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.