[DeTomaso] More Holley Tuning

[Daniel C Jones]

I got the chance to spend a few more hours tuning the 735 Holley on the
351C in my Pantera.  As initially installed, it way very rich across the
board but would go lean under initial part throttle, as if the accelerator
pump was not working but a visual inspection showed that not to be the
case.  Even so, I replaced the accelerator pump diaphragm as this engine
had been sitting on the shelf for several years.  To lean out the carb and
make sure I had no vacuum or power valve leaks, I did a number of things:

 replaced vacuum caps and the distributor vacuum line

 tightened the 90 degree vacuum fitting on primary metering block

 removed the metering blocks and squirted carb cleaner through passages,
 blew dry with compressed air

 replaced the inlet needles and seats, along with float adjusting nuts
 and plastic sealing washer

 replaced the idle mixture needles (large head style) and cork gaskets

 leaned idle mixture screws from 1 1/2 turns out to 1/2 turn out

 lowered the float levels on primary and secondary, the adjuster nut is
 flush with the plastic top and bottom washers in place

 reduced primary jets from #66 to #64

 replaced the 4.5" power valve with an unmarked power valve (likely
 a 6.5") from an AED rebuild kit, AED power valve is window style but
 has fewer windows than previous power valve

 bumped timing from 8 to 13 degrees BTDC to compensate for slower burn
 of leaner mixture

These changes had the desired effect of leaning out the carb but I went
too far and ended up with the carb lean across the board and cruise
mixtures in the 16's.  The previous owner installed a larger radiator
and fans which were wired up to a manual switch.  With the carb
calibrated rich, there was never a need to turn the fans on, even idling
in traffic it would stay cool.  With the very lean mixture, temperature
crept up in stop-and-go traffic and I could hear the coolant pulsing to
the overflow bottle when I turned the engine off.  Also, with the extra
timing, it would kick back slightly on the stock starter when hot.  With
the fans on, the engine stayed cool and no coolant noises were heard when
the engine was switched off.

For the next round of tuning, I went to 3/4 turn out on the idle mixture
screws and raised the primary float to richen the idle and primary
mixtures.  That ended up a little richer than desired.  Setting the idle
mixture screws back to 1/2 turn got me pretty close to where I want to
be.  It's now at low to mid 15's cruising in 5th gear between 60 and 80
MPH and drops to mid 12's at WOT.  Low speed cruise is in the high 14's,
low 15's and drops to low 12's WOT at low speed.  Should yield good
fuel economy and WOT power.

To tune a Holley for best power and fuel economy, you usually have to
alter the power valve channel restrictors (PVCR's).  The power valve
only determines the opening point when the additional fuel is added.
It's the PVCR's that how much fuel is added.  In general, PVCR's should
be no larger than one half the diameter of your main jets.  This ratio
of orifice size to jet size will give you a 25% increase in fuel flow,
approximately the difference in fuel flow required to make the difference
between a stochiometric (14.7:1) and a best power fuel air mixture.
Unfortunately, standard Holley carbs provide no easy way to change the
PVCR's so you have to modify the metering block, either drilling the
passages larger or epoxing them shut and re-drilling smaller.  Many
performance Holleys have huge PVCR's which make it impossible to get
reasonable WOT mixtures on milder (and smaller displacement) engines
but the 735 Holley was set up by Ford with reasonably sized PVCR's.
Also, there are aftermarket metering blocks and carbs (QFT is one
example) that have replaceable PVCR's.

The mystery vacuum leak was traced to the PCV grommet in the Boss 351
style Ford Motorsport cast aluminum valve covers.  The previous owner
installed the valve covers and used a push-in grommet that allowed the
PCV to pop out.  The first couple of times it happened, it was just
partially out and I assumed I knocked it loose when I re-installed the
air cleaner.   It finally popped all the way out which clued me in.
I replaced the grommet with the proper twist-in style (Ford part number
C7AZ-6A892-B for finned cast aluminum valve covers with 1 1/4" hole)
and now it stays put.

I had set the idle for maximum vacuum (1 1/2 turns out) which may be
okay for automatic transmissions (in gear with foot on brake) but is
not so good on an unloaded manual transmission.  With the wide band
hooked up, I could see the idle was too rich.  With the carb off the
car, I initially set the idle screws at the suggested 1 1/2 turns out.
In retrospect, it makes sense I would need to have the idle mixture
screws leaner.  A low compression 351C would need less idle mixture
volume than the high compression 428CJ the carb was designed for.
This particular carb has a bulky 90 degree vacuum fitting directly
above the passenger idle mixture screw.  That and the electric choke
make the mixture screw difficult to see which makes getting a screw
driver on the screw tricky.  I discovered the best way is to set a
small flashlight to illuminate the screw head and look through the
tinted 1/4 window.  That made it easy to change the idle mixture.

I had initially set the floats level with the carb off the engine.
When rocked, fuel would dribble out the sight hole as it's supposed
to but I believe that contributed to the initial rich mixture.  The
Pantera bolts the transaxle directly to the engine and the assembly
is mounted level in the car.  Most production cars have the engine
installed with the nose high, so most intake manifolds have a cant
angle that levels the carb out.  In the Pantera that tilts the the
carb nose down which affects the float level.  Lowering the float
level had the desired effect on the mixture but I still need to run
the car around to make sure the engine doesn't stall under hard
braking and cornering.

While setting the timing, I discovered a loose distributor wire.
Someone had simply taped a bare wire end to a connector blade.
I installed a matching connector on the bare end.

Given the idle vacuum, I could probably switch to a taller power
valve but what's in there now should work fine and be a bit better
for fuel economy.  Next on the list is playing with the secondary
spring.

Dan Jones