Esprit2
Member
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srq2002 wrote:
While under the bonnet adjusting the Strombergs I revved the engine and saw what appeared to be a light white mist coming OUT of the carb intakes. What could this be (Snip)... It seems this thread has gotten away from the original question.
There's a difference between crankcase oil fumes/ smoke, and fuel mist. I'm not sure which you're seeing. Hold a tissue in the mist, then sniff it. Does it smell more like petrol, or oil fumes/ smoke.
If fuel mist:
It's easy to think of inlet flow as being straight inward, but there's some very funky flow dynamics going on in the manifold. That includes reverse flow and reversion waves. The reversion waves can be strong enough to result in a standing wave of fuel vapor just outside of the inlet.
The strongest visual example I recall seeing was watching the old Can-Am cars go roaring buy. They'd touch 200 mph by the end of the front straight at BIR, yet there would be a standing cloud of mist about the size of a loaf of bread, stationary above the air horns. Well, stationary at 200 mph... the point is the cloud traveled with the car and never blew backwards. The cloud was due to reversion waves in the inlet runners.
Have you ever seen a picture of a fighter jet breaking the sound barrier, and there's a donut cloud of vapor surrounding the fuselage. The vapor cloud is due to a compression wave, it travels with the plane and doesn't get blown back by the supersonic wind. Loosely similar, but different.
Fuel injection can be more tolerant, but with carburetors, if the reversion wave blows back into the throat, it can play havoc with fuel metering. Manifold and carb designers use a number of tricks to try to control the reverse flow and reversion waves.
On the old Weber IDA 2-bbl down-drafts, as used on the Ford GT-40, there was an obvious smaller diameter tube sticking out of the middle of the air horn, and about 3/4" above it. That was an extension of the secondary venturi inlet that allowed it to breath smooth air from out beyond the reversion wave. That smoothed the airflow through the venturi and made jetting much easier.
Some Weber DCOE / Dellorto DHLA tuners apply the same trick, using K&S brass tubing to fabricate venturi inlet extensions.
Lotus' own die cast soft mount spacer has a tapered bore, like a funnel. On the down stream end, it's smaller than the carb or manifold bores, and looks like it would be a restriction. But going backwards, upstream, it forms a dam to the reverse flowing boundary layer, and interrupts it before it gets into the carb throat. It might be a restriction, but it's overall benefits outweigh it's losses.
Anyway, without seeing what you saw, I'm just suggesting it might have been something more esoteric than crankcase breather fumes.
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