Detuning a Carb

FordRacing302

5 Year Member

Oct 1, 2005

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Cleveland, TN

Dec 10, 2005

#1

• Dec 10, 2005

• #1

I have a 302 with a pretty worn looking holley 600 sitting on top and I know it has to be shot...So I have this holley 750 double pumper sitting doing nothing and I know its too much of a carb for a 302 but is there a way that I could tune it down a bit to where it doesnt drown the motor out???

 

1

10secgoal

Active Member

Dec 1, 2003

2,801

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49

San Diego

Dec 10, 2005

#2

• Dec 10, 2005

• #2

Could take the parts from the 750 depending on the condition and swap them.
Or you could jet the carb down. But it will still be too big for the motor and it's going to get horrid mpg.

 

FordRacing302

5 Year Member

Oct 1, 2005

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Cleveland, TN

Dec 10, 2005

#3

• Dec 10, 2005

• #3

Well if MPG is going to be the only problem, then it would be safe to just throw the carb on there??? My main goal is just to see the car run

 

S

steel1212

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Jun 24, 2004

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Dec 10, 2005

#4

• Dec 10, 2005

• #4

I think if you just want to see it run then jet it down to the 600 and it'll run. But if possible swap everything over from the 750 to 600 if you can.

 

FordRacing302

5 Year Member

Oct 1, 2005

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Cleveland, TN

Dec 10, 2005

#5

• Dec 10, 2005

• #5

The 600 is a real piece of junk I wouldnt give that carb to my worst enemy, There's not even a choke on it...So your saying I can pull the jets off the 600 and install them on the 750??

 

FordRacing302

5 Year Member

Oct 1, 2005

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Cleveland, TN

Dec 10, 2005

#6

• Dec 10, 2005

• #6

So all I have to do to change out main jets it to unscrew them from the old carb and screm them into the new one??? And how about the power valve should I leave that alone???

 

S

steel1212

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#7

• Dec 10, 2005

• #7

whats junk about the 600? PM 10secgoal he is more knowledgable about carbs than me but if they are the same models I would Pull everything off the mainbody from the 600 and put it on the 750 as well as the jets and that should get you close.

 

D

D.Hearne

New Member

Sep 29, 2000

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south louisiana

Dec 10, 2005

#8

• Dec 10, 2005

• #8

You CANNOT "jet down" a 750 to a 600. Jetting it down, simply leans out the mixture, making it worse for the motor. If all you want is a smaller carb, then simply disconnect the secondary linkage, this will in effect make it a 500 cfm, 2 bbl carb. the 750 DP has the same size throttle bores as the 500--2 bbl, (the 2 bbls are flow rated at a different pressure differential) and is jetted two sizes below the primary side of a 750. Change the jets from 75's ( if that's what's in there now) to 73's. The Holley lists some 750's as having #70 jets, in that case, you may need to jet up 2-3 sizes. If you want to fine tune it, then after doing this, you may need to change the jetting on the primary side. In doing so I usually go up or down 2 sizes at a time., depending on the need for such after reading the plugs.

 

S

steel1212

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Dec 10, 2005

#9

• Dec 10, 2005

• #9

as I said just ask D. Hearne he knows more about carbs than me

 

FordRacing302

5 Year Member

Oct 1, 2005

384

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Cleveland, TN

Dec 10, 2005

#10

• Dec 10, 2005

• #10

Well you shoulda spoke a little sooneramnit: ...I just got done pulling the old carb switching out the 2 jets and installing the 750...So your telling me this is going to be bad for my motor!!!

 

FordRacing302

5 Year Member

Oct 1, 2005

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Cleveland, TN

Dec 10, 2005

#11

• Dec 10, 2005

• #11

And since you say I just leaned out the mixture then cant I just adjust the air screw down and the fuel screw up a bit??

 

D

D.Hearne

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#12

• Dec 10, 2005

• #12

FordRacing302 said:

And since you say I just leaned out the mixture then cant I just adjust the air screw down and the fuel screw up a bit??

Click to expand...

Don't understand carbs, do you? Those "jet" screws aren't jet screws, the only thing they do is affect the fuel mixture while the idle circuit is in operation. To affect the mixture off idle ( there's more than two circuits in a Holley carb, but we'll just concentrate on these two for now) you HAVE to change the jet screws inside the fuel bowls. The circuit this affects is the main metering circuit. Get a Holley carb book and read it, you'll find a lot of useful info there.

 

FordRacing302

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Oct 1, 2005

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Cleveland, TN

Dec 10, 2005

#13

• Dec 10, 2005

• #13

Ok Ok...Im sure the book would help me alot, but Im just gonna try to get by without spending any money...So I guess Ill pull the carb off change the jets back to normal and put the 750 back on there and deal with the bad gas mileage...The only thing I want is to see this car run

 

D

D.Hearne

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#14

• Dec 10, 2005

• #14

Just change the jets back to what they were ( should be73-75's or 70's if it's a -1,2 or 3 list number) , disconnect the secondary linkage, and enjoy. Those adjustable screws on either side of the main metering plate should be screwed in all the way, then back them out 1-1/2 full turns. There's an idle adjustment screw on the left side primary linkage, adjust this till it just starts to open the throttle blades. Start the motor, adjust the timing and then adjust the idle speed screw to where you want it, then fine tune the idle with the mixture screws.

 

69Rcode_Mach1

Active Member

Apr 20, 2004

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Salt Lake City, Utah

Dec 10, 2005

#15

• Dec 10, 2005

• #15

I also recommend disconnecting the secondary linkage she will really have some kick if you do. Won't drown and will run way better than if you do.

 

1

10secgoal

Active Member

Dec 1, 2003

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San Diego

Dec 10, 2005

#16

• Dec 10, 2005

• #16

What I was referring to maining is DP are usually around 3-5 jets higher than a carb of the same size, non DP form.

 

FordRacing302

5 Year Member

Oct 1, 2005

384

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Cleveland, TN

Dec 15, 2005

#17

• Dec 15, 2005

• #17

Update: I took the 750 off again and replaced it with the original jets after that I had a buddy of mine come over and help drop the gas tank so I could empty out the old gas, The gas was dark brown...So we put it back in put some fresh gas in, sprayed some starting fluid down the carb and as soon as we turned it over it flamed out the carb...So what this means to me is I got good gas and obviously good spark but its not timed, but I went about timing it right

But while doing all this I ran into another problem, the car barely wants to turn over and my battery is at 12.7 volts...and everytime it tries to turn over the negative battery cable smokes at the battery...The car was turning over perfect a few days ago, I dont understand it

 

D

D.Hearne

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Sep 29, 2000

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south louisiana

Dec 16, 2005

#18

• Dec 16, 2005

• #18

Got a bad ground or fried starter.

 

LMan

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Aug 10, 2002

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Dec 16, 2005

#19

• Dec 16, 2005

• #19

FordRacing302 said:

Well you shoulda spoke a little sooneramnit: ...I just got done pulling the old carb switching out the 2 jets and installing the 750...So your telling me this is going to be bad for my motor!!!

Click to expand...

Wow, I feel terrible....it's all Stangnet's fault. Whadda shame.