got a 327 with a holley 750 double pumper carb and a edelbrock air gap intake. The thing bogs/stumbles anytime you give it gas at low rpm (there is no stumble or bog when you're above 3000-3500rpm) I have not been able to get it tuned right and finally i just threw on a 50 primary and 45 secondary squirter with the hollow screws..it cleaned up quite a bit and i threw a 50cc secondary on and this is the best its ever driven. It doest make sense that I need that much pump shot and it must be covering up some other problem...any ideas?
darkostoj
You pretty much said it all: "got a 327 with a holley 750 double pumper carb...." Nothing wrong with that carb that a 454 won't fix. 
Just to be clear: That carb is a race car carb meant for aggressive large displacement engines. It's way too big for your 327 at any RPM under 6000. when you stab the gas it opens up and dumps half again too much dry air into the manifold, vacuum and velocity go to zero. If the mains were operating they quit, no air velocity to build the vacuum signal to pull fuel from the bowls. So, it's running on the pumps, probably pig rich in some cylinders and lean in others as the huge pump shot does not vaporize well or distribute to the individual cylinders well. Why doesn't it vaporize well? because the manifold is at atmospheric pressure, no vacuum.
Essentially, a traditional 750 DP is a lot like a faucet on a small block.
What you need is a 600CFM vacuum secondary like an 1850.
Did I mention spark plugs? Buy plenty as yours are going to be fouled out regularly by the huge pump shot, liquid fuel entering the chambers. Do you know what gas fumes look like? Before gas stations had vapor recovery systems you could see the clear gasoline vapor coming out of the filler neck as the tank filled. That clear vapor is what burns in an engine. Liquid fuel and the wet white fog stuff don't burn. If the fuel isn't vaporized to that clear state before the intake valve closes it's lost, ends up going out the exhaust as unburned hydrocarbons. These are the real-world reasons why a too-big carbs are a loser deal.
More than you ever wanted to know?
Wanna know the main reason guys don't like the LIST #1850 600CFM carb (an excellent all-around hot rod carb BTW)? Because of the square fuel bowls. It doesn't look racy. Holley has done a lot of dumb stuff over the years but one of the worst is not marketing the 600 Vac Sec with a pair of center hung float bowls and a secondary metering block. 99% or more of the hot rod crowd wouldn't be able to tell it wasn't a 750 or something else perceived as cool.
Just to be clear: That carb is a race car carb meant for aggressive large displacement engines. It's way too big for your 327 at any RPM under 6000. when you stab the gas it opens up and dumps half again too much dry air into the manifold, vacuum and velocity go to zero. If the mains were operating they quit, no air velocity to build the vacuum signal to pull fuel from the bowls. So, it's running on the pumps, probably pig rich in some cylinders and lean in others as the huge pump shot does not vaporize well or distribute to the individual cylinders well. Why doesn't it vaporize well? because the manifold is at atmospheric pressure, no vacuum.
Essentially, a traditional 750 DP is a lot like a faucet on a small block.
What you need is a 600CFM vacuum secondary like an 1850.
Did I mention spark plugs? Buy plenty as yours are going to be fouled out regularly by the huge pump shot, liquid fuel entering the chambers. Do you know what gas fumes look like? Before gas stations had vapor recovery systems you could see the clear gasoline vapor coming out of the filler neck as the tank filled. That clear vapor is what burns in an engine. Liquid fuel and the wet white fog stuff don't burn. If the fuel isn't vaporized to that clear state before the intake valve closes it's lost, ends up going out the exhaust as unburned hydrocarbons. These are the real-world reasons why a too-big carbs are a loser deal.
More than you ever wanted to know?
Wanna know the main reason guys don't like the LIST #1850 600CFM carb (an excellent all-around hot rod carb BTW)? Because of the square fuel bowls. It doesn't look racy. Holley has done a lot of dumb stuff over the years but one of the worst is not marketing the 600 Vac Sec with a pair of center hung float bowls and a secondary metering block. 99% or more of the hot rod crowd wouldn't be able to tell it wasn't a 750 or something else perceived as cool.
I used an AED 750 DP Holley on my 331" road race motor and it worked very well at all rpm's. It was pretty much a stock 4779 with the choke horn milled off and some rework by AED. I don't think you need more than a .031" squirter and a 30cc pump unless you've got other problems
Earl Parker of Earl Parker Carburetion sent me the following setup memo to get the off-idle response real crisp.
One method of setting up your idle system is a follows:
To start with, invert the carburetor and check the position of the throttle
butterflies. Turn the idle speed setting screw to set the bottom edge of the
primary throttle butterflies about .020" from the bottom edge of the transfer
idle slot. Don't worry about measuring anything - your eyeball is good
enough. Positioning the throttle butterflies near the bottom of the transfer
idle slot at curb idle is absolutely critical for maximum acceleration.
Next, turn the idle mixture needles in until they are lightly seated. Excessive
force here will damage both the needles and metering block and make the
idle fuel mixture difficult to set with any accuracy. After seating them turn
them out 1 1/4 turns, which is a good baseline setting. Now you're ready to
reinstall the carburetor and setup your idle system.
Before you start the engine, examine the fuel bowl side of the throttle body.
Hopefully you'll see a little tube, covered by a rubber plug. This vacuum port
connects with a passage in the throttle body that 'sees' manifold vacuum.
Remove the plug, attach a good vacuum gauge to the port and position the
gauge where you can see it clearly. Don't forget to zero out the gauge.
Without touching the carburetor, turn the engine over until you have
pumped fuel into the bowls. Work the throttle a few times then start the
engine. If it dies, which is likely, you'll have the turn the idle speed setting
screw to increase the RPM to get it to idle while it's cold. Since throttle
butterfly position is critical, count the turns and fractions of turns so you'll
know exactly where you're at. The whole idea is to be able to return the
throttle butterflies to the position you originally set them at. As the engine
warms up it should gain rpm, so you should be able to reduce the throttle
opening at least somewhat without the engine dying. Now the fine tuning
begins.
With the engine idling, pick one of the idle mixture needles and turn it in 1/4
turn while you're watching the vacuum gauge. Give the idle a few seconds
to stabilize. If manifold vacuum increases repeat the process, letting the idle
stabilize each time, until it starts to decrease. If turning it in hurts manifold
vacuum then try turning it out. When you've found the 'sweet spot' (i.e. the
manifold vacuum is as high as you can get it) repeat the process with the
other idle mixture needle.
Presumably you'll be able to pick up enough idle speed by optimizing the idle
fuel mixture that you can close the primary throttle butterflies down to their
original position near the bottom of the transfer idle slots.
As a final check give each idle mixture needle a slight turn in then a slight
turn out. If any motion hurts manifold vacuum, you know that needle is set
properly. At this point if the idle is stable and the engine responds quickly
when you just crack the throttle, you should be good to go.
One final note: Make sure your timing is set correctly before starting this
process.
If your distributor has a mechanical advance system there is a much better,
though more involved, way to setup the idle system.
Position the throttle butterflies and idle mixture needles as described above,
attach the manifold vacuum gauge to the vacuum port and start the engine.
Turn the idle speed screw to increase the RPM, again taking note of exactly
how much you have to turn the screw to open the throttle butterflies
enough for the engine to idle while it's cold. Allow the engine to warm up,
the close the throttle butterflies as much as reasonably possible without the
engine dying. Attach a timing light, check to see how much initial ignition
advance you have and make a note of the figure.
Next, loosen the distributor hold down clamp and turn the distributor so as
to increase the initial ignition advance. When the initial ignition advance is
increased the RPM should rise as well, allowing you to reduce the throttle
butterfly opening. Simply turn the distributor to increase the initial ignition
advance and continue to reduce the throttle butterfly opening until they're
in the original, correct position and the engine is idling at the desired RPM.
Lightly snug the hold down clamp to make sure the distributor can't move,
then adjust the idle mixture needles for best manifold vacuum. Once they're
properly set if the idle RPM is higher than desired, loosen the hold down
clamp and turn the distributor slightly to achieve the desired idle RPM.
Recheck the idle mixture needle position then tighten the hold down clamp.
Once the idle system is setup you'll need to correct the distributor's
advance curve. The first step is to attach a timing light and recheck the
initial ignition advance. Let's say, for example, that it was originally 15° and
now it's 22°, a 7° increase. If your total ignition advance was originally 35°,
in order to keep that figure the advance curve will have to be shortened by
7°. Assuming you have a centrifugal advance system you'll have to limit how
far the advance weights can move outward, which will limit the total
advance. The method required will vary from distributor to distributor, so I
won't get into that here, but any competent technician with a good
distributor machine should be able to do it for you.
If you don't have access to said technician/distributor machine and you can
come up with a way to limit the outward motion of the advance weights,
you can do the same thing using your engine as form of distributor machine.
Limit the motion of the weights somewhat, make sure you have the correct
initial ignition advance then check to see how much total ignition advance
you have. If the total ignition advance is still too high, just continue to limit
the motion of the advance weights until you achieve the desired total figure.
AED has some good tips on carb setup, also.
http://www.aedperformance.com/Tuning Tips.htm
Earl Parker of Earl Parker Carburetion sent me the following setup memo to get the off-idle response real crisp.
One method of setting up your idle system is a follows:
To start with, invert the carburetor and check the position of the throttle
butterflies. Turn the idle speed setting screw to set the bottom edge of the
primary throttle butterflies about .020" from the bottom edge of the transfer
idle slot. Don't worry about measuring anything - your eyeball is good
enough. Positioning the throttle butterflies near the bottom of the transfer
idle slot at curb idle is absolutely critical for maximum acceleration.
Next, turn the idle mixture needles in until they are lightly seated. Excessive
force here will damage both the needles and metering block and make the
idle fuel mixture difficult to set with any accuracy. After seating them turn
them out 1 1/4 turns, which is a good baseline setting. Now you're ready to
reinstall the carburetor and setup your idle system.
Before you start the engine, examine the fuel bowl side of the throttle body.
Hopefully you'll see a little tube, covered by a rubber plug. This vacuum port
connects with a passage in the throttle body that 'sees' manifold vacuum.
Remove the plug, attach a good vacuum gauge to the port and position the
gauge where you can see it clearly. Don't forget to zero out the gauge.
Without touching the carburetor, turn the engine over until you have
pumped fuel into the bowls. Work the throttle a few times then start the
engine. If it dies, which is likely, you'll have the turn the idle speed setting
screw to increase the RPM to get it to idle while it's cold. Since throttle
butterfly position is critical, count the turns and fractions of turns so you'll
know exactly where you're at. The whole idea is to be able to return the
throttle butterflies to the position you originally set them at. As the engine
warms up it should gain rpm, so you should be able to reduce the throttle
opening at least somewhat without the engine dying. Now the fine tuning
begins.
With the engine idling, pick one of the idle mixture needles and turn it in 1/4
turn while you're watching the vacuum gauge. Give the idle a few seconds
to stabilize. If manifold vacuum increases repeat the process, letting the idle
stabilize each time, until it starts to decrease. If turning it in hurts manifold
vacuum then try turning it out. When you've found the 'sweet spot' (i.e. the
manifold vacuum is as high as you can get it) repeat the process with the
other idle mixture needle.
Presumably you'll be able to pick up enough idle speed by optimizing the idle
fuel mixture that you can close the primary throttle butterflies down to their
original position near the bottom of the transfer idle slots.
As a final check give each idle mixture needle a slight turn in then a slight
turn out. If any motion hurts manifold vacuum, you know that needle is set
properly. At this point if the idle is stable and the engine responds quickly
when you just crack the throttle, you should be good to go.
One final note: Make sure your timing is set correctly before starting this
process.
If your distributor has a mechanical advance system there is a much better,
though more involved, way to setup the idle system.
Position the throttle butterflies and idle mixture needles as described above,
attach the manifold vacuum gauge to the vacuum port and start the engine.
Turn the idle speed screw to increase the RPM, again taking note of exactly
how much you have to turn the screw to open the throttle butterflies
enough for the engine to idle while it's cold. Allow the engine to warm up,
the close the throttle butterflies as much as reasonably possible without the
engine dying. Attach a timing light, check to see how much initial ignition
advance you have and make a note of the figure.
Next, loosen the distributor hold down clamp and turn the distributor so as
to increase the initial ignition advance. When the initial ignition advance is
increased the RPM should rise as well, allowing you to reduce the throttle
butterfly opening. Simply turn the distributor to increase the initial ignition
advance and continue to reduce the throttle butterfly opening until they're
in the original, correct position and the engine is idling at the desired RPM.
Lightly snug the hold down clamp to make sure the distributor can't move,
then adjust the idle mixture needles for best manifold vacuum. Once they're
properly set if the idle RPM is higher than desired, loosen the hold down
clamp and turn the distributor slightly to achieve the desired idle RPM.
Recheck the idle mixture needle position then tighten the hold down clamp.
Once the idle system is setup you'll need to correct the distributor's
advance curve. The first step is to attach a timing light and recheck the
initial ignition advance. Let's say, for example, that it was originally 15° and
now it's 22°, a 7° increase. If your total ignition advance was originally 35°,
in order to keep that figure the advance curve will have to be shortened by
7°. Assuming you have a centrifugal advance system you'll have to limit how
far the advance weights can move outward, which will limit the total
advance. The method required will vary from distributor to distributor, so I
won't get into that here, but any competent technician with a good
distributor machine should be able to do it for you.
If you don't have access to said technician/distributor machine and you can
come up with a way to limit the outward motion of the advance weights,
you can do the same thing using your engine as form of distributor machine.
Limit the motion of the weights somewhat, make sure you have the correct
initial ignition advance then check to see how much total ignition advance
you have. If the total ignition advance is still too high, just continue to limit
the motion of the advance weights until you achieve the desired total figure.
AED has some good tips on carb setup, also.
http://www.aedperformance.com/Tuning Tips.htm
What Tom said is exactly correct. (Well written I must say) When you added the larger squirters you helped the delay when the mains actually stopped, when your air velocity stopped in your intake. At 3000+ rpms the velocity in your intake is much stronger and faster and the double pumper is much happier then. If you run a loose enough converter it will help get your rpm's up quicker and help the huge bog area. You can get a 670 Street Avenger that looks cool with the center hung floats and dual feed line.
Dave
Dave
Its a 4 speed car and the timing is at 20 initial and 38 total.
my buddy has a 65 mustang with a mild 302 and he has a 3310 750 cfm carb on his and a 28 squirter in the primary. His car probably has the most crisp throttle response out of anything i've driven....does all this stuff not apply to vac secondary carbs?
my buddy has a 65 mustang with a mild 302 and he has a 3310 750 cfm carb on his and a 28 squirter in the primary. His car probably has the most crisp throttle response out of anything i've driven....does all this stuff not apply to vac secondary carbs?
that's a vac sec carb designed for street driving. A very different critter from a 750DP.
Truth is, I ran low on usuable carbs because a I sold a couple. I dug out an old 3310-1 I've had since the mid-70s and installed it. Its a 331 in a 57 Chev pickup, stock TH400 and 3.31. it's doggy off the line just driving around and is just sort of doggy all around under about 3000. I looked on the box, I last re-built it in 1993. But, now digging around in the shed looking a DZ code Holley I have I found a nice shiny #1850, it'll go on this weekend. It'll bring the 331 back to a snappy, good response mode that it always had.
Truth is, I ran low on usuable carbs because a I sold a couple. I dug out an old 3310-1 I've had since the mid-70s and installed it. Its a 331 in a 57 Chev pickup, stock TH400 and 3.31. it's doggy off the line just driving around and is just sort of doggy all around under about 3000. I looked on the box, I last re-built it in 1993. But, now digging around in the shed looking a DZ code Holley I have I found a nice shiny #1850, it'll go on this weekend. It'll bring the 331 back to a snappy, good response mode that it always had.
a 331 at 6000RPM at 90% V.E. needs about 520CFM. at 100% VE it needs 574CFM but you won't get 100% in any reasonable facsimile of a street car. 90% is a good number.
I agree with Tom's post. That 750 carb is way to big for that motor. Maybe if had a bunch of compression, big cam, and heads that really flowed, it might work out ok. but for a mild 327 that 750 is dumping so much air into that small motor, that adding the bigger squirter is just making up for it. I bet if you put a stock 600 vaccum secondary on it, you would be shocked how much throttle response it had. And reason I know this is because I had to learn this leason first hand.
Back in high school I had a 65 chevelle with a pretty basic 327. It had camel back heads with 202 intake valves, and a victor jr. intake. I always ran this carb shop 750 I had, which my dad told me was the most stupid thing I could do. But of course I was young and dum, and thought my dad didn't know anything about the newer carbs, and was just refusing to change with the times. I SHOULD OF KNOWN BETTER!!
One day my dad came over with a 600 vaccum secondary, (which he took off a customers car at work).. And told me if I didn't take the 750 off, and swap it out for the 600, that he was going to steal my car and run me over with it...:sad: Nice guy!!! Anyway to make a long story short, the car went from running 13:40 at 103 to running 13:15 at 106, just from the change in carbs..
I guess the moral of the story is that 1. bigger is not always better.
and 2. don't argue with a man that has 35 years of experince working on cars, and several fast Mopars through out his life time.
He still to this day reminds me of that story
sorry for rambling on.
Back in high school I had a 65 chevelle with a pretty basic 327. It had camel back heads with 202 intake valves, and a victor jr. intake. I always ran this carb shop 750 I had, which my dad told me was the most stupid thing I could do. But of course I was young and dum, and thought my dad didn't know anything about the newer carbs, and was just refusing to change with the times. I SHOULD OF KNOWN BETTER!!
One day my dad came over with a 600 vaccum secondary, (which he took off a customers car at work).. And told me if I didn't take the 750 off, and swap it out for the 600, that he was going to steal my car and run me over with it...:sad: Nice guy!!! Anyway to make a long story short, the car went from running 13:40 at 103 to running 13:15 at 106, just from the change in carbs..
I guess the moral of the story is that 1. bigger is not always better.
and 2. don't argue with a man that has 35 years of experince working on cars, and several fast Mopars through out his life time.
He still to this day reminds me of that story
sorry for rambling on.
>> "He still to this day reminds me of that story"
LOL, some things never change.
bet it drove better around town too.
LOL, some things never change.
bet it drove better around town too.
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