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post #1 of 4 (permalink) Old 06-20-04, 01:37 PM Thread Starter
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confused Thirsty Pony

I am trying to determine why I am getting only 9 to 10 mpg fuel economy and whether I should install a Holley 750 cfm double-pumper carburetor on my 5.0L Mustang.

Here's the background:

I installed a B&M 144 supercharger several years ago with a Competition 268H-10 cam/hydraulic lifters.
Gross Valve Lift .456 Intake .456 Exhaust
Duration at 0.050 tappet lift 218 Intake 218 Exhaust
Duration at 0.006 tappet lift 268 Intake 268 Exhaust
Lobe Separation 110
With stock heads, Cyclone AR-1 headers, and a Holley 600 cfm vacuum secondary & #68 orifices, I was typically getting between 16 and 19 mpg.

This past fall, I rebuilt the engine and bored the cylinders 0.030 over. I replaced the #68 orifices in the Holley with #72's. I also replaced the camshaft with a Competition XE262H-10 cam, springs, and lifters.
Gross Valve Lift .493 Intake .500 Exhaust
Duration at 0.050 tappet lift 218 Intake 224 Exhaust
Duration at 0.006 tappet lift 262 Intake 270 Exhaust
Lobe Separation 110

I WANT 16 to 19 mpg again. My son has given me a Holley 750 cfm carburetor for Father's Day. There was fear that the vacuum secondary would cause my air/fuel ratio to go lean during acceleration with the blower. Is the 750 too big? Will it further hurt my fuel economy? Is the Holley 600 not working correctly?

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post #2 of 4 (permalink) Old 06-20-04, 11:22 PM
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Re: Thirsty Pony

The 750 can handle more air and fuel and hence power.
If the 600 is tuned and good for the job, then there would be no need for the 750.
If you can dyno it and stick an air/fuel ratio meter up the exhaust, this will tell you if the carbs not right.
If you were happy before, you shouldn't have modified it. But if you were after more power than this new fuel economy is the price you will pay for it.
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post #3 of 4 (permalink) Old 06-21-04, 04:27 AM
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Re: Thirsty Pony

Did you actually replace the 68's with the 72's because it was running lean?
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post #4 of 4 (permalink) Old 07-06-04, 10:54 PM Thread Starter
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eek Re: Thirsty Pony

I installed a new Holley 750 double pumper. The front jets are 76's and the rear jets are 80's.
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