Most likely it was not rebuilt properly. Missing or misplaced parts. Incorrect idle speed screw or mixture screw settings. Incorrect float height. etc.
This is a difficult question. The easy answer is the qjet is better on mileage. But there are so many variables that its really hard to consider one or the other being better for all engines. Your 'typical' qjet - 750-800cfm spreadbore, to a 750-800 holley squarebore. Both of these carbs are more complicated to tune than an edelbrock, and both will likely give more power. Edelbrock will likely give the best transition when the secondaries come on. Holleys being more complicated also have more tuning capabilities (about the same as a qjet, and both more than the edelbrock), and being that the bores are the same size, when the secondaries open up the power is a nicer transition than the qjet. The rochester qjet (thermoquad is carters equivalent) has smaller primaries because it is a spreadbore. This means when driving around 90% of the time in a street car, you use less gas per cfm. Holleys are still being developed today, they offer many goodies and some safety features you wouldn't find in a remanufactured qjet. I much prefer holleys on an engine which couldn't use the cfm flow of the big qjet in the first place. Now when we're talking about 383 ci and bigger (or a really thumping 350) I would take the qjet for the street. Basically, if your engine size, cam, intake, etc cannot use 750-800 cfm, there's no point in getting the qjet. Also the qjet, having larger secondaries will really make some noise and you'll feel the power coming on harder when the secondaries open up. This isn't saying it makes more power, just how it is. (mechanical secondaries will give more kick, so again you can have similar results with a holley with nicer transition - better torque curve) Edelbrocks to me are weekend warrior or general mechanic carbs, someone who doesn't want to spend the time or money to learn how to tune correctly.. The downside with chosing those is holleys are going to make more power for about the same money. The downside with holley is you may have to spend a few hundred $$ on gauges, tuning tools and supplies, and start educating yourself on how an engine and carbs work. And if you have that hopped up cam, you may want to consider the holley for free top end hp. There are a number of aftermarket companies that have sprang up and based their designs on holleys. Quickfuel, proform, and a handful of others. Some of these advertise better power, but who knows. Seems to me they're all just the same as a racing style holley without choke or choke horn etc. If you know how to work around a carb and tune already, then grab a qjet and put it on the engine of the correct size. Now you've got some big power and some reasonable mileage. If your car is a 1/4 mile or track car, get the holley. If I remember correct, holley did make some smaller spreadbore carburetors for marine applications. You could look into those for even better mileage on a smaller engine, say a v6 or small v8. If you could not find a spreadbore for those sizes, get a smaller cfm holley it will be more efficient and similar mileage as the qjet. I haven't even discussed vacuum advance vs mechanical and dual pump vs single accelerator, different style chokes etc there are a lot of variables to consider, and that's not even talking about the drivetrain/engine/vehicle the carb is in ...one other thing to consider is if you have a really lead foot and you're going into and out of the secondaries all the time, a squarebore may have better mileage than a spreadbore. Think a crazy teenage driver here.
If what you have is a stock 350 with a stock intake manifold and no other modifications other than the cam, you are probably around 300 horses. To make some changes that will enhance the cams potential, you will need to do some additional things. First, if you can't recurve the distributor yourself, have a shop do it. with that cam I would aim for limiting total mechanical advance to about 20 degrees and I would use springs and weights that will make the distributor hit maximum advance at about 2400-2500 RPM. Then I would set the initial timing at 15-16 degrees. Small block chevies seem to do best with a total of 36 degrees total advance. I would also set the vacuum to the distributor so it is manifold vacuum, not ported vacuum,..That way, there is always vacuum advance and that will greatly enhance low and mid RPM performance at part throttle. The next thing that I would do is add an exhaust system that increases flow. A lot of people don't like headers, but I strongly believe their advance far and away outweighs the hassles that they can cause you. The next thing that I would do is to get a dual plane performance intake manifold, and run a500CFM Edelbrock or Qjet carb on it. With these cahanges, you are probably in the 375-400 horsepower range. Good luck
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