Reply to "Need new carb"

> have installed a 750 double Pumper holley on a stock 351C. Changing jets,
> power valves, etc it ran OK and a little rich. Replace the 750 double
> Pumper holley with a 700 double Pumper holley again changing jets, power
> valves, etc and it ran much better and was not rich at all.

There's more to it than just the carb CFM rating. The size of the PVCR's,
the idle mixture setting (volume) and the float level can have a huge
difference. The booster style can have an effect as well. An example is
the 735 Holley I'm tuning now on a near stock 351C (low compression CJ
with Edelbrock Perfomer intake and Hall big bore headers and exhaust).
I originally set the floats so fuel would drip out the sight hole when
the car was gently rocked and the idle for maximum vacuum. The carb was
rich across the board. By lowering the floats and leaning the idle mixture
(screws at 1/2 turn out), the carb is now lean across the board. Ideally,
I'd like a lean cruise (for fuel economy) in the 15:1 range that goes rich
(12.5:1 to 13:1 range) at WOT when the extra fuel from the PVCR is added in.
With the size of the PVCR's in this particular carb, that appears possible.
On many Holleys, that is not the case. Many Holley's have very large
PVCR's which make them run very rich at WOT. It's possible to re-size
the holes but few do. Luckily, there are Holley style carbs on the market
now with replaceable PVCR's. The magic of a wide band O2 sensor makes
this sort of fine tuning practical.

Be aware when you lean the mixture out, you'll need to increase the timing
to offset the slower burn of the lean mixture. If you don't adjust the
timing, it will run hotter (even at the same water temp, the piston tops
will be hotter). A sronger spark from a capacitive discharge ignition is
a big help when running lean mixtures too.

Dan Jones