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Re: Oil filler cap mystery

To: F.vanDalen@LR.TUDelft.NL (F. van Dalen)
Subject: Re: Oil filler cap mystery
From: vortex@worldaccess.nl (Bert Palte)
Date: Fri, 29 Sep 1995 17:32:30 +0100
F. van Dalen wrote:

>My friend lifted the oil filler cap to improve the 
>sound quality. What happened then is unspectacular but I just can't figure it
>out: we got a very noticable increase in RPM. How the heck is that possible?

Well, there  IS an explanation for this phenomenon.
On my 1970 B, there is a flexible hose from one of the tappet covers  to the
rear end of both carburettors.
You didn't mention the year of your car, but I assume yours is similar (also
made
for the European market, I'm not sure whether USA cars are the same in this
respect).

This is called a PCV (Positive Crankcase Ventilation ) system.
Its objective is to keep any blow-by gases (leaking from the combustion area,
bypassing the piston rings, remember there is a high pressure there) out of
the Great Outdoors by burning them again. 

There is a vacuum at the rear end of the carbs, especially at idling.
So a bit of air will be sucked through the filler cap (hence the filter in 
this cap) and get into the inlet manifold.

So your engine gets some more air (i.e. there is less air resistance in the
 filler hole) when the cap is removed  - hence the higher speed.

You might justly object that, since  the piston and jet in the carb are
bypassed, 
the mixture will be leaner - I quite agree - but you add:

>I had one carburettor running too rich at the time.

,so I don't think that mattered too much.

BTW, if you put your hand on the oil filler opening  - cap removed - you
should get 
a vacuum.
On worn-out engines, you get a positive pressure, most of the time accompanied
by white or brownish 'goo' (= H2O and oil) on the inside of the rocker cover.
So this is a quick check to get an impression on the wear and tear on an 
unknown engine.

Hope this is of some interest to you.


Bert Palte

<palte@rt.el.utwente.nl>

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Bert Palte                                       Vortex Balanceertechniek B.V.,
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