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Author Topic:   holley carb help,please.
posted June 01, 2003 03:12 PM
I just switched over from alcohol to gas and need help tuning carb. I am running a 750 holley double pump 4150-6 with power valve in front block off in rear .I am still using the barry grant mechanical bypass valve and the big holley mech pump with 2.5 to 3 psi at idle not sure at full rpm but i know over 10. went to track with 74 and 75 jets ran lean. I was told that I need 4-5 jet sizes bigger in rear to make up for no power valve.Jetted up 79 -82 for heat ran rich car bogging bad off corners. Went down to 77-81 still stumbles then takes off.Are my rear jets to big causing motor to run rich when secondarys are opened or do you think big pump is pumping to much fuel and when letting off in corners bowls are being over filled. Thanks in advance.

posted June 01, 2003 08:14 PM
First, you are pumping way too much fuel for a gas application when at wide open throttle. That's part of the problem. Any time pressure goes over 9lbs, a Holley float won't be able to close or hold back the needle. Higher pressure works with alky since the volume used is so much greater than with gas, that the higher pressures are not a problem for the floats since it's getting used fast enough that the float doesn't have to hold it back.
Next, your jetting there probably will wind up in the 76 primary and 80 or so secondary, depending on such things as accelerator pump cams and total volumes, and air quality (temp, altitude, moisture content, etc).
Fix your fuel delivery mismatch and jetting will become much easier as well.

Hope that helps,

gerald berry jr
posted June 03, 2003 11:03 AM
kp has right answer but along with it i'd put a power valve back in the rear running gas you don't need that much fuel volume thats the only reason to plug it

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