I thought getting a smaller carb would help with mileage... I rarely even hit 3000 rpm when I drive. Anybody have any ideas? Combo's in the sig. Cam is stock at .435/.460, 212/222 @ 0.50; CR is under 9.5 and rear is stock 2.73's. Thanks in advance.

 

Yes, that double pumper is taking the economy away. Ive got a friend with the exact same crate engine, runs a standard Vac sec 600 holley and gets 15MPG in the city.

 

Is there any way it can be tuned for better mileage? Right now its stock out of the box. I was told by many people not to get a vac secondary carb with a manual trans.

 

Lean down the primary jetting on the double pumper.

Go down in size till driveability goes away. Then go back up two sizes. as you lean down the primary side, cold weather drivability will require a functioning choke.

What is the jetting you have now?

Try #62 primary jets.
Check ignition timing,, should be something like this:

12 to 16 initial, with 32 to 36 total mechanical by 3500 rpm,
with 15 to 20deg vacuum advance at high speed cruise.

 

F-Bird'88, I've never really messed with timing, so I'm not really clear on that. As for the jetting, I found this:

"Double Pumpers and Gas Mileage (or lack thereof...)

Have you heard people complain about the gas mileage they get with performance-type double pumper carbs? There is a reason that the 0-4776 through 0-4781 double pumpers get bad gas mileage. It's the jets! Surprised? Holley sizes the jets and air bleeds on these carbs so that they run on the rich side at cruise speeds. They make more power this way, at the detriment of gas mileage. These are competition carbs, and they are supposed to work this way. Why don't we just put leaner jets in them to get mileage? Because the PVCR's are small, and under power conditions, the carb will supply an overall lean condition. What you can do is lean the jets out, then enlarge the PVCR's to compensate for power situations.

The way to do this scientifically is to measure the diameters of the openings of stock jets and PVCR's using drill bit diameters. Then calculate the total area of all the openings, and add them up.

A = 3.1415 x dia x dia / 4

Decrease the main jets for proper cruise mixture, and enlarge the PVCR's until you get back to the original area of all the openings. This way, your cruise mixture will give you good gas mileage, and under power, the engine will have proper mixture."

It came from this site:

http://www.bob2000.com/carb.htm

I also talked to Holley and they said to get a VS carb. I contacted Summit and they said stick with a double pumper. I know you shouldn't match a VS with a manual, but I've never heard WHY exactly. Does anyone know??

To add to my confusion, I looked and the secondries don't even open until after 3000 rpm. I stay around 2000 (2500 max) so I'm only running on 2 barrells! 8 mpg with 2 barrells... Holley advised against modifying the pvcr's but I may have to. Has anyone ever gotten decent mileage from a double pumper? Ddi I get the wrong carb?

 

I know how ya feel man with a 750 edelbrock manual sec. I get 6-8mpg no matter wut, it even seems liek its worse if i shift at like 1500 and keep it under. I duno wuts up with it. my carb is tuned real good and my timing is at 12degrees. seems to keep getting faster to more i advance it and still doesnt ping lol. well if any1 knows how to get good gas milage without fuel injection hook a brotha up lol