Re: [alfa] air fuel ratios

[James Montebello <jamesm@xxxxxxxxxxxx>]

The rich dip at 5000 rpm is very probably caused by cam timing.
The intake open time and the overlap, along with the length of the intake
tract (and to some extent, the length of the exhaust manifold), all add
up to a wave that passes through the carb three times, once forwards
and again backwards (and again forwards).  This picks up fuel each time,
as the carb can't tell which direction the air is flowing.

This used to be a common problem back in the days of carbs and wild cams.
On carbs with open trumpets, you could often see a cloud of fuel vapor
hovering at the mouth when the engine was doing this.  Bike guys are
generally more familiar with this, simply because bikes have had wild
cams and carbs much more recently than cars have.

Fixing it will require going to someone who really knows how to tune a
whole engine package, not just fiddle with carb adjustments.  No amount
of jet swapping and screw twiddling will fix this.  You'll need different
cams and/or have them timed differently, perhaps a different header.
All of these will come with their own downsides, of course.  You usually
end up just moving the problem, not fixing it.

jamesm

On Tue, 2 Dec 2003, Peter Kemos wrote:

> Have been following the 'bigger injectors' thread with some interest, both
> for my aspirations of warming up my Verde, but also as it relates to tuning
> the Webers on my recently carb converted GTV track car.
>
> The question is this:  what air/fuel ratio is considered optimal for a
> fairly well warmed up 2.0 GTV, with about 10.5:1 compression?
>
> Reason i ask is that my Weber tuning 'expert' set them to where they are
> right at 13:1 on the top end, which seems a little lean to me, but worse
> still, there is a big dip in the rich direction (approx. 10.5:1) right at
> 5,000 rpm which is resulting in a significant and corresponding dip in the
> torque curve, right where i need it the most, and that is after spending
> (pun intended) 2 hours on a chassis dyno.  Sigh.
>
> Thanks,
> Peter Kemos
> Santa Cruz
> 89 Verde
> 73 GTV
> 66 Super
>
> _________________________________________________________________
> Need a shot of Hank Williams or Patsy Cline?  The classic country stars are
> always singing on MSN Radio Plus.  Try one month free!
> http://join.msn.com/?page=offers/premiumradio
> --
> to be removed from alfa, see /bin/digest-subs.cgi
> or email "unsubscribe alfa" to majordomo@domain.elided
--
to be removed from alfa, see /bin/digest-subs.cgi
or email "unsubscribe alfa" to majordomo@domain.elided