Help with Jetting
 

Hello, I am new to the hobby and looking for some direction on how to get started tuning my carb. I have a pontiac 428 with a Holley 750 4779. The carb has had the choke horn milled off. It has Doug's headers, edelbrock aluminum heads, and this CAM

The carb was freshly rebuilt by a reputable shop, but it's still not running right. It cranks right up, but dies right away unless I hold the gas pedal until it warms up a bit. Once warmed up it will idle fine. It bogs off of idle and seems low on power. Sometimes when its fully warmed up it doesn't want to re-start. Plugs look pretty black and I have a feeling its running rich. The exhaust is pretty strong and burns my eyes.

I have adjusted the 4 corner idle to the highest vacuum, although the vacuum is pretty weak. I'm assuming it's due to the cam.

Im looking for suggestions on which jets would be a good place to start with for my setup. I havent pulled the carb yet to see what's installed on the secondary side, but I took a peek at the primary side. There are 77 jets in there now with no power-valve, just a plug. Im wondering if I should install a power valve and jet down a few sizes. The accelerator pump nozzles look to be #25, they are hard to read so not 100% positive on that.

The car will not be tracked and will be used mostly for cruising.

Thoughts?

A carb without a choke on anything but a full time race car is your first mistake.

There set up far too rich to make up for having no choke.

I inherited this setup, just trying to make it run better. I realize not having a choke isnt optimal, but Im not at the point of wanting to replace the carb yet.

I have a 4779 on a 400, and a demon 825 on a 461. Neither has a choke tower. Both engines start just fine, but require a bit of throttle feathering for a minute or so until the engine develops a bit of heat. Then idles just fine.

I would start with 72-primaries and 76-78 secondary. Is this a vac-secondary
or mechanical? I usually run a vacume secondary on my 400-street motors.
With a ram air iv cam with 72-78 jets works pretty good for me.

GT

The first big mistake here is to running a blocked power valve in any Carb that needs to live any amount on the street.

What Cam is in the motor and who put that Cam in?

What Intake Manifold are you using?

Do you have a vacuum gauge?

With the motor warmed up and finally idling how much vacuum do you have

Jetting has not one dam bit of anything to do with

1) Cold start conditions

2) the idle

3) the off idle transition performance.

Why folks are offering you suggestions on jetting which is not part of your issue is a mystery.

Find a youtube video on how to check and adjust the holley float levels.
This is a baseline foundational carburation step on which nothing else carb related matters unless this is right, ... it could be flooding on you when it stalls, .. fuel filer anywhere?

Jetting, power valve, squirters google the list# and start with baseline parts they may not be optimal but functional


The car is new to you and you care about it, The prior owner may of had the best intentions but not the best results time to check everything ignition and timing wise as well as fuel and spark are often misdiagnosed for the others failures

Start here

http://www.forums.maxperformanceinc....ght=DP+jetting

I've been running a 4150 Holley with no choke for years. Cold start routine for me is run at 1800rpms for 15 seconds with idle set around 950. Dont chug it at low rpms, that will definitely foul your plugs. Holley has some nice tutorial videos. I'm guessing your squirter size is 35 not 25? Your car should scream with that CID/cam/heads

I mean you've probably got three unrelated problems. The cold start stuff is going to happen without a choke, especially with that cam in a 428. The off-idle bog is probably a lean condition at light throttle due to low signal to the carb from that cam. And not wanting to restart when hot could be a vapor lock issue or fuel boiling off.

I would disconnect the secondaries and put them out of your mind while you play with it, so you know they aren't involved. Most likely the jetting has nothing to do with your issues. I'd try a bigger primary nozzle first and play with the pump cam, they make a variety of them for different tuning issues. Is it a steady-state light throttle issue or is it during the movement of the throttle off-idle?

Also, get a wideband! It will teach you a lot.

Here is a link to the cam. It was installed by an engine builder
Click Me

Intake is an edelbrock p4b

Yes, vacuum is about 11 in hg

Im pretty sure they are 25's. Here is a picture.

Pic

It happens from movement of the throttle off-idle

Mechanical secondaries.

What are your timing settings (initial and total).
Idle rpm?

do the primary (front) nozzles squirt fuel IMMEDIATELY when you just barely move the throttle? if not, try adjusting your accelerator pump linkage.

Initial is 18, total is 31. Idle is around 900 RPM. CR is 11:1

are you using vac. advance. if so, limit it to about 12-14 deg. if not, install one. it is absolute necessary on a street engine, and works in conjunction with the power valve you are about to install.. without a power valve, you are running a wide open throttle fuel mix at all throttle openings. sometimes it is helpful to think of the power valve as an economy valve. as the power valve closes, do to less throttle and therefore more vacuum, the vacuum advance unit starts pulling in more timing to compliment the now leaner mixture. a good way to guesstimate on a power valve value is to record engine vacuum @ 60MPH, assuming you have vacuum advance.

I'm closing out for bit . modem problems

for reference, the stock jetting , with power valve, is 71/80. the stock squirters are .028/.031. I would, considering your C/R, start with a 73 pri jet and 82 sec. jet.
if your 60mph cruise vac is 16-18 in., I would use a 8.5"/10.5" PV. less vac, a proportionally lower PV.
the amount of secondary throttle opening , at idle, is part of the idle system. make sure it is to spec.
thank to I82 for jogging my memory about the accelerator pump. depress the accl pump lever all the way , let the excess fuel burn off , carefully check the off idle reaction.. if it stumbles , you might want to try the stock size squirter . if it takes the throttle without hesitation, you may need to adjust a gap between the arm and the lever, or put the pump cam in the "summer" position

gonna let the modem cool off. I'll be back.

for reference, the stock jetting , with power valve, is 71/80. the stock squirters are .028/.031. I would, considering your C/R, start with a 73 pri jet and 82 sec. jet.
if your 60mph cruise vac is 16-18 in., I would use a 8.5"/10.5" PV. less vac, a proportionally lower PV.
the amount of secondary throttle opening , at idle, is part of the idle system. make sure it is to spec.
thank to I82 for jogging my memory about the accelerator pump. depress the accl pump lever all the way , let the excess fuel burn off , carefully check the off idle reaction.. if it stumbles , you might want to try the stock size squirter . if it takes the throttle without hesitation, you may need to adjust a gap between the arm and the lever, or put the pump cam in the "summer" position

gonna let the modem cool off. I'll be back.

Sorry-- "summer" is the #1 position on the pump cam.
the 73/81 jets I suggested are probably a little rich ,but I want to keep NALVAREZ on the safe side of ping while performing the drivability tuning.

all of previous responders are assuming the carb hasn't been improved by an "expert"
POWER--. Two items are holding back the power output.
1: Intake manifold. designed in the mid sixties as an aluminum replacement for a stock '65 intake, you're chocking off about 30-40 HP.
2: timing. The Edelbrock heads need about (depending on version} need about 38- 41 deg. total timing for max performance. I am assuming timing is set to 31 total to suppress detonation.

Should a 428, with 11:1 compression, 18* of base timing and a XE cam with a 236 intake lobe, only make 11" of vacuum at 900 RPM?

sounds reasonable to me?

i82--me too.
What's your guess , Chip?

My gut reaction would expect 12-13" of vacuum on that combo, properly tuned and warmed up. Maybe the P4B, a non degreed cam and a generous PCV is reducing the number?

Maybe the carb's idle circuit is still way rich, and it would pick up if it were to be leaned some. That motor is likely going to want a 32-33 IFR and 70-ish LSAB, mixture screws 3/4" of a turn from seated. Just the fact that it's got a blocked power valve and a milled airhorn makes me wonder what else funky was done to it. That engine should pull hard to 5700. If it's not scaring the owner when he stomps it, something is seriously wrong.

Once he has been helped in getting his primary side set up to work well then it’s on to his second side that likely has a blocked power valve also.

As mentioned somewhat his combo of aftermarket heads flowing far higher than stock iron heads and a P4B intake is not the ticket for power production.

The runner expansion rate. ( port area ) is in the wrong direction.
So there’s two problems with that.
1) the motor will not make the hp power or torque you would expect, and the power band the motor does have will only extend over a narrow rpm range.

Get wide band O2 sensor for the AFR to put in the exhaust and you can see exactly what the carb is doing vs guessing! You can easily tune every circuit.

Start on the primary side first. Your bog off idle and low power feel is from lack of a front power valve. There is really no amount of pump shooter and cam that can overcome the missing power valve. People don't realize that even light throttle tip in will involve the power valve even for just a second or two and that also helps to cover the lean spot you created when you cracked the throttle open. Someone was messing with that carb that didn't know what they were doing, they never have a blocked power valve in the front for a street application, or even a street strip hot engine application.

First thing I would do is get a power valve in the front half of the carb. Typically start with a 6.5. If you go too high it may open too frequently at tip in and light throttle cruising. Typically a 6.5 is a good starting point and works well on engines that make 10-13 inches of vacuum. This will solve the issue of trying to cover a lean spot with a whole bunch of pump squirter.
After that knock the front jets down to about 72's, you don't need a bunch of front jet with a power valve. From there you'll have to make sure your front blades aren't exposing too much of the transfer slot so it's not idling off the main jet on top of the idle circuit and instead should be idling off the idle circuit only.

The power valve setup in front will solve your lean tip in stumble and weak power. Start there and report back. Then we can move on to other things.

Off idle even up to 2500 also involves the idle circuit as well as power and main circuits. If tuning idle by rpm ans or vacuum be sure if an auto it is ion gear and the convertor loaded.

Which Intake manifold would you recommend ? I can't use anything any taller than the p4b, as it barely clears the hood.

The carb came from DaVinci Performance Carburetors in Houston.

Right now, the mixture screws are about 1.5 turns out. At 3/4 turns out it wont even idle. It definitely does NOT scare me when I stomp it.

There is not currently a vacuum advance. It is capped off. Once I get the carburetor at least close, I plan to hook it up. Should I use ported or manifold vacuum? I read so much conflicting information on the subject.

Im actually going to order an afr this week.

I want to thank everyone for all of their insights. I have ordered some various jet sizes (70-80) and a 6.5 power valve to get started. I am also ordering an afr gauge to help with tuning. Based on everyone's responses, here's what I'm thinking of starting with.

73 primaries with a 6.5" power valve. 81 secondaries.

What about the squirters? What would be a good starting place for those? It has 25's in there now.

I looked up the carb. It's a garden variety 4150 double pumper, mechanical choke, non-adjustable airbleeds. The new ones are listed as 28 squirters in the front, 31s in back. A 428 is likely going to want a 31-34 squirter in front with a fast pump cam. Squirt speed depends on how tight the converter/gearing is.

I don't see mention of the transmission or rear gears. A double pumper is going to want a loose converter and/or deep gears to respond properly. Otherwise it's going to have a lazy response until the engine builds RPM.

I would suggest re-reading the thread I linked to in my first response. It's a detailed tuning thread for my QF double pumper on my 400. It'll save me from re-typing it here.

The idle circuit on a holley feeds the motor until 2500 RPM. So start there. I see where you mention that the car won't idle unless the mixture screws are 1.5 turns out. That's WAY too much for a 4-corner carb. A 4-corner carb will typically end up at 1/2-1 turn out in most applications. The QF on my 400 is at 5/8 of a turn from seated on all 4 screws.

I assume this means the transition slot is too exposed on the primaries, in addition to the secondaries being set improperly. This usually results in the idle screws being turned out too far, a high, unstable idle, blackening the plugs quickly, and a big hesitation when opening the throttle. The transition slot/idle throttle plate opening sets the table for the entire rest of the carb tuning.

Well, It turns out I have a new problem. I somehow managed to crack the baseplate on the carb. I used a torque wrench and only tightened it to 12 ft lbs... but I guess that's all it took. Looks like I will be getting a new carb. Im thinking Im going to try out a Quick Fuel SS-780-VS. I'll get that bolted on and report back with how it's running.

The P4B is actually about a 1/2” shorter than the factory 67-79 4bbl intakes, so clearance shouldn’t be a problem.
https://youtu.be/nHfmYUH4C-g?si=7o3JCxfOIVFibfvF