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Vintage Mustang Member
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97 Posts
Discussion Starter · #1 ·
Does anyone know what to set the idle screws at on a new Holley 750 cfm 4V? Seems that some settings use more gas then others...horribly...we're talking using 1/4 tank in just 5-10 miles. One time I even used half a tank just sitting in the driveway idleing trying to adjust the carb. A little help, please?
 

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Vintage Mustang Member
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70 Posts
The air bleed screws have little effect on gas mileage. They are located on the side of the float bowls and are best adjusted with an anyalizer or vacuum gauge. They idea is to set the screws to achieve the smoothest idle.
 

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I agree with superdave.It is doubtful the idle air bleeds would have that much impact on fuel consumption.Be sure the float levels are correct and the carb is jetted correctly for your engine.
 
G

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I third that thought, as soon as you touch the gas pedal the carb comes off idle and the idle mixture screws have no effect. Jetting of the carb is possible. It also lists that you have a 302. Even if you were putting out 400hp at 6000 rpms and up that would be too much carb for your engine. That or tuning things like the squirters, cams, float levels etc.

I have a 650 double pumper on my hopped up 289, but expect I am putting out 350+ hp. A normal 289/302 needs a 500-600cfm carb. Let us know what you know!
 

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Vintage Mustang Member
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97 Posts
Discussion Starter · #5 ·
carb settings

Well I have been working on the carb for a while now. I adjusted the float levels, i had them up to normal level, but I lowered them a little. I will admit that I am no good with carbs, but in my experiences, the lower the float level, the less gas it used. This also increased my top speed and acceleration, but I am wondering why the carb does so much better with a less then normal level. Also, I know only 2 barrels drive the car untill you go up a hill or something that kicks the other 2 barrels in, but how do i adjust the carb to kick the other 2 in sooner?
 
G

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Ok, don't know what you have but am suspecting if its a 750 it is a double pumper or a mechanical advance secondaries. What you are thinking about when you say "2 barrels drive the car untill you go up a hill or something that kicks the other 2 barrels in" you are discussing a vacuum advance secondary. In the vac adv seconday units they use a vacuum line that senses the manifold pressure of the motor (load) and opens them up and closes them off accordingly. With the car off, when you push the pedal to the floor olny the primaries will open and the secondaries will stay closed. On a mechanical secondairy/double pumper (mean the same thing) the secondaries are mechanically staged to open with the primaries. If the car is not running and you push the pedal all the way to the floor, both primary and secondary butterflys will open. Gas milage sucks cause you dump a lot of fuel everytime you hit the throttle, but the engines response to the pedal is much better. One is more for performance, the other is more for effiancy.

To adjust a vac advance sec car you have to change the springsthat govern the vacuum assembly, acclerator squirters, and acclerator pump.

On mechanical advance carbs you change the cam, springs, acclerator squirters, and acclerator pump.

Remember though, performance and gas milage are opposite concepts!!!!

Don't know about dropping the float levels and more high end power. Seems to me that you should set them and thats that. Anyone else know??
 

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The float levels should be set to the correct hight or close to it.It does seem to me that the carb may have to have the correct main jets fitted to suit your engine.It may have overly large main jets.There is also a power valve in the carb that may need checking.Is this a new or used carb.I would leave secondary barrel settings as they are or you will open another can of worms.If you want more carbing than the primary /secondary setup then there is a double pumper type that operates all 4 bls at once.Personally for a road car i like the vacum secondary setup.What size engine is this on as 750 is quite large.
 

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Vintage Mustang Member
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97 Posts
Discussion Starter · #8 ·
The carb (Holley high performance 750 cfm) was purchased brand new from an auto store over the summer. I originally put the carb and a high performance intake on the 289 in the car untill the engine blew. Before the engine went, the carb ran the car fine. Now its on a 302 with the same intake and a high performance airfilter. Do spacers make any difference using them or not? Also, the carb when purchased, was not set at factory settings for some reason, someone already went through the box it had been in.
 

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cougar/bulletbird member
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74 Posts
that carb is way too big for your application and the guy who sold it to you needs to be shot. even a well tuned hi-po with good mods does not need a 750. to make this carb work will take some tuning. if dropping the floats makes it run better it is because it is running too rich either due to plugged air bleeds (if the carb has sat for more than a month or two on a shelf after it had gas in it this is common) or it is jetted too big. the first thing to do is make sure all of the air bleeds in the air horn are clear. if they are all clear then start jetting down and test driving. if the car starts pinging, you have gone too small and are now running lean and hot (great gas mileage, but not good for the engine or what will be left of it if you drive it this way!) if this does not do the trick you may have plugged emulsification tubes and a soak in carb cleaner is your only hope (and i say hope because this does not always unplug them) other than sending the carb back to holley for a full overhaul. as for getting the secondaries to open sooner, if it is a vacuum secondary carb how do you know they are not opening? if a carb is set up right it will be a smooth transition and you should not notice any change. if the carb is a mechanical secondary then something is broken. hawkrod
 

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Vintage Mustang Member
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97 Posts
Discussion Starter · #11 ·
The engine in it now is a 302. But what im really confused on is why it would run my 289 fine using hardly any gas. I dont think the carb is clogged at all because it is brand new, it hasnt sad long when the engine was out of my car so I dont think its dirty. Though, what I am going to do is jet it down. For the secondaries, I am guessing this because the car has good performance going up hills, but its still not what it should be. The 2 barrel carb on my 289 with bad rings did better. This is why im guessing that the holley carb is not set up right.
 

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Vintage Mustang Member
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Discussion Starter · #12 ·
Ok since I am going to jet it down, what size jet should I use? Does anyone know the standard size for a 302? Thank you all for your help.
 
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Ok, mine is not completely tuned yet, but it will give you some numbers to play with and compare:

current primary jets /secondary jets
jet history
67 73
current primary squirter /secondary squirter
35 31
 

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cougar/bulletbird member
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sorry to take so long to respond, i have been having 'puter trouble today. there is no right jet size for a given engine. vacuum and flow characteristics determine fuel demand. vacuum is affected by cam choice, manifold, exhaust and so much more. just changing the mufflers can require a jet change if the back pressure is affected significantly (i like the guys who put headers, 2 1/2" pipe and flowmasters on there otherwise stock 289 and now it doesn't run right!). the trick to jetting is to find out what you have and go up or down appropriately. what you do is change the jets, take her for a spin and then "read" the spark plugs. i am sure you can find a website that goes into detail on this area. the real trick is to take the car out and drive the way you do normally, that is if you are a moderate driver don't go racing and then check the plugs or if you are a racer don't sit in traffic and then check the plugs. you need real world info or you run the risk of damaging the engine. good luck, hawkrod
 

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Archimedes said:
The engine in it now is a 302. But what im really confused on is why it would run my 289 fine using hardly any gas.
This also has me scratching my head as you would expect if it ran good on the 289 it should also be ok or close to ok on the 302. Be sure the carb is clean and gaskets are good etc etc. It will be interesting to find out how you get on. Maybe the 302 is pulling quite a bit more vacum down low. Good luck.
 

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Vintage Mustang Member
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Discussion Starter · #16 ·
I have diagrams on what the plugs should look like. The exhaust set up right now is a single, would it be recommended to get a dual? thank you all for your help, I will let you know what happens.
 
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