List,
Keith has solved his problem by changing carbs, but could have
corrected the original with a little work.
The problem was due to being lean in the 'off idle' or 'transition
circuit'. This overlooked and misunderstood circuit is responsible for
almost all off idle flat spots.
Hold your Holley above your head right side up and notice the idle fuel
port clearly below the primary butterflies. Also note the transition slot
partly covered by the butterfly (throttle closed). If at your best curb
idle speed adjustment more than 40 thou of this slot is exposed below the
butterflies you will very likely get an off idle flat spot (lean) until the
main circuit kicks in. As George suggested the best way to get this
butterfly to transition slot phasing corrected if you need lots of throttle
opening to feed a low vacuum engine at idle, simply drill a hole in each
primary butterfly close to the transition slot to allow more air through
and close the butterflies to correct position.
Keith was obviously running on this lean circuit which can give a bunch of
revs with little throttle opening on an unloaded engine. To richen this
circuit you can decrease the idle/transition air bleeds in the top of the
main body. They are the outer larger holes which altering is only becoming
an option on new racing style Holleys and BG's. Alternately open up the
idle fuel restrictors in the primary metering block face, facing into the
fuel bowl as in most double pumper carbs. If these restrictors aren't
fitted there, they are under the 2 outer core plugs on the top edge of the
metering block. Just make sure you have replacement plugs before you tear
them out. Some tap these core holes and tap for threaded plugs.
If your not the patient type, you can often cover an off idle flat spot by
increasing the accelerator pump shooter nozzle size and/or volume, but this
is only a band aid deal.
Can't comment on your black plug 3 x 2's Doug. I ran progressive 3x2s on my
first rod. They were 3 1956 Ford 2V's on an old 57 Ford 'Y' block. Half
of the central carb gave me 70 mph and superb (for its day) acceleration
when I hit it and the outside jugs came on line. Really great deal, never
lost tune once set up.
Chris Harris. New Zealand.
|