense. I too wondered about the function of the spring inside the cap and the flame arrestor explanation also makes sense.

Nice work!

David

most of it is not a dissertation.
just thinking out loud on the way to the last post.
the last post is the succinct summary of prior ramblings and dismissal of my own errant thought projections.

its your fault.
you sent me off on a tangent.
can't stop until i get to the end.
blame it on my post graduate training in the USA.
its their fault as well.

i was talking to my mate max tonight on the phone after diinner.
he said that mgs had these types of oil caps and were round in australia.
said they were famous for turning into chimneys.

never mucked around with an mg and never will.
but ...... if we knew some mg guys they probably could have told us about this.

@djway is right on the money.
you should not be able to blow smoke out of that pinhole.