Nathan Ruffcorn wrote:
>
> Hey all-
>
> The weather was great today! I've been able to get the roadster out for
> the past three days straight. The bugs are slowly getting worked out,
> making each run more enjoyable. As always, it attracts lots of attention.
> I'm especially enjoying the 3.9 rearend/2L combination. <grin> It's easy to
> accidentally bark the tires when pulling away from a stop light. For
> leasure driving, out of traffic, I routinely start in 2nd and skip to 4th.
>
> In an effort to get everything just right, after each run I have been
> pulling the # 1 and # 3 spark plugs to judge the mixture of each carb. The
> plugs have been whitish/grey so I've been gradually richening it up.
> Yesterday, out of curiosity, I decided to pull all the plugs. I was REALLY
> surprised to find #1 and #3 the same color (whitish/grey) and #2 and #4 the
> same (brown). What am I missing?! I admit, I'm no master tuner but I know
> that cylinder #1 & #2 and #3 & #4 should at least correspond together.
>
> Other factors (symptoms):
> -Rebuilt carbs from Z-therapys
> -Seems to surge a bit at highway speeds (65-70mph)
> -Hesitates under gradual acceleration, but seems to immediately "come alive"
> with a heavier foot.
>
> Please let me know if I'm missing anything really obvious.
> Man these cars are fun!!
>
> Thanks!
>
> Nathan
> http://albums.photopoint.com/j/AlbumList?u=123299&Auth=false
You just discovered that the firing order and manifold design can lead to a
differential in mixtures between the outer two cylinders and the inner two
cylinders. On various cars this effect can range from hardly noticeable to
extreme enough to justify radical manifold redesigns. When thinking of firing
order as it relates to mixture, it is best to look at the firing order
differently than the way it is most often expressed. So as to start with
cylinder #1, the order is normally expressed as 1342, but to see how the firing
order affect mixture organize the order as it relates to the carbs, begin with
cylinder 2 instead. Then the order is 2134. Now you can see that when cylinder
#2 is on the intake cycle it is drawing through a manifold runner/carb that has
been inactive for two intake cycles. This means the air/fuel mixture in the
intake tract is barely moving and must be accelerated rapidly. This causes a
lean out as the air reacts more quickly than the fuel, and therefore cylinder 2
runs a bit lean. Then cylinder #1's intake cycle begins. But it is now drawing
through a manifold/carb that was operational on the very last cycle and so the
column of air/fuel in the tract is already moving. It suffers no lean out as the
air/fuel mix was already moving and did not need to be accelerated, plus it gets
some of the slow reacting fuel from cylinder 2's intake cycle, so it runs a bit
richer yet. Same thing goes for the intake cycles for cylinders 3 and 4.
Marc Sayer
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