Changing Carb
- Thread starter leomovitz
- Start date
Lee_in_KC
Active Member
Leo, 750 for that 355 seems a little large to me. I talked to Edel when I was setting up my motor, and they said they recommend a 600 or 650 for a 350-355. That 750 came out of the box set up for a bigger motor, so you are over-jetted right now.
Edels are dirt-simple to work on. Look at your manual (available here http://www.edelbrock.com/automotive_new/misc/tech_center/install/1000/1407_manual.pdf). There are charts in the back to guide you on tuning the carb with jets, rods and springs. Pretty simple to do with common tools, although you need a #20 or #22 torx driver to get the lid off the carb (only necessary if you are going to change the secondary jets).
Be sure to check your fuel pressure. Edels don't like more than about 5-5.5 pounds. More than that can overcome the float valves and cause over-rich or flooding.
I love my Edels (two 1405s) and wouldn't swap them for anything else.
BTW, your T turned out spectacular! Can't wait to see it live.
Edels are dirt-simple to work on. Look at your manual (available here http://www.edelbrock.com/automotive_new/misc/tech_center/install/1000/1407_manual.pdf). There are charts in the back to guide you on tuning the carb with jets, rods and springs. Pretty simple to do with common tools, although you need a #20 or #22 torx driver to get the lid off the carb (only necessary if you are going to change the secondary jets).
Be sure to check your fuel pressure. Edels don't like more than about 5-5.5 pounds. More than that can overcome the float valves and cause over-rich or flooding.
I love my Edels (two 1405s) and wouldn't swap them for anything else.
BTW, your T turned out spectacular! Can't wait to see it live.
Screaming Metal
Active Member
Going down 1 size on those dice might get you a couple of mpg's! Quad is a good carb, so is a Edel. Its all in the tuning! A quad is more at home in mild apps where you want milage.
This time around I went smaller on my 283 (bored to 301), decided on a 570cfm avenger. A buddy of mine put one on his cobra and he was really impressed with how it was setup right out of the box. He welded on a bung in the collector and I hooked up my wideband to it and it was really on the mark for a carb. BTW, one of the best running smallblocks I ever had --had a Qjet on her. With the puny primaries they really shine for mpg.
Steve
Steve
Hotrod46
Member
I agree with going to a smaller carb.
It's been my experience with Edelbrocks that they tend to be jetted rich out of the box. I've gone as far as 2 steps lean on the cruise with no problems. Read your plugs, if they're black, you're to rich.
If you're after a mileage increase, are you running vacuum advance? It makes a big difference in cruise milage.
Mike
It's been my experience with Edelbrocks that they tend to be jetted rich out of the box. I've gone as far as 2 steps lean on the cruise with no problems. Read your plugs, if they're black, you're to rich.
If you're after a mileage increase, are you running vacuum advance? It makes a big difference in cruise milage.
Mike
BTW, your bucket is a beauty :win:My T Bucket now has a 355ci / 400 Turbo and a 750 Edelbrock the mileage sucks.The 411 reared doesn;t help,but with the quickchange I can make it what ever I want. I'm leaning toward a newly rebuilt Quadrajet whats you'alls opinion?
It's getting about 10 looking for 20
Differen T
Member
Mileage is based on several main variables assuming there is not excessive wear in the drive train;
1- The amount of air that flows through the engine at a specific RPM
2- The amount of fuel in the air at a specific RPM
3- The ignition spark curve
4- The overall drive ratio (turns of the crank shaft to one turn of the rear tires
5- Weight of the vehicle
6- Rolling resistance of the tires ( type of tread and construction as well as wheel alignment )
7- DRIVING HABITS (the most important item, not a reflection on you)
The maximum flow of a carb can never exceed the maximum flow of the engine (if you have a 750 CFM carb, but the engine can only use 650 CFM the carb will only flow 650 CFM). Therefore the carb with 750 CFM is jetted for its maximum CFM to have a specific air/fuel ratio and most likely needs to have its main jets size reduced. It probably needs the complete fuel curve reduced which with your carb is done by changing the metering rods as well as jets. Once the fuel curve is determined then the spark curve may need to be modified.
To determine if a carb is too large for the engine does the engine bog at rapid throttle opening? Does it take a large increase in accelerator pump fuel volume to cover the bog? If you change from a low rear gear ratio to a high ratio a large bog develops?
At a steady speed in high gear surging occurs? Most likely if the carb is too large more that one of the above usually happens, but if only one happens it may indicate a fuel curve or timing problem may be the cause.
Of course a computer controlling the timing and fuel delivery will solve these issues in real time and improve mileage.
Al
1- The amount of air that flows through the engine at a specific RPM
2- The amount of fuel in the air at a specific RPM
3- The ignition spark curve
4- The overall drive ratio (turns of the crank shaft to one turn of the rear tires
5- Weight of the vehicle
6- Rolling resistance of the tires ( type of tread and construction as well as wheel alignment )
7- DRIVING HABITS (the most important item, not a reflection on you)
The maximum flow of a carb can never exceed the maximum flow of the engine (if you have a 750 CFM carb, but the engine can only use 650 CFM the carb will only flow 650 CFM). Therefore the carb with 750 CFM is jetted for its maximum CFM to have a specific air/fuel ratio and most likely needs to have its main jets size reduced. It probably needs the complete fuel curve reduced which with your carb is done by changing the metering rods as well as jets. Once the fuel curve is determined then the spark curve may need to be modified.
To determine if a carb is too large for the engine does the engine bog at rapid throttle opening? Does it take a large increase in accelerator pump fuel volume to cover the bog? If you change from a low rear gear ratio to a high ratio a large bog develops?
At a steady speed in high gear surging occurs? Most likely if the carb is too large more that one of the above usually happens, but if only one happens it may indicate a fuel curve or timing problem may be the cause.
Of course a computer controlling the timing and fuel delivery will solve these issues in real time and improve mileage.
Al
Mike
Well-Known Member
Look at that, look at that, look at THAT! Al "gets" it. :thumb: Post Of The Year nominee, right there.
It sounded so nice, I'm sayin' it twice -
I found a gentleman in Texas who is a carb rebuilder that worked for Holley for many years.He's rebuilding a 1406 for my T he sets up the carb
based on this questionnaire then dyno's the carb to set the fuel curve.Guarantees his work 100%.
TYPE OF CAR OR TRUCK.
WEIGHT.....
CUBIC INCHES...
ALTITUDE YOU LIVE AT...
GEAR RATIO..............
TYPE OF TRANSMISSION........
CAM SIZE............
HEADER TYPE AND SIZE............"
MAX RPM....
PERFER MORE POWER OR BETTER FUEL MILEAGE
Thought I give him a try...
