>Glenn Wrote:
>So here's the questions, 1) My car
>has always seemed to run a little lean even when the carb adjusted fully
>rich, the problem seems to stem from the breather tube from the valve cover
>allowing or forcing too much air into the carb. It has a restrictor where
>the hose meets the carb with about a 1/8" hole in it, but I think it's
>still diluting the mix too much and causing a miss, has anyone else seen
>this problem?
Glenn,
If your crankcase breather tube is entering the induction system upstream of
your carburetor then it has nothing to do with a lean running condition. Any
air entering before the carburetor is metered air, i.e. the carburetor is
adding fuel for the given air volume. Positive pressure at this point is not
relevant. This crankcase ventilation air would pass through the venturi the
same as the ambient air being brought in through the air cleaner. Always
remember this as a rule: any air entering the induction system prior to the
venturi is metered and any air entering after it's un-metered. This
condition, in fact, with warm low density air being delivered by the PCV
system usually richens the A/F mixture slightly. This is because the
carburetor does not compensate for the warmer low density air. Carburetors
are a volume based rather than mass based fueling system. This is only a
problem with high blowby engines and most people wouldn't know the
difference.
A side note:
A PCV system can more frequently cause a rich condition rather than lean.
During a cold start, with a rich A/F mixture, lot's of excess fuel can be
put into the oil due to the rich mixture and poor ring/piston sealing during
the cold start. What then happens, once the engine and oil warm to operating
temperature, is that the fuel evaporates from the oil. These fuel vapors are
then removed from the crankcase via the PCV system and conveniently
delivered to the ambient side of the venturi. The venturi then adds more
fuel for the given air volume, as it's designed, which in turn produces a
rich A/F ratio. This same condition can happen during full power wide open
throttle runs. High cylinder pressures push some of the rich A/F mixture, as
it should be at high loads, past the rings and again into the crankcase. At
this point the fuel vapors are immediately carried back through the PCV
system and cause the same rich A/F mixture as described above.
I would look elsewhere. I'm not sure what carburetor your using but there
are probably other mainjets available for it. You may be interested in
purchasing a book on your particular carburetor. It is important to know
what section of the carburetor is being used when doing your tuning. You
certainly don't want to be changing main jets when the idle circuit is being
used for your evaluation. Have fun.
Regards,
Matthew Van Benschoten
mvb@voyager.net
http://my.voyager.net/mvb
-----Original Message-----
From: Glenn Trunnell <trunnell@mindspring.com>
To: spitfires@autox.team.net <spitfires@autox.team.net>
Date: Sunday, October 24, 1999 10:50 AM
Subject: a couple of questions
>
>Hi everybody,
>
>I've been lurking for the last few months, but I need to get back into the
>fray now. This past summer I rebuilt my spare engine for autocrossing and
>I've got about 3000 miles on it now. So here's the questions, 1) My car
>has always seemed to run a little lean even when the carb adjusted fully
>rich, the problem seems to stem from the breather tube from the valve cover
>allowing or forcing too much air into the carb. It has a restrictor where
>the hose meets the carb with about a 1/8" hole in it, but I think it's
>still diluting the mix too much and causing a miss, has anyone else seen
>this problem?, 2) I have a slight tap that sounds like a rocker arm tap,
>but I can't be certain where it's coming from, I've adjusted the rocker
>arms to .005 for a few minutes to see if I could make it go away and I can
>still hear it. My main concern is a problem in the bottom of the engine
>such as a spun bearing, but I believe that this would be loud and obvious,
>does anyone have any experience with that? Thanks as always!
>
>Glenn
>
>
>
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