I was wondering if anybody could tell me what the small screw inside the
plastic cap on the right side of the carb does. The Haynes manual I have
refers to this screw as a "fine idle adjustment screw." The reason i'm
asking is that i'm trying to get the car to pass emissions ('75
Spitfire).
I took the car through emissions once and my CO level was 412ppm, the
max allowable for my state is 300ppm. I did realize the exhaust had a
smell like the car was on the rich side of the mixture. I did as the
manual suggested, and lifted the slide on the carb slightly (1/16"), and
the engine did indeed speed-up. The manual said that this was the sign
of an overly rich mixture. I then proceeded to adjust the previously
mentioned screw, it was screwed fully clockwise when I started. The
manual said this screw was for fine adjustment of the mixture in one
part of the book, and fine idle adjustment in another. Anyway after
backing out the screw a few turns, I got the motor to run evenly and not
to rev so much when lifting the the slide. The exhaust seems to be less
rich, hard to tell without an analizer, and the car runs much better on
the road. However I cannot get the idle to below 1000rpm, even with the
stop screw for the butterfly backed all the way out.
I guess my real question is should I switch to adjusting the needle
itself, and then move back to the external screw? And what is the
correlation between the needle adjustment, and the "fine idle adjustment
screw?" Is this screw a mixture adjustment, or an idle adjustment?
I took mine completely apart (including the parts they warn you not to
disassemble) and have spend many weeks battling it. With some help from two
experts, I learned how to battle this.
The fine (and coarse) adjustments are bypass bleeds. As you back the screw out
you are leaning the mixture by allowing more and more air around the piston and
butterfly. Yes, it leans the mixture, but as you learned, it revs the engine
up too. It's really a backaswards way to adjust the mixture, or more, to
comensate for an incorrectly adjusted mixture.
After much dinking around, I did what I was told and fully seated that screw
(and the coarse, becuase in my case it had been (gasp!) tampered with). Set
the main needle until the engine ran right, and only then started tinkering
with the idle bypass screw. Which actually gets ahead of me. In that settin
the needle where I was told (flush with the bottom of the piston), the engine
now runs so nicely I haven't done anything. It idles just fine, and is only a
little ragged. It's so much nicer then it's been for weeks that I'm just
enjoying what I've got right now.
Hope this helps some.
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