1FSTLS1

iTrader: (9)

Any one know about what size is good for the accelartor pump dishcharge nozzle on a mild built 455 with a Holey 750? I have a .031 and I think I need to go bigger. It is a olds motor in a Trans Am

I have a mild 455 with a Holley 750 on it. Brand new motor seems to idle fine my problem is when I give it full throttle it either dies or hesitates then picks up. When driving it is fine after you get going. So after research I am thinking the accelerator pump nozzle is too small. It is a .031. So with no place to buy a jet I was able to find some super small drill bits. The bit that fit in there snug was .029 according to my caliper measureing device thingy. So I used the next size up .031 and drilled out the accellerator pump nozzle. I cleaned it out when I was done ect. This is probably not a common practice but I figured what the hell. Got it back in and it seems worse if anything not better. Did I just not drill it out far enough? Do I need to buy a .034-.035 jet? How about the accelerator pump cam should I be messing with that? Thanks for any input this is my first time with any carb and first motor build

sunnyd 95

From what you say its doing, I would say its more like the accelerator diaphram is bad.
Shut the car off take off the air filter and look down the carb. while your looking down the carb. move the carb linkage back and watch for fuel to come out of the squitters, it should come out as soon as you move the throttle linkage.
If it does'nt come out right away the accelerator pump(diaphram) is bad.
Try what I'm saying it might solve your problem. Good luck

1FSTLS1

iTrader: (9)

The carb is brand new. Fuel comes out of the nozzle right away when you move the linkage.

1 FMF

iTrader: (1)

drilling the discharge nozzles bigger will only cause the fixed volume of fuel supplied by the accelerator pump to dump quicker in a bigger stream. After drilling them and you say it is worse, then I suspect you might have been too rich on that circuit to begin with.
also, since you say it's your first time with carb, best advice would be to pick up a book on holley carbs off amazon for around $20.
I recommend Motorbook's "carburetor performance, how to tune & modify" and also "how to build horsepower vol.2" by david vizard. both books are excellent and will explain everything you need to know in order to tune your holley.
you say carb is brand new, can you provide a list # to it? that way we'll know what size jets are in it, the size of the accelerator pump, the power valve number, secondary diaphram spring, etc. Most people (because they don't know carbs) fixate on the accelerator pump being the problem when the car doesn't accelerate well, when most of the time it can be a number of other things.
fortunately you should be able to buy new squirters to replace the one's you drilled,
in most cases you should never need to physically drill out something to where it's a permanent irreversible modification. There will be a replacement part of a different size if there is reason for it to be changed.... such as the accelerator pump cam like you guessed.
also know, the purpose of the accelator pump is to supply an adequate amount of fuel only when the throttle is rapidly opened. in this situation when you rapidly open the throttle, it causes a sudden loss of vacuum single to the carb which has no way to supply extra fuel needed at that moment, so the accel. pump circuit is set up to supply a strong squirt of fuel. what happens a lot of times is an acceleration problem is corrected by incorrectly adjusting the accel. pump to supply fuel, when the real problem elsewhere such as too low a float setting, incorrect secondary diaphram spring or wrong power valve, etc.

what i would do is define the acceleration problem more specifically. At what rpm and engine load does it happen at? And are you rapidly mashing the pedal to the floor?
If you give it easy consistent pedal to wide open throttle and it bogs then it most likely is not the accelerator pump. And you mentioned mildy built, if you have a long duration high lift cam, the vacuum signal to the carb can get a little hokey and actually cause a rich condition as you accelerate (and then modding the accel. pump to shoot more only worsened it). On the other hand, you might just be leaning out which could simply be corrected by richening the idle mixture slightly, going up one jet size, or changing the power valve to a higher number.

1FSTLS1

iTrader: (9)

This is the carb I have:

Holey 4160

http://store.summitracing.com/partde...0&autoview=sku

When in gear (TH350) with foot on the brake (because I dont have tags and the police man is home) when I mash it to the floor it stalls out. When I bring the RPM up real slow then get to about 1600-1700rpm then mash it to the floor the tires brake loose and no problems. (all this is with the air cleaner on the car) So I am thinking that my .031 acceleator pump nozzle is too small

xx_ED_xx

iTrader: (17)

Whats DA in maryland? When I had the 454 in my 67 firebird I had to step up to the biggest pump cam and it helped fix some of my issues

1 FMF

iTrader: (1)

http://www.amazon.com/gp/offer-listi...3356028&sr=1-1


http://www.amazon.com/gp/offer-listi...3356158&sr=1-1

from that description, what i would try because it's easy and to start ruling things out,

loosen the nut on the accelerator pump so there is a liitle slack in the arm, this will cause less fuel to be squirted when you mash the pedal. Try a few times loosening it more and more so less and less fuel is squirted, if it helps then that may be indicating you're too rich and dumping too much fuel. first thing is make sure you're not dumping too much fuel.

if that doesn't do it, restore the accelerator pump arm to original and check your fuel level in the bowl. You have adjustable floats, so adjust it so there's slightly more fuel in the bowl and see if that helps the problem, if it does then you're having a general lean out condition due to lack of fuel which would be most likely. And if it's from an 800-1000 rpm idle & you mash the pedal quickly then that is the accel. pump circuit. What you can do here in addition to slightly raising the fuel level in the bowl is also adjust your idle mix screws- turn in CW to lean it till idle drops, then turn out CCW to richen it (past original position) to where idle drops again. Remember the middle point between that range you just turned the screws, that's where you should typically set it, instead go slightly richer around 1/4 to 1/2 turn CCW, but don't go too far CCW to where it idles terrible. If the higher float setting plus this richer idle mixture helps the problem then it's indicating fuel starvation. If you have the 30cc accel. pump then you most likely need the 50cc pump, not sure if it comes in the trick kit or if it is its own part, i know the accel. pump cams come in the kit.

holley list # 80508 =
model 4160, 750cfm, renew kit 37-754, trick kit 37-933,
main jet = 122-72, secondary metering plate = 134-21, power valve = 125-65,
pump nozzle size = 0.025",
and i believe it's a 30cc accel. pump.

----
from holley catalog:
http://www.holley.com/data/TechServi...ech%20Info.pdf

The accelerator pump system consists of three main components:
the pump diaphragm, the pump cam and the pump nozzle.
This is the carburetor system that is most responsible for having
good, crisp, off-idle throttle response. Its purpose is to inject a
certain amount of fuel down the throttle bores when the throttle
is opened. By accomplishing this purpose it acts to smooth the
transition between the idle and main circuits so that no stumble,
hesitation or sluggishness will be evident during this transition
phase.
The first adjustment to check is the clearance between the pump
operating lever and the pump diaphragm cover’s arm, at wide
open throttle. This clearance should be around .015". The purpose
for this clearance is to assure that the pump diaphragm is
never stretched to its maximum limit at wide open throttle. This
will cause premature pump failure. Once this clearance has
been set take a good look at the pump linkage and work the
throttle. Make sure that the accelerator pump arm is being activated
the moment that the throttle begins to move. This will
assure that pump response will be instantaneous to the movement
of the throttle. These adjustments can be made by turning
the accelerator pump adjusting screw that is located on the
accelerator pump arm together with the pump override spring
and lock nut.
The amount of fuel that can be delivered by one accelerator
pump stroke is determined by the pump’s capacity and the profile
of the pump cam. The period of time that it will take for this
pre-determined amount of fuel to be delivered is affected by the
pump nozzle size.
A larger pump nozzle will allow this fuel to be delivered much
sooner than a smaller pump nozzle. If you need more pump
shot sooner, then a larger pump nozzle size is required. During
acceleration tests, if you notice that the car first hesitates and
then picks up, it’s a sure bet that the pump nozzle size should be
increased. A backfire (lean condition) on acceleration also calls
for a step up in pump nozzle size. Conversely, if off-idle acceleration
does not feel crisp or clean, then the pump nozzle size
may already be too large. In this case a smaller size is
required.
Holley accelerator pump nozzles are stamped with a number
which indicates the drilled pump hole size. For example, a
pump nozzle stamped “35” is drilled .035". Pump nozzle sizes
are available from .025" to .052". Please note that whenever a
.040" or larger accelerator pump nozzle is installed the “hollow”
pump nozzle screw should also be used. This screw will
allow more fuel to flow to the pump nozzle, assuring that the
pump nozzle itself will be the limiting restriction in the accelerator
pump fuel supply system.
NOTE: When changing the pump nozzle it’s best to jump
three sizes. For example if there’s currently an off-line hesitation
with #28 (.028") pump nozzle, try a #31 (.031") pump
nozzle. If you must use a #37 (.037") or larger pump nozzle,
then also use a 50cc pump.
The same applies to the accelerator pump cams. Once a
pump nozzle size selection has been made the accelerator
pump system can be further tailored with the pump cam.
Holley offers an assortment of different pump cams, each with
uniquely different lift and duration profiles, that are available
under Holley P/N 20-12. Switching cams will directly affect
the movement of the accelerator pump lever and, subsequently,
the amount of fuel available at the pump nozzle. Lay out the
pump cams side by side and note the profile differences. This
little exercise may help to better explain the differences
between the cams and their effect on pump action.
Installing a pump cam is straightforward. It’s a simple matter
of loosening one screw, placing the new pump cam next to the
throttle lever and tightening it up. There are two and sometimes
three holes in each pump cam, numbered 1, 2 and 3.
Placing the screw in position #1 activates the accelerator pump
a little early, allowing full use of the pump’s capacity.
Generally, vehicles which normally run at lower idle speeds
(600 or 700 RPM) find this position more useful because they
can have a good pump shot available coming right off this relatively
low idle. Positions #2 and #3 delay the pump action,
relatively speaking. These two cam positions are good for
engines that idle around 1000 RPM and above. Repositioning
the cam in this way makes allowance for the extra throttle rotation
required to maintain the relatively higher idle setting.
Pump arm adjustment and clearance should be checked and
verified each and every time the pump cam and/or pump cam
position is changed.
Lastly, a 50cc accelerator pump conversion kit is available under
Holley P/N 20-11 when maximum pump capacity is desired.

1FSTLS1

iTrader: (9)

Thanks for all the info I ll check it out one day this week

1FSTLS1

iTrader: (9)

Apparently the butt hole who put the nozzle back in had it upsidedown. So I fixed it and is seems better less hesitation but still not perfect. I also tried backing off the spring tension on the acellartor pump arm and that made it worse so I think my problem is lack of fuel. I need to get a bigger nozzle and see what happens. Then get some tags on this bad boy so I can drive it hard on the road and get this new motor broke in.

LM97Z

iTrader: (4)

Was this a new carb??, putting the nozzle on upside down is a 1st. for me, especially when the nozzle has a flat end & tapered end for the screw, & you can clearly see the discharge holes . If this carb was jacked with, theres NO telling what else might be screwed up, before you do anything to the squirters, make sure the fuel level is set, & the air bleeds are clear, another thing is if your running a PV, make sure its the right size for the amount of vacuum your pulling, check the vacuum "in gear", then pick a PV 1.5 too 2 sizes below that reading, so if your pulling 9.0*, get a 7.0 too 7.5 PV, you may not necessarally need a larger nozzle, you may just need a longer duration, i myself like the 50cc pump & brown cam (#1 hole) up front with a 31 squirter & 30cc in back, or pink cams F&R (#1 hole) with 33 or 35 squirters, another trick is in-largeing the PVRCs, power valve restrictor channels. theres many little things that can cause the carb to stumble IF the obvious stuff is set right (float levels & mixture screws). If you need any tips or advice feel free to PM me.

1FSTLS1

iTrader: (9)

Thanks no telling how many beers I had when I put that nozzle in upside down. The carb is brand new.