Posted By: Clayton
biggest carb for a 273 - 03/16/12 12:16 AM
Well my pos eddy carb is evidently junk and I was wondering if I could get away with running a holley 750 if I jetted it down. I know its a little big for a 273 but its all I have and if I drop the jets down from the 72's that are in it do you think there is any way it will work and if so what size jets should I go with? Thanks, Clay
Posted By: ahy
Re: biggest carb for a 273 - 03/16/12 12:52 AM
If it has a dual plane intake it would probably work OK. Dual planes are tolerent of big carbs. No idea on the jet size.
Posted By: 71yelladustr
Re: biggest carb for a 273 - 03/16/12 12:55 AM
I think you are kind of missing the boat on the thinking of jetting down a 750 carb for use on a 273. Its not the jet size thats the issue, its the capability of the carb to flow fuel AND air. A smaller carb with smaller throttle blades, venturies, etc... will make the car have crisp thottle response and run better than a carb that is too big and does not proudce enough vac. signal on your engine. Unless you are planning on turning your 273 some ungodly RPMs, I think a 750 is too big. Id say common consensus would suggest a 500 to 600 CFM carb.
Posted By: DUFFMAN
Re: biggest carb for a 273 - 03/16/12 01:33 AM
Does the Holley have mechanical or vacuum secondaries? If it has vacuum secondaries you should be fine jetting it down as long as the secondaries are adjusted/operating correctly because your engine will only open them as far as it needs or can use.
As far as throttle response that depends on the size of the primaries. A spread bore will give you better throttle response because generally the primaries are smaller.
Remember, the CFM rating is the carb's maximum ability to flow, it doesn't have to flow that much.
Posted By: Clayton
Re: biggest carb for a 273 - 03/16/12 02:38 AM
It has vacum secondaries which also led me to believe I might get away with it. gonna try it with the 72 jets and see how it acts but don't know how small to go on the jets
Posted By: dogdays
Re: biggest carb for a 273 - 03/18/12 06:35 AM
The carb doesn't know what size of an engine is under it, all it knows is the velocity of the air going through the venturis and especially the venturi boosters. As the air flows through the boosters it speeds up and because of Bernoulli's equation we know that pressure at that point drops.
This pressure, which is below atmospheric, allows the atmospheric pressure plus the pressure of the height of the fuel in the float bowl to push the fuel through the jets and boosters into the air stream. So what the carb reacts to is air velocity. Period.
It stands to reason that a carburetor set up to provide a correct air/fuel ratio over a wide flow range will do so on various sized engines. This is the long way of telling you that if the carb is set up right in the first place you shouldn't have to rejet just because you put it on a smaller engine. Air velocity will be quite slow and you may need to adjust your accelerator pump shot because small differences in throttle will produce small differences in fuel flow that might not keep up, although that's an area that you have to experiment.
The front two barrels of a 3310 750 Holley are about the same size as a 500 cfm Holley two-barrel. I suspect you will not be seeing the vacuum secondaries open very often.
But I suggest this talk of "jetting down for a smaller engine" is out of widespread ignorance of what the carburetor actually does. I suggest you stick the carb on and see what happens. Use the non-stick gaskets so jet changes will be easier when you start experimenting.
The other question is why you think your Edelbrock carb is junk. Did you break it somehow? I bought a carb that someone had turned the idle screw in so far that it broke the casting. I don't know how to fix that! But almost everything else can be repaired or replaced.
Edelbrock settings for carbs are easy to replicate as their info is all over the Web. Their electric choke carbs are lean for mileage, their manual choke carbs are rich for power. They are easy to change and the charts tell you exactly what to do.
Good luck!
R.