Mike:
Drilling a hole in the butterfly is for gnarly cams that won't idle,
and need
to have the idle speed turned way up. On downdraft Holleys and Quadrajets,
opening the butterfly valve too far to get the idle speed up activates the
transition circuit, making the idle mixture rich so the car bogs when leaving
a stop light. Drilling a hole allows you to back off the idle setting while
keeping the idle speed decent, and thus avoid activating the transition
circuit.
Since there is no transition circuit in the Zeniths, you probably
should look
elsewhere. Are your dashpots topped off? Is your timing correct (10 BTDC with
the dizzy vacuum disconnected)? Are your carb diaphragms in good condition?
Are your air valves moving up and down freely, with no binding?
If all of these are correct, here is my suggestion - is your carbon
canister
still connected? If so, you have a vacuum leak. The factory cam creates enough
idle vacuum that the air flow from the canister is well tolerated. But with a
larger cam, vacuum drops and the engine will not tolerate the vacuum leak from
the canister. Try plugging the larger hose between the canister and the carbs
at the *canister* end. You will probably need to reset the idle speed. If that
helps, you can fabricate a restrictor to fit in the hose to reduce the vacuum
leak. That has worked very well for me with my 270 degree cam, and leaves the
emissions systems operative (we get EPA tested here in Oregon). I do not
recommend simply plugging the hose permanently. Don't forget to reset your
mixture. The loss of air from the canister will cause your mixture to go
slightly rich, so you will need to lean it to get it back to the correct
ratio.
BTW, using the "lift the slide" test to set your mixture is invalid if
you
have cammed your engine. You *MUST* use a Color Tune, O2 sensor, or plug color
to set the mixture with any sort of road cam. The lower engine vacuum
invalidates the results of the lift the slide test, and will give you (most
likely) a rich mixture.
Vance
Vance Navarrette
Cogito Ergo Zoom
I think, therefore I go fast
-----Original Message-----
From: 6pack-bounces@autox.team.net [mailto:6pack-bounces@autox.team.net] On
Behalf Of michael lunsford
Subject: [6pack] Lurching/clunking
I have been trying to run
down an annoying tendency my car has to lurch when I get off the throttle or
under light constant throttle.
<snip>
I am beginning to suspect
that the problem may be related to the S-2 cam and lightened flywheel I
installed during my engine rebuild. My son said lurching of this sort was a
problem on Mustangs with high lift cams and that the fix was to drill a small
hole in the butterfly of the carb. Any thoughts on the butterfly hole idea
or
any other suggestions as to the source of the problem?
Mike Lunsford
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