i was thinking about putting a 4 barrel carb on my 240. i want to know if a q-jet would be too big. im only thinking about a q-jet because i know they are cheap and easy to work on.

 

A small block configured Q-Jet should be just fine as long as the secondaries are adjusted to the needs of your engine. I like spread bores...small primaries for driveability and economy and more than enough left in the secondaries for the "Exxon" moments.

 

BTW, a Holley Economaster (see my signature) is a spread bore replacement for QJets and work quite well. They can be found on eBay and rebuild kits are available for them.

 

The q-jet should work fine on the 300. I have one on a 4cyl marine engine (224 cu in) and that is factory installed. The q-jet was used on a wide variety of engines, from 4 cyl to V-8's. The q-jet has mechanical secondaries that open when the primary's pass a certain point. However that doesn't mean the secondaries start to work at that point. Above the secondary throttle plates are the secondary air flaps. They are spring loaded plates like a choke that are opened by the air flow through the secondary throttle plates. This makes the carb essentially a variable CFM carb. The air flap will open only as much as the engine can use. I would find a marine carb from a 305-350 (usually Chevy). These carbs are very basic no frills carbs, without all the pollution calibrations. Some of the marine carbs have the choke on the carb and it is electric, most however have divorced chokes (Chevy) and will have to be converted ( there are several kits available to do this) to on carb electric choke. There are some parts available ( Rochester quit making them 15 years ago)--jets-some primary and secondary metering rods-some secondary metering rod hangers --and of course rebuild kits of all kinds. The early 70's Olds carb would be a good choose as it has an on carb hot air choke that can be easily converted to electric choke. If you are going to use the Q-Jet get the book--Rochester Quadrajet Carburetors by Cliff Ruggles--it tells you everything you need to know and is up to date.

 

thanks for all the info. im probably going to pick up one from the junkyard and start messing with it.

 

Cliffs High Performance Quadrajets :: Qjet Carburetor Rebuilding, Bushing Kits and Parts

Ruggles has the parts for rebuilding too.

 

See my signature- Q-jet on a 300. I bought mine rebuilt from NationalCarburetors.com for $210 delivered. They did all the right stuff including new bushings and epoxy on the places that leak. Use an "M4ME" model carb- it's a later version with an electric choke.

Only thing I can say is learn how to tune it. This is where I hear most people go wrong and start calling these carbs "quadrajunk". Primaries are tuned with rods & jets. Rods are stepped- the "cruise" portion of the rod varies in diameter from rod to rod. The WOT tip of the rod is always the same size. If your cruise mixture is wrong, you have the change the rods- WOT won't be affected. If your WOT mixture is wrong, you have the change the rods and the jets, because changing the jet for WOT will affect cruise tune. When changing rod & jet sizes, don't "match" based on diameters. Q-jet rods & jets are tuned using the open area of the jet, and the blocked area of the rod. Remember PI-r-squared, etc.. Secondaries have a fixed jet, so only the rods change. The last thing is to tune the timing of the secondary air valves. There's a screw you can adjust to change tension on a spring on the air valves. This acts against a vacuum "solenoid" to determine how much secondary gets delivered for a given engine load.

Good luck!

 

What intake do I need to make this work? I have a 1976 F-100 4x4 with a 240 in it, and I would LOVE to make this work! I have 8 q-jets sitting around doing nothing.

 

looks like you need an adapter plate and fab up some kind of throttle linkage.

 

Offy C, DP, or Clifford 4v intake and an Edelbrock or TransDapt spread bore/square bore adapter. Obtain a Lokar 4' throttle cable, fabricate or obtain the bracket and start putting it together. fmc400 had a bracket fabricated to work with an Edelbrock 500 using the stock cable and posted pics a while back. Same thing can be used for just about any 4v carb.