Holley carb diagnosis

440 engine, about 375 -400 HP--- Edelbrock RPM performer---750 CFM double pump , mechanical secondaries, out of box jets, power valve etc. ---electronic ignition , set up right ---3:23 sure grip.
Rolling start from idle, If I mash the gas at anything below 2000 RPM , I get a major bog. 2000 RPM or anything higher it gets right with it. No problem If I ease into the gas pedal. If I launch from stopped, I have to spin the tires to not get below 2000 RPM. If I use tires that get enough traction to not be able to spin the tires ---major bog.
Is the mechanical secondaries the problem -flopping the secondaries open to far and to fast, and would vacume secondaries with changeable diaphram springs solve the problem? I notice that Holley does not recommend mechanical secondaries for anything but auto trans . lighter weight cars...............................MO

On my holley carbs I run the jet extensions with the notched float in the rear bowl, and a rear power valve block off, then just increase the rear jetting. This would be if you think the fuel is sloshing away from the jets. More likely, it is just the mechanical secondary opening too fast. I usually try a different accelerator pump cam and if needed, start opening the accelerator nozzle diameter to get a larger and longer accelerator pump shot.
The Vacuum secondary carb would be my choice, but you already have the carb, so may as well tune on it.

Cornpatch MO said:

I notice that Holley does not recommend mechanical secondaries for anything but auto trans

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Holley recommends double pumpers for manual transmissions, or automatics with high stall speed converter.

Go up in primary jet size ... that's assuming your accelerator pump is absolutely dialed in, and timing correct ... primaries on that carb are what, 72? I'd go to 74 and see what happens ...
Also, I would disconnect the secondary linkage to verify if the stumble is on the primary side or secondary. If it's still there with the secondary disconnected you know its in the primary side.

I would look at accelerator pump arm adjustment first and then float level. fuel pressure can play a big part. that engine should easily handle a 750dp. 750dp's only have about 6sqin's of venturii area, pretty small for 440 cubes

Mechanical carbs can be made to work just fine with 727 & heavier cars. It takes an understanding of how the carb functions, adjustment parts(different shooter sizes, accelerator pump cams, jets, power valves) and patience. You need to try different combinations and several iterations may be needed. Changing one piece affects other pieces, which is why you need to understand the function of each sub-system in the carb. As said, good fuel pressure, proper float levels, idle adjustment screw setting. The jet extensions might help. One other overlooked "trick" is the idle speed (throttle position) adjustment for the secondaries. That's usually a set screw under the base plate. Small adjustments there can make real changes. Proper torque converter selection is also key with a "heavier" car & taller gears. You may not need to

Don't forget to try advancing the timing a little. I had the same issues. For me, alot of it was timing, and accelerator pump shooters. Sometimes you have to try all the ideas here, one at a time, so you know what helped.

66Satellite47 said:

Mechanical carbs can be made to work just fine with 727 & heavier cars. It takes an understanding of how the carb functions, adjustment parts(different shooter sizes, accelerator pump cams, jets, power valves) and patience. You need to try different combinations and several iterations may be needed. Changing one piece affects other pieces, which is why you need to understand the function of each sub-system in the carb. As said, good fuel pressure, proper float levels, idle adjustment screw setting. The jet extensions might help. One other overlooked "trick" is the idle speed (throttle position) adjustment for the secondaries. That's usually a set screw under the base plate. Small adjustments there can make real changes. Proper torque converter selection is also key with a "heavier" car & taller gears. You may not need to

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What he said. You would have had less of a "time" with a vacuum secondary carb, and that's the right design for your combo.

451Mopar said:

On my holley carbs I run the jet extensions with the notched float in the rear bowl, and a rear power valve block off, then just increase the rear jetting. This would be if you think the fuel is sloshing away from the jets. More likely, it is just the mechanical secondary opening too fast. I usually try a different accelerator pump cam and if needed, start opening the accelerator nozzle diameter to get a larger and longer accelerator pump shot.
The Vacuum secondary carb would be my choice, but you already have the carb, so may as well tune on it.

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I have tried all of these things , and eventually returned everything back to stock out of the box. Only thing I didn't try was the jet extensions, but I don't think it is rear bowl fuel starvation. If I launch at above 2000 rpm and let the tires spin enough to keep at or above that, --no problem
Thanks, I am mostly interested in opinions on if vacume secondaries might help. .........................MO

Photon440 said:

Holley recommends double pumpers for manual transmissions, or automatics with high stall speed converter.

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Yes, agreed. But in both cases , launching with higher RPM. I am concerned with rolling and punching it at idle speed . Thanks......................MO

What is the exact timing you are running?
Are the float levels set correctly?

If the answer is yes, I would not think it's a carb issue. You do not have much rear gear at 3.23. Do you run a stock converter? Your explanation has me thinking the car wants more gear, and/or more converter. Has this set-up run for you in the past? What changed?

65-440 said:

Go up in primary jet size ... that's assuming your accelerator pump is absolutely dialed in, and timing correct ... primaries on that carb are what, 72? I'd go to 74 and see what happens ...
Also, I would disconnect the secondary linkage to verify if the stumble is on the primary side or secondary. If it's still there with the secondary disconnected you know its in the primary side.

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I thought about doing the secondary dis-connect linkage, but with my low vision and arthritic fingers, I would most likely loose the tiny clip! May have to get a helper . Thanks.........................MO

moper said:

What he said. You would have had less of a "time" with a vacuum secondary carb, and that's the right design for your combo.

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I have tried about everything that has been mentioned, but a 4 speed heavy car is a different breed of cat than an automatic with some stall. Thanks for your input on maybe using vac, secondary carb................................MO

So the car's a 4-speed. Missed that if you mentioned it. Disregard my last post. Gears would help!

Frustration said:

What is the exact timing you are running?
Are the float levels set correctly?

If the answer is yes, I would not think it's a carb issue. You do not have much rear gear at 3.23. Do you run a stock converter? Your explanation has me thinking the car wants more gear, and/or more converter. Has this set-up run for you in the past? What changed?

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Keep in mind, this is a 4 speed. The last one of those I messed with was a light car with 4:10 rear gears, many years ago when a screw in the secondary linkage slot is what we did to get mechanical secondaries!............................MO

In general, a "bog" is too much air and/or too little fuel. Most of what's already posted is good, sound advice. Here's my 2-cents.
1. I prefer vacuum secondaries (better mileage) but a 750 double-pumper on a 440 should be fine.
2. I suggest you triple check your timing & carb settings
3. Are you getting any "squirt" in the back barrels? - stuck float, float setting too low, car lifts front end big-time & starves rear jets, rear squirter plugged up/not working?

After triple checking everything, then maybe start stepping up (slowly) your rear accelerator pump cam and/or rear squirter? With a double-pumper, you can't limit the air flow in the back barrels when you stomp on the pedal from a dead stop (like vacuum secondaries) to decrease air flow, so all you can do is increase fuel flow.

Timing has to be right, Eliminate that first. My opinion is that you would be happier with a vacuum secondary carb. I had a double pump carb on a 440 charger a few years ago and I hated it. It had progressive linkage and I never did get it dialed in right. I ended up with a Holley Street Avenger, opinions vary but I like these carbs. You get an electric choke, quick change covers make it easy to change the secondary springs and you can get them in 670, 770, or 870 cfm flavors. If it were mine, that's the carb I would swap to. I can't stress timing enough though, it has to be right or no matter what you do it will never run like it could.

too lean will bog, but too much fuel can do the same. big 50cc pumps and large squirters aren't friendly on small carbs.

Just spit-balling here; sorry in advance for the short book
{?} not enough Ignition timing {too retarded} could very well
be a contributing factor here too
{it's not uncommon, especially for a lower compression engine,
especially with a big cam to want like 20* initial advance {38*+ total}
vs the normal 10*-12* initial advance {32*-34* total many say to do}
like a stock or higher compression build, it's kind of a band-aide (advancing),
usually kind of for a miss matched/wrong parts combos}
what is you timing set at ?
What is the initial & total advance in degrees ?
when is the advance all in ? & at what RPM ?
you said it has a Chyrsler EI, does it have an adjustable vacuum advance ?
if so, how much timing is added by the centrifugal &/or vacuum advance ?
what box controller are you using orange ? chrome ? or some other aftermarket box
does it have any modifications to the centrifugal weights ?
or the advance springs {lighter} to get the timing in a bit quicker/earlier ?

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just for curiosity;
how big of a camshaft does it have ? what is it ? what was it set at ?
when installed & was it degreed in ? a what deg. &/or advanced or retarded ?
if it's retarded & low compression it could be an issue
{albeit most modern cams are ground with advance for timing chain stretch like 2*-4* advance}
does it have headers & free flowing exhaust ?
what other mods were done to the engine ?
does it have adjustable rocker arms ? if so are they adjusted properly ?
{proper length pushrods}
did you do "any other or multiple changes" before trying the said carb ?
does it have a free flowing air cleaner element ? {like a K&N or is it paper}
& have you tried it without the air-cleaner ? {does it improve any}

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bear with me here, I know these are kind of **** questions,
some of the stuff is for general knowledge,
some of it could seriously affect your type of tune...
more information we can get the better,
it will help & not have to go back & forth Q&A

does it {lean} surge when you cruising ?
or does it hold a constant rpm when you hold it at like 2500-3k rpm in neutral ?
no surging, while trying that ?
if it does any of these 2 above, it's probably too lean or a vacuum leak
or a serious ignition issue somewhere

like said before by a couple people,
too lean can cause it to bog bad too
& so will way too fat can too
not enough Hp, {honestly not trying to be a prick} to get a bit of tire spin/wheel-speed,
dead hook can bog even a hi-hp manual combo at too low of an RPM
{probably realistically not the issue here}
or missed mixed/matched parts, just thrown together, parts/combos
not liking each other, can cause a serious bog or tuning issues too...

IMO a Holley 750cfm DP should run like a raped ape,
it's a bit small, but should work OK {850-950cfm could be better}
it may not be jetted correctly, bad accelerator pump charge etc.
when you look down the carb & goose it does it have a
"nice full spray/even stream, out of both the squiters" {discharge nozzles}
also are you at any real altitude ?
or the float levels are set too low for it flowing/running condition
it should be trickling out of the fuel level bowl sight when it's idling
{also if it's overfull or going past the needle & seat (what ever causing it) it /fuel could
come out the top vents on top, you can add a piece of rubber hose to them, to keep it/fuel
from slouching out & into the carb, that can cause a bog on hard launches too}
do you have a fuel pressure gauge ? if so what's it doing when you nail it ?
If not I'd suggest to get one & if nothing else for tuning purposes...
Does it have an electric pump ? if so which ?
does it have a mechanical pump ? if so which ?
have you tested the fuel flow/gpm ?
you can do it running into a can & see how much fuel it flowing
{probably not the issue, but lets cover all basis's...
do you have a regulator inline ? if so what's it set at ? what style ?
is it a bypass or dead head style ?
{bypass with a return line back to the tank for unneeded fuel is always better}
either (should be set between 5.5psi & 7psi usually/flowing pressure preferably)
have you tried a vacuum gauge, see where it's at ?

you can Google or Bing search Tuning a carburetor with a vacuum gauge
for some basics

check for a few simple things

check the clearances on the accelerator pump, make sure it's not sloppy
maybe change the acc. pump cam so it gets a better initial squirt,
more fuel right off idle
also maybe a little bigger "squiter" acc. pump nozzle go up 2 sizes
if it makes it worse, than go the other way
you can also slightly bend {Straighten IIRC} the secondary linkage
to have the secondary's open latter
but that's just a band-aide covering up another tuning issue usually

I hope some of my short book helps directs you in the correct directions
I hope I haven't confused the **** out of you too...

I highly recommend

Getting my mechanical 850 demon a better fuel squirt on fast throttle opening (at any rpm) was needed in my case..(3680# race weight 440/727/3.73 car) Got it smoothed out with a blue cam up front and green in the back, and re-adjust the pump arms. I've found that mechanical secondaries with a progressive type linkage work best for me, regardless of transmission type