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>
(signature under construction)
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Bert Palte Vortex Balanceertechniek B.V.,
Roggeveld 25 Industrieweg 21 / P.O. Box 107
Soest, The Netherlands Soest, The Netherlands
NL-3764 ZB NL-3760 AC
tel. +31 35 602 3949 (home) tel. +31 35 601 3816
tel. +31 35 601 3816 (office) fax +31 35 602 6288
<palte@rt.el.utwente.nl> <vortex@worldaccess.nl>
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