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going fuel rich??

PostPosted: September 2nd, 2013, 6:16 pm
by grumpyvette
z28 wrote:Okay, so I got a Skip White 406 short block and added AFR 1054 heads, Victor Jr. intake & Quick Fuel 850SS carb (as recommended by S.W.) It is 10.4:1 compression.
After running only a 12.9 @ 106 I thought something was wrong.
So I got a wideband o2 sensor hooked up & the a/f gauge reads 12.5 - 12.9. That's great but....
It is running rich under acceleration. From the point of even touching the pedal and accelerating, it drops down to between 10 & 11:1 and stays there until I let off the gas to where it returns to the 12.5 - 12.9 range.
The carb has size 85 jets. Any thoughts? I would hate to mess with the mixture screws to screw up the idle/cruising performance to compensate for wide-open throttle performance. It would be nice to see and use full potential of this motor. Thanks in advance!



There are several potential causes but a bit of testing will isolate the source, at this point it sounds like the power valves adding too much fuel too soon, Id suggest leaning it out a bit and check your fuel pressure is consistent at the carb inlet port and float levels are correct, if your not running a return style fuel pressure regulator it may be related to fuel flooding the fuel bowls as the throttles opened rapidly allowing the needle/seat to open and fuel pressure holding it open longer than ideal. if your cars got a fuel pressure gauge Id suggest setting the carb inlet pressure at 4.5-5 psi,and checking your fuel bowl float levels.
don,t guess do a few tests and document the result of changes made.




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Re: going fuel rich??

PostPosted: September 2nd, 2013, 6:58 pm
by grumpyvette
Z28 wrote:When I ran it, I ran a 13.2 with 4.5 fuel pressure. I raised it to 6.5 and ran the 12.9. I didn't try for higher, I thought 6.5 was pushing limits. I do have a return line.

Another thing, the total timing is set at 34.

Fuel float bowl levels are right at the bottom of sight glass.

Through a couple of trial & error tunes and backfires through the carb, I replaced the power valve with a brand new one. This was all prior to the race track. I'll research that a bit more though.


are you referring to power valve NUMBERS or fuel pressure measured at the carb inlet port?
fuel pressure should not exceed 5 psi in most carb applications
if you want to lean it out youll want a lower numerical power valve, IE if your now getting richer as the throttle opens Id go back to a 4.5 power valve and slightly larger primary carb jets, keep in mind jets provide the base line ,power valves ADD fuel as the vacuum drops as air flow increases.
btw Ive generally found you would be best served trying for about 13.7:1-14.2:1 fuel/air ratio at idle with a consistent and predictable but slow enrichment curve that gets you close to about 12.6:1 at wide open throttle above about 3500rpm, this tends to reduce plugs getting fouled

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POWER ENRICHMENT SYSTEM
The power enrichment system supplies additional fuel to the
main system during heavy load or full power situations. Holley
carburetors utilize a vacuum operated power enrichment system
and a selection of power valves is available to “time” this sys-
tem’s operation to your specific needs. Each Holley power
valve is stamped with a number to indicate the vacuum opening
point. For example, the number “65” indicates that the power
valve will open when the engine vacuum drops to 6.5" or
below.
An accurate vacuum gauge, such as Holley P/N
26-501,
should be used when determining the correct power
valve to use. A
competition or race engine which has a long
duration high overlap camshaft will have low manifold vacuum
at idle speeds. If the vehicle has a manual transmission, take
the vacuum reading with the engine thoroughly warmed up and
at idle. If the vehicle is equipped with an automatic transmis-
sion, take the vacuum reading with the engine thoroughly
warmed up and idling in gear. In either case, the power valve
selected should be 1/2 the intake manifold vacuum reading
taken. EXAMPLE: 13” Hg vacuum reading divided by 2 = 6.5
power valve. If your reading divided by 2 lands on an even
number you should select the next lowest power valve. EXAM-
PLE: 8” Hg vacuum reading divided by 2 = 4 power valve.
Since there is no #4 power valve you should use a 3.5.
Most of the popular Holley “Street Legal” and “Street
Performance” carburetors incorporate a power valve blow-out
protection system. A special check valve is located in the throt-
tle body expressly for this purpose. This check valve is
designed to be normally open but will quickly seat to close off
the internal vacuum passage when a backfire occurs. Once
closed, the check valve interrupts the pressure wave caused by
the backfire, thus protecting the power valve.
If you have a carburetor older than 1992 (or you have experi-
enced an extreme backfire) and expect a blown power valve,
use this simple test. TEST: At idle turn your idle mixture screws
(found on the side of the metering block) all the way in. If your
engine dies the power valve is not blown

Re: going fuel rich??

PostPosted: September 4th, 2013, 4:26 pm
by grumpyvette
Z28 wrote:when run with 4.5 pounds of fuel pressure at carb inlet:
R/T was .296
13.221 @ 104.27. 1.976 60' time

Run with 6.5 pounds of fuel pressure at carb inlet:
R/T was .122
12.990 @ 106.40. 1.882 60' time


theres two distinctly different issues here!
lets look at what your saying, and apply a bit of logic, at a fuel pressure of 4.5psi the car runs a 13.221 @ 104.27. 1.976 60' time, yet
If your running 4.5 psi and the carb is running the slightly rich but nearly correct 12.7-12.9:1 f/a ratio, it should continue to do so under acceleration unless the fuel supply from the pump is restricted, in which case the sensors and plugs would, or should indicate a LEAN MIX RATIO, but instead its running RICH, at nearly 10:1 at the sensors, but if it was really 10:1 your power would be reduced.
yet the car seems to run better, when the fuel pressure increased to 6.5 psi ,this is simply indicating your car runs better with more fuel, or a richer fuel air ratio,than the carb is currently supplying at the 4.5 psi fuel setting, but your not getting that increase in power from the richer mix indicated in a controlled manor or the sensor would not read a 10:1 F/A ratio, because at a true 10.1 ratio power would fall, from a properly adjusted carb, your getting it by having the carb basically flooding at wide open throttle, or the power valve your using is adding a good deal more fuel than ideal, yet your getting better power, you should be having a rather consistent fuel air ratio closer to that 12.7:1 ratio, if your getting better power at the higher fuel pressure either the sensors giving you the wrong ratio info or its being doused with raw fuel, not a controlled fog from a properly adjusted carb, making the sensor read rich in error.

REMEMBER WHAT YOU POSTER EARLY IN THE DISCUSSION??
Z28 wrote:
So I got a wideband o2 sensor hooked up & the a/f gauge reads 12.5 - 12.9. That's great but....
It is running rich under acceleration. From the point of even touching the pedal and accelerating, it drops down to between 10 & 11:1 and stays there until I let off the gas to where it returns to the 12.5 - 12.9 range.
The carb has size 85 jets. Any thoughts? I would hate to mess with the mixture screws to screw up the idle/cruising performance to compensate for wide-open throttle performance. It would be nice to see and use full potential of this motor. Thanks in advance!

think about it, you want to run that 12.7:1 fuel air ratio, because thats where an engine makes the most torque, and from your statements ,your sensors read rich because the fuel is either not being correctly mixed or atomized,or the power valves adding too much fuel, because the sensors read closer to a 10:1 F/A ratio, but at 6.5 psi of fuel pressure the carbs most likely effectively flooding and supplying close to what you actually need in a fuel air ratio (close to that 12.7:1 but not correctly atomized hence the sensor reads 10:1,and the rapid increase in air flows effectively partly masking the problem and by chance helping to correct what would be a lean mix other wise, with the 4.5 psi fuel supply.
This can be corrected with a bit of testing, but a 10:1 f/a ratio will eventually cause problems, get the fuel/air mix consistent at about 12.7:1 and in a controlled manor your times and speed will improve even more!

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