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]

I think we have a bit of a red herring here. Based on that geyser of smoke, it looks like you're pumping some serious pressure into the crankcase. As such, it will find a way to escape and that pin hole is definitely where it should be escaping. Further, you have the hose that runs from the fill neck to the intake boot, which appears to have Olympic level clamping so, well done on that!

I would set aside the smoke show and go back to the original symptom that prompted your investigation e.g. start-up cranking. Have you checked your points gap and your timing? Second, when cold starting, is your idle sluggish for the first several minutes or does it climb up and then settle as the engine warms up?

[nihil44]

@wonkipop

Yep. I read the Samba article and it did my head in and am not much wiser. The cap is a little bit of a mystery . I inserted a fine probing wire into the hole and it engaged what seemed to be an annular balloon. Very difficult to see and it may indeed perform some pressure control / gatekeeping function for the crankcase. I will have a look more closely in the next day or so. Laid up at the moment

@Van B

I will park the smoke show aside for the moment. I was only using a couple of psi for the smoke.

The start problem. As described, the car used to start straight off the key even after not having run for a month or two. It always made me feel quite smug.

Now - after not running for a few weeks, crank for 4 or 5 secs and no start. Fuel pressure gauge is installed in engine bay and it shows operating pressure after this initial cranking.
Crank again and it starts and idles as it would from a warm start and drives perfectly.

The cold start injector may be a player here and I haven't performed the test of removing the injector and cranking the engine to see if it is working. It is such a bitch to get to the cold start injector without removing more components than I want to.

I will take your advice Van and check the point gap etc

Thanks for the help