Have you checked your float level - if too low - no matter where you set the
mixture it will be lean.
----- Original Message -----
From: Matt Van Benschoten <mvb@voyager.net>
To: <spitfires@autox.team.net>
Sent: Monday, October 25, 1999 12:27 AM
Subject: Re: a couple of questions
>
> >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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