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I've recently replaced the carb and had some performance enhancements done on the engine. Now I'm only getting 6 to 8 mpg around town (12 on highway).with very reasonable driving style. Is that normal or is something not dialed in right? Here's the setup. 69 Firebird 400 CI engine, mostly stock except below. Holley 4150 double pumper 750CFM carb Butler Edelbrock D-Port, 72cc, Hyd. FT. Top End Package (intake, heads, mild cam) https://butlerperformance.com/i-307...-ft-top-end-package-bpi-tep-dp-72ft.html700r4 transmission and 3.73 differential. The shop says everything is set up correctly but that 6 to 8mpg seems just incredibly low. Any thoughts?
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The gas mileage conversation is one that we love around here... So the answer to your question is maybe. First off for reference my bone stock well worn 350 with grocery getter gears and a 2bbl carb gets between 10 and 12 mpg around town. Not super far off but you should be doing better even with bigger cubes and enhancements. Some guys claim horrible gas mileage and others seem to rival a Prius.
My question would be how is it running? Do things seem good? Is it running rich? Idle good? In other words if you never measured the gas mileage would you have reason for concern?
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I would recommend doing some of the "common sense" checks first. Record mileage 2-3 times and take the average mpg. Look at a few of your plugs and check color to see if you're running rich. Check air pressure, PCV valve and air cleaner. Double check your timing and check your fuel pump for leaks. Verify your carb air/fuel mixture with a vacuum gage and make sure you're getting good engine vacuum at idle (15mg+) 20 is better. Once you verify the basics and if all checks out then that's probably all you're getting in mileage. If it was me I would also switch out the Holly for a well tuned Quadrajet, you'd have 750 as well and I'll bet you'll get better mpgs.
Last edited by ramair68; 12/14/20 12:37 PM.
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Double pumpers are meant to provide massive amounts of fuel to high power engines. They are not meant to be thrifty.
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In addition to what was said above, are you running a distributor hooked up to vacuum advance? And since you bought a package from Butler, ask them for a recommended distributor curve (static and mechanical advance). Any time you make volumetric efficiency changes like compression, camshaft, heads, etc., your distributor has to be re-evaluated, just as the carburetor settings do.
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Joined: Jul 2018
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My question would be how is it running? Do things seem good? Is it running rich? Idle good? In other words if you never measured the gas mileage would you have reason for concern?
Unfortunately I wouldn't have any basis for comparison since I don't have any other 400's to drive  Anyway, I took it in to the shop today and it looks like it was running a bit rich. Made some changes and will see.
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Joined: Jul 2018
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I would recommend doing some of the "common sense" checks first. Record mileage 2-3 times and take the average mpg. Look at a few of your plugs and check color to see if you're running rich. Check air pressure, PCV valve and air cleaner. Double check your timing and check your fuel pump for leaks. Verify your carb air/fuel mixture with a vacuum gage and make sure you're getting good engine vacuum at idle (15mg+) 20 is better. Once you verify the basics and if all checks out then that's probably all you're getting in mileage. If it was me I would also switch out the Holly for a well tuned Quadrajet, you'd have 750 as well and I'll bet you'll get better mpgs. Thanks, I've started to check. The mpg is already an average across several hundred miles.
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