Edelbrock Carb
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Logistics Pro
Joined: Oct 2002
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From: Valley Center, CA
Thanks guys for the info. I am tweeking the carb little by little as we type. On Saturday I measured the vacuum of the carb and it is 19 inches. Edelbrock says to cut that in half and then install the appropriate springs on the metering rods. The only snag is that the rating only goes up to 8, not 9.5, so I installed the silver springs which is what an 8 corresponds to. The truck actually runs better! And the next step is to experiment with the accellerator pump lever. The catalog says that it may help in low rpm cruising to change the hole the lever is attached to. So if this works, great, and if it doesn't then I will move on to the other suggestions. Thanks, John
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Joined: Aug 2001
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From: Delaware
At idle the throttle plate is closed, there is no air fuel mixture through the venturi's. It is through the idle circuit which can meter small amounts of fuel. The problem with large venturis occurs with the throttle plate partially open. Due to the large circumference it opens too quickly and the vacuum drops drastically with a small motor at low RPM. A 289 at 1200 RPM pulls only 100 CFM. The pressure drop across the large venturis at 100 CFM is too low for proper flow and atomization.
Post Fiend
Joined: Feb 2001
Posts: 6,328
Likes: 124
From: Iowa
Originally posted by danlee
At idle the throttle plate is closed, there is no air fuel mixture through the venturi's. It is through the idle circuit which can meter small amounts of fuel. The problem with large venturis occurs with the throttle plate partially open. Due to the large circumference it opens too quickly and the vacuum drops drastically with a small motor at low RPM. A 289 at 1200 RPM pulls only 100 CFM. The pressure drop across the large venturis at 100 CFM is too low for proper flow and atomization.
At idle the throttle plate is closed, there is no air fuel mixture through the venturi's. It is through the idle circuit which can meter small amounts of fuel. The problem with large venturis occurs with the throttle plate partially open. Due to the large circumference it opens too quickly and the vacuum drops drastically with a small motor at low RPM. A 289 at 1200 RPM pulls only 100 CFM. The pressure drop across the large venturis at 100 CFM is too low for proper flow and atomization.
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Joined: Aug 2001
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From: Delaware
fatfenders,
I can't argue with success. Do you have a 3.0:1 rear ratio?
The Edelbrock 1406 is the preferred carb on the 400 cu inch M block motors used in many F150's.
I still haven't gotten the RPM of the motor at 25 MPH.
The springs will raise the metering rods when the manifold vacuum drops below the value of the spring. This will richen the mixture to the Power Mode ratio. If using an 8lb spring improves the performance, then your cruise mixture is too lean. You can't use a heavier spring, because you will be in power mode all of the time. You should change the rods to get richer in cruise mode.
I can't argue with success. Do you have a 3.0:1 rear ratio?
The Edelbrock 1406 is the preferred carb on the 400 cu inch M block motors used in many F150's.
I still haven't gotten the RPM of the motor at 25 MPH.
The springs will raise the metering rods when the manifold vacuum drops below the value of the spring. This will richen the mixture to the Power Mode ratio. If using an 8lb spring improves the performance, then your cruise mixture is too lean. You can't use a heavier spring, because you will be in power mode all of the time. You should change the rods to get richer in cruise mode.
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Logistics Pro
Joined: Oct 2002
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From: Valley Center, CA
Sorry Danlee, I forgot about the rpm part of the previous question. I don't have a tach, so I am guessing that at 20 mph the engine is running around that 1200 rpm number you threw out. I hear what you're saying about the springs perhaps making the normal operation too rich. This weekend we are going on another shake-down run of maybe 50 miles. Afterwards, I will check the color of the plugs. 'fenders, I don't know about mileage numbers yet. I ran one tank through it and it was 10 mpg. But that was with all the idleing in the garage and short neighborhood trips. On the freeway I am hoping for 16-17. 
But we'll see. Thanks for the help guys, John 
But we'll see. Thanks for the help guys, John
Post Fiend
Joined: Feb 2001
Posts: 6,328
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From: Iowa
John
Keep us posted. I think you'll eventually end up wanting more gear as Dan suggests. Somewhere around a 3.50 ratio would be very noticeable. It will never be a powerhouse with a 289 and high gears, but that may not be a big priority for you anyway. It really isn't to me. I should be able to get a good mpg figure this spring. I will be happy with anything over 15. That would be better than either of my daily driver Ford trucks.
Keep us posted. I think you'll eventually end up wanting more gear as Dan suggests. Somewhere around a 3.50 ratio would be very noticeable. It will never be a powerhouse with a 289 and high gears, but that may not be a big priority for you anyway. It really isn't to me. I should be able to get a good mpg figure this spring. I will be happy with anything over 15. That would be better than either of my daily driver Ford trucks.
Posting Guru
Joined: Jul 2001
Posts: 2,119
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From: Southern New Hampshire
How tall are your tires? If you are running a common 28" tire then you are only at 900 rpm at 25 mph..
Here is an easy way to calculate the rpm.
"http://www.quiknet.com/~ke6vut/rpmcalc.html"
Many years ago I ran a 600 AFB on a stock 69 302, C4 in a 64 Falcon and never got it right, went with a 500 Holley (This was when you could find anything in a boneyard).
Later on the AFB went on a 327/350 hp cammed Chebby, it hauled butt with minimal tuning.
I'd consider dropping down to .086 primaries, leave them alone and tune from there. Go up one step and redo, etc until it works.
My favorite tool is a vacuum gauge in the drivers compartment, it tells you a lot.
Dont overlook the ignition, set the timing by the book. If you are still running points upgrade to electronic.
Here is an easy way to calculate the rpm.
"http://www.quiknet.com/~ke6vut/rpmcalc.html"
Many years ago I ran a 600 AFB on a stock 69 302, C4 in a 64 Falcon and never got it right, went with a 500 Holley (This was when you could find anything in a boneyard).
Later on the AFB went on a 327/350 hp cammed Chebby, it hauled butt with minimal tuning.
I'd consider dropping down to .086 primaries, leave them alone and tune from there. Go up one step and redo, etc until it works.
My favorite tool is a vacuum gauge in the drivers compartment, it tells you a lot.
Dont overlook the ignition, set the timing by the book. If you are still running points upgrade to electronic.
Last edited by 286merc; Mar 16, 2004 at 06:43 PM.
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Postmaster
Joined: Aug 2001
Posts: 4,270
Likes: 5
From: Delaware
John,
The carb is lean in cruise mode. That is a classic problem when there is not enough carb vacuum to atomize the fuel. The springs are probably OK, but you need to change the metering rods and/or jets to go one step richer in cruise mode.
The carb is lean in cruise mode. That is a classic problem when there is not enough carb vacuum to atomize the fuel. The springs are probably OK, but you need to change the metering rods and/or jets to go one step richer in cruise mode.
Post Fiend
Joined: Feb 2001
Posts: 6,328
Likes: 124
From: Iowa
"John,
The carb is lean in cruise mode. That is a classic problem"
I know Danlee and I have been debating back and forth on this thread, but I think he is dead on. Although you may believe you are running rich, trust us on this one unless you are seeing any black smoke out the tails. Try tuning it a step or two richer. With your gears, your low speed cruise RPM is a fast idle. Even backing out the idle mixture screws may help the transition just a bit. It may not either, but it won't hurt to try if you are lean.
The carb is lean in cruise mode. That is a classic problem"
I know Danlee and I have been debating back and forth on this thread, but I think he is dead on. Although you may believe you are running rich, trust us on this one unless you are seeing any black smoke out the tails. Try tuning it a step or two richer. With your gears, your low speed cruise RPM is a fast idle. Even backing out the idle mixture screws may help the transition just a bit. It may not either, but it won't hurt to try if you are lean.
Thread Starter
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Logistics Pro
Joined: Oct 2002
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Likes: 5
From: Valley Center, CA
Good news, We went cruising on Saturday with good results all day until the evening. Then the truck started running rough under excelleration. I pulled a plug and saw that it was running rich. The color was not black, but dark grey so I figured if I kept going it would soon be black. As a cure I went one step leaner on the metering rods. The followup crusing was fine with no engine sputtering. After about a ten mile loop, I pulled a plug and found it to be light grey. 
Mileage on Saturday was only 10mpg with a 50-50 mix of freeway neighborhood driving. (A/C on) So we are going to try it again this weekend and see if the mileage is better. Things are looking up. 
John
Post Fiend
Joined: Feb 2001
Posts: 6,328
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From: Iowa
Sounds good John. Keep the needle and seats in mind if you get any further intermittent rich condition. I'm not too impressed with the stock Edelbrock arrangement. The off road needle and seat is a good upgrade IMO.
55forder