Bull.... Trevor. I just have to wade into this, probably since I started the
whole thing off. The discussion is not over what is right or wrong, emission
wise, but what is the best solution to fix a problem with a given configuration.
The whole issue centres around the use of the Weber DGV (and as Joe pointed out
the DCOE). It does not have the same vacuum port as in the standard carb, the
ZS. It has only one vacuum attachment and that is used for the distributor. This
port leads into the throat just after the throttle plate and the opening is very
small, just a few thou, at the most a 1/16". It could never be used to vent the
crankcase.
The ZS has a much bigger vacuum port and it draws a significant vacuum under
higher RPMs. That level of vacuum appears to be needed to relieve the crankcase
pressure. Just try removing the hose from the valve cover and run like that for
a while. I will guarantee that you will get the same problem, oil pumping out
the dip stick.
So the air flow is significant enough, far more than the DGV can pull in through
the air filter connection. In fact, it really has next to nothing in the way of
vacuum at this point.
Joe is right and he is wrong. Right in the belief that his "fix" works because
he is directly venting the crankcase, avoiding the valve cover as the air flow
somehow is restricted from the crankcase into the valve cover. He is also wrong
in the fact that the air pump does not come into play here. AFIK, there is no
link between the air pump and the crankcase. It just pumps fresh air directly
into the exhaust ports.
Since there doesn't seem to be a way to increase the vacuum on the DGV, and my
examination of same shows that there is no place where one ever existed (in some
other application), the catch can is a good solution, vented directly to the
crankcase, and the valve cover, with a tube to the air filter connection to
catch the vapors.
Vic Whitmore
Trevor Boicey wrote:
>
> Joe Curry wrote:
> > > Excess pressure in the crankcase is drawn out of here
> > > by the vacuum in the carb body, then pulled through the
> > > carbs and through the engine.
> >
> > In theory it should work that way. But in practical application it does
> > not.
>
> Literally hundreds of millions of cars sold worldwide
> use similar systems for crankcase venting, including to this
> day. (most modern cars have a one way valve called a PCV
> valve in the line, that's the only real change)
>
> Logically, I'm going to have to beleive that at
> least somebody beleives the system works in practical
> applications. You don't see new cars puffing oil out
> of their dipstick holes until somebody drills a hole
> and hangs a jar down there!
>
> > My suspicion is that the pushrod tube channels
> > are not sufficiently large to allow all the built up pressure to work its
> > way up to the valve cover and vent through the valve cover vent.
>
> I think this is highly unlikely, because the amount of air
> flow actually required is VERY low. Again, plug your vent and
> hold your hand over the oil filler and you'll feel. Unless
> your engine is REALLY knackered and blowing-by like crazy,
> the airflow is very low.
>
> The factory emissions tubing actually contains what is called
> a "restrictor" in the vent line, which is a fancy way of
> saying a plug. It's a white plastic thing shaped like
> a thimble that you put over the valve cover vent and
> then slide the hose over it.
>
> This plug closes off most of the 1/2 inch hose except
> for a pinhole down the middle.
>
> All of the air required to vent the crankcase passes
> through this little pinhole comfortably, it's really
> not much. You would certainly have no trouble getting
> that much air through the pushrod tubes.
>
> Compared to the amount of air that the carbs are
> gulping in to run the engine, it's basically
> an irrelevent trickle.
>
> > So necessity being the mother of
> > invention, I got inventive and that is what I came up with.
> >
> > And believe it or don't, It really does work!!
>
> Well, it depends how you define success really.
>
> An equally "successful" fix would just be to leave
> the oil filler cap off all the time. That would save
> you the trouble of having the jar, but still, the
> question would be "why would you want to".
>
> --
> Trevor Boicey, P. Eng.
> Ottawa, Canada, tboicey@brit.ca
> ICQ #17432933 http://www.brit.ca/~tboicey/
> "You can't crush cars with a university degree."
--
Vic Whitmore
76 Spitfire
Thornhill, Ontario
http://www.vicwhit.com
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