Hot Rod Forum banner
1 - 6 of 6 Posts

·
Registered
Joined
·
3 Posts
Discussion Starter · #1 ·
Is it possible to jet down an Edelbrock 750(1407) to Edelbrock 600(1405) specs? I have chevy 350 that the 750 would be too much for, and don't want to have to buy a new carb. Any help would be great.
 

·
Cruzin w/Elvis in Bigfoots UFO
Joined
·
247 Posts
The 600 vs. 750 carbs are jetted differently to work with the different sized venturis, boosters, metering orifices and other physical differences between the two. I would suggest you don't try and 'jet down' the 750, you can't make it smaller with tuning parts.

Try the 750 and see how it does, but if it's doggy you'll be better off finding something smaller.
 

·
Registered
Joined
·
17,613 Posts
I don't know who started all this "jetting down" business.
Every time I see this statement, it gives me pause, wondering what area of science it came from. According to everything I've learned over the past 50 years, the throttle bore size and venturi size dictate the CFM of a carburetor, not the jetting. The jetting you choose only changes the relationship between the amount of air compared to the amount of fuel that is being pushed into the ports by atmospheric pressure with the intake valve open. A 650 CFM carb is a 650 CFM carb, no matter how you change the ratio of air to fuel by changing the jets.
 

·
Registered
Joined
·
15,298 Posts
orrbmx said:
Is it possible to jet down an Edelbrock 750(1407) to Edelbrock 600(1405) specs? I have chevy 350 that the 750 would be too much for, and don't want to have to buy a new carb. Any help would be great.
I'm lost in the fog. CFM ratings and jet sizes are like apples and oranges.

CFM is the carbs ability to flow air, this is controlled by the size of the venturis and the obstructions there-in like the booster and the throttle blades and shaft and is independant of fuel jet sizes.

Jet sizes control the maximum amount of fuel that can pass from the float bowls into the main well before being ejected from the boosters into the passing air stream. Reducing the jet size of a large carburetor does not affect the venturi's air flow capacity other than if the mixture gets too lean or rich the engine will go into miss-fire and be unable to rise to the RPMs that would demand all the air capacity the carb is capable of.

In theory, the engine will use what air flow it needs for a certain throttle blade position to hold the commanded road speed. The fuel mixture would be correct for that setting whether the carb was rated at a max flow capacity of 750 or 650 CFM with their respective jet sizes. In reality it isn't quite that simple, the 750 with jets sized to make a correct mixture at the max rated air flow will prove to run rich when the average CFM flow demand is too far below this point. The design goal is to match maxflow CFM capacity closer to the max engine requirement or for a street engine rather less than max demand. When you get too far from these guidelines holding the correct mixture across the operating range of the engine gets sloppy.

But all that said, you can get acceptable fuel economy/performance out of the 750 by fuel jetting it like a 650 and putting it on an engine that is mild enough that it would never demand the CFM flow capacity of the 750. Will this be as tunable as a smaller carb like the 650 on such an installation, No! But it will work pretty well.

Bogie
 

·
Registered
Joined
·
3 Posts
Discussion Starter · #5 ·
techinspector1 said:
I don't know who started all this "jetting down" business.
Every time I see this statement, it gives me pause, wondering what area of science it came from. According to everything I've learned over the past 50 years, the throttle bore size and venturi size dictate the CFM of a carburetor, not the jetting. The jetting you choose only changes the relationship between the amount of air compared to the amount of fuel that is being pushed into the ports by atmospheric pressure with the intake valve open. A 650 CFM carb is a 650 CFM carb, no matter how you change the ratio of air to fuel by changing the jets.
1403 500 .086" .095" .065" x .052" orange (5") .0935" 7/16" 1-1/4" middle hole .028" 1-3/16" 1-9/16" 1-7/16" 1-11/16"
1404 500 .086" .095" .065" x .052" orange (5") .0935" 7/16" 1-1/4" middle hole .028" 1-3/16" 1-9/16" 1-7/16" 1-11/16"
1405 600 .100" .095" .070" x .047" orange (5") .0935" 7/16" 1-1/4" middle hole .028" 1-3/16" 1-9/16" 1-7/16" 1-11/16

If that holds true, Edelbrock makes a 500 cfm carb with the exact primaries, secondaries, and venturi sizes. But when the jetting and metering rods are changed it magically becomes a 600.
 
1 - 6 of 6 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top