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Original Message
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| More info.... |
| By John - 12/29/2004 10:58:34 PM; IP 142.177.99.152 |
I did a search and found some of the stuff I once wrote....hope it helps. If you have any specific questions you'd like me to look up, feel free to e-mail me for a quick response, or eventually I will see your question in this forum. Here goes:
The linkage needs to be adjusted so the second carb comes on
after the first carb gets to about 1/3 open throttle. Then the second carb advances at a
faster rate so that both wind up fully open together. So, you may need to modify the
second carb bellcrank as the linkage pin is in the wrong spot on carbs designed for
single operation. Also, a air balance tube must be added. Instead of getting new
vacumn covers, you can drill and tap each existing cover for a grease fitting. Use the
90 degree ones and remove the balls and springs. Tighten and adjust till they point
towards each other for a tidy vacumn hose installation.
Pressure is nice about 7 to 8 psi. However you could have a fuel pump which is
inadequate (possibly) or a fuel filter which is restrictive (most likely), or even too small
fuel lines anywhere from tank to carb (so the experts say, but I havn't had any trouble
with lines smaller than 1/2". About those 600's though. The ONLY difference is to the
idle circuit as the idle adj is for fine tuning only. Two carbs give off twice the fuel they
should at idle with the mixture screws in the normal position, but will run OK, but still
rich in the fully closed position. The progressive linkage neeeds to be set up correctly
and make sure the carbs are monted in the right position as the belcranks are
different front vs back carb. If the fuel supply turns out to be OK, sounds like your
driveability is in the accel pumps or possibly the vacumn advance. Are the vacumn
advance connected together for synchronous secondary operation? If you use regualr
carbs, you can drill and tap and add fittings to the vacumn chamber caps and add a
balance tube. As far as the accel pumps go, check the colour of the cam and make
sure both are the same and the nominal type (I forget that colour, but any Holley
book will tell you that) and the 50cc pumps are a better bet than the smaller ones if
you use an after market HP cam. Maybe the spring rates should be changed, but this
hasn't had much affect in my experience.
The problem with 2X4 Holleys is, as noted earlier in this thread, the idle circuits.
Although the engine may draw the right mixture at speed, as the air fuel ratio is
governed by the main venturis, the idle circuits are set with the assumption that only
one carb is on the engine. Two carbs...twice the needed fuel at idle. The mixture
screws are only a "fine" adjustment, so you can turn them in all the way and it's still
too rich. The idle jets must be reduced in size until they are close enough to allow the
mixture screws to lean out the engine to a stall. They can be replaced, or a small wire
bent into a "V" with little feet bent on the ends, can be inserted in the jets. The
secondary idle circuit (used to bleed off fuel in a lightly used car so it doesn't get
stale) don't really require the same attention. You can fiddle around with the wires till
you get it right and probably spend $900 in float bowl gaskets, or you can get
someone to give you out of the box units with the correct vacumm balance covers.
Pony Carbs supplied me with two units that work perfectly with my Offy 360Deg
manifold (which many hold in disdain). You guys with other FE's out there might be
faster, but I havn't seen a Chevy yet that was. |
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