> I've never been good at carb tuning.
If you don't have a wideband O2, you're just guessing. Weld a bung
into a tail pipe just aft of the collector and buy or borrow a
wideband.
> I got a 750 Dp on the car now. It seems to be running pretty good for the
> moment.
I've tuned carbs that ran flawlessly (no hesitation, flat spots, or bogs)
but were very rich or dangerously lean. Seat-of-the-pants won't tell you
what the air-fuel ratio is.
> When adjusting the idle mixture,the engine seems to run best (highest
> vacuum/idle speed) with the nedles turned almost all the way in.
Though it is often recommened, highest vacuum/idle speed is not a good
way to set the idle mixture. It works a bit better in a vehicle with
an automatic transmission (in gear with foot on brake) but is not good
for an unloaded manual transmission. The usual starting point for idle
mixture screws is 1 1/2 turns out. On my Pantera, that also provided
the maximum vacuum but, with the wide band 02 sensor hooked up, I could
see the idle was too rich. Leaning the screws to 1/2 turn in provided
the desired idle mixture. 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.
> Does this mean that I'm running rich and need to down size my jets?
> This goes for all four corner adjustments.
No. The primary jet has no effect on the idle air-fuel ratio. If fuel
is dripping out of the boosters at idle, you have other problems. The
float level setting can have a big effect on idle, though. The float
level sets the pressure that drives the idle fuel. On my car, I
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
the Pantera bolts the transaxle directly to the engine and the assembly
is mounted level in the car. The intake manifold I have has a canted
carb pad so the carb sits nose down which affects the float level.
I had to lower the float level quite a bit to lean sufficiently lean
the idle mixture. Be aware if you go too far, the engine may stall
under hard braking and/or cornering. Check the condition of the little
idle needle gaskets.
You want to run lean of stoichiometric at cruise for fuel economy and rich
of stoichiometric at wide open throttle (WOT) for maximum power. 14.7:1
is the stoichiometric ratio for gasoline. It's the "chemically ideal" ratio
where there is no excess fuel or oxygen left after combustion. Leaner means
there's excess oxygen left after combustion. Richer means there's excess
fuel left. Rich of stoichiometric at wide open throttle will make better
power and lean at cruise will yield better fuel economy. There's no single
ideal ratio that applies to all engines. Some engines make best power at
13:1, others closer to 12.5:1. 12.8:1 is a good WOT ratio to shoot for.
Note that the air fuel ratio is by weight. 13:1 means 13 pounds of air
are mixed with 1 pound of fuel. The usual target values for normally
aspirated 4 stroke engines are about 12.5 to 13 for WOT, 14.5-15.5 at
part-throttle cruise and 13.5-14.0 for part throttle acceleration (or
climbing a long hill, pulling a load, etc.). If you want to lean out the
mixture at cruise for best fuel economy, be aware that you'll also need to
adjust timing. Combustion gets much slower under lean conditions and if
you don't adjust spark timing, the combustion occurs much later and exhaust
temperature climbs. That's bad for the exhaust seats and valves. However,
if you adjust for MBT spark at each A/F ratio, exhaust temperature will
actually decrease relative to stoichimetric (rich will still be somewhat
cooler). For typical gasoline engines, the range or ratios is:
A/F Characteristics
Ratio
5 Rich burn limit. Combustion is weak and/or erratic.
6-9 Extremely rich. Black smoke and low power.
10-11 Very rich. Some supercharged engines run in this range at full power
as a means of controlling detonation.
12-13 Rich. Best power A/F for normally aspirated WOT.
14-15 Chemically ideal. At 14.6 the A/F is at the theoretical ideal ratio
with no excess fuel or oxygen after combustion. Good A/F target for
part throttle cruise and light to moderate acceleration.
16-17 Lean. Best fuel economy A/F ratio. Borderline for part throttle
drivability (worse than borderline if EGR is used).
18-19 Very lean. Usual lean limit (Driveability).
20-25 Lean burn limit. Varies with engine.
The previous owner of my car had the carb exceedingly rich. Spending
some time with the wideband, I was able to get my mixture in the low to
mid 15's cruising in 5th gear between 60 and 80 MPH and with 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. The car drives much better than it did before and MPG
has improved by more than 5 MPG. Last weekend on a 500 mile round trip
I got 20 MPG running 75+ MPH.
To tune a Holley for best power and fuel economy, you often 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, including the 750 DP you have, have huge PVCR's
which make it impossible to get reasonable WOT mixtures on milder (and
smaller displacement) engines. You can epoxy these shut and drill
them out to the desired size or there are aftermarket metering blocks
and carbs (QFT is one example) that have replaceable PVCR's. The carb
I used was a 735 Holley from a 428CJ application that was set up by
Ford with reasonably sized PVCR's.
Dan Jones
