Dave :
I've "played around" a bit with the sort of instrument he was talking about,
but never done anything serious with it. In my case, I drilled and tapped the
stock exhaust manifold (which I was planning to discard anyway) for a common O2
sensor from a modern car, which I then connected to a DVM. What I found was
that :
1) The stock needles do 'pretty good' for my basically stock engine. However,
there is relatively little enrichment for low speed power, since SU carbs don't
"know" the difference between low speed, wide throttle and high speed cruise.
This suggests to me that another carb design (with a power valve) might give
better low speed power and better cruise fuel economy. Unfortunately Holley
never made sidedrafts (AFAIK).
2) The 'lift the piston' test hits the 'perfect' mixture reasonably close.
3) My carbs (recently rebuilt by an amateur) seem to always go lean just as you
crack the throttle, then appropriately rich. Heavier oil (20W50) in the
dashpots helped, but did not eliminate this effect. Straight 50 weight
resulted in still going slightly lean, then very rich.
I'll add that my other readings have indicated that the common O2 sensor is
really only accurate very close to stoichiometric (perfect) mixture, which is
too lean for best power, and too rich for best economy. There exists a
wide-band O2 sensor which can accurately cover the entire mixture range, but
it's significantly more expensive. I plan to buy the kit next year ...
Have you checked that your vacuum and centrifugal advance mechanisms are
working correctly ?
Randall
59 TR3A daily driver
On Tuesday, October 24, 2000 4:28 PM, David Willett & Carol Gray
[SMTP:willgray@region.net] wrote:
>
> With help and advice from various people on the List (Thank you!) I
> have cleaned, rebuilt and balanced/"tuned" my SU carbs, but am still
> not entirely comfortable that I've got the mixture correct, as for
> one thing the gas consumption seems a bit excessive.
> Someone on the List referred me to an article at
> <www.team.net/www/morgan/tech/tuning.html> on "Tuning vs Setting SU
> Carburetors" by Fred Sisson, in which Mr. Sisson claims that the ONLY
> way to tune SU's is by means of an "on-board exhaust analyzer".Quote:
> "Anything else is groping in the dark"
> Has anyone had any experience with one of these things?
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