Has anybody ever noticed a signifigant difference between the two? I can't beleive how fast my gas gauge moves...I though I was looking at the tach! Probably doesn't help that my foot is pressed against the floor alot :D .

My power valve is blocked in the back and there is 80's back there. I was thinking of going back to stick settings.
Thanks

Make that 82's in the back!

With me it was a huge difference in mileage.I took a 750DP off ,that was getting 8 MPG and put a 780 0-4803-1 OE vac sec carb and got 12 MPG .

Between the 650 DP I have and a 750 VS I see 1.5-2mpg improvement. I have the DP set pretty lean on the main circuit but have to use a real big acc. pump circuit to cover the 'hole'.

Vince how much cam is in your motor? If it's at all mild stick a Q-jet on it. Not only will it run better at low speed but the mileage will be much better.

Mike,
Its a 454 ho thats not even broken in yet. Just got it running this weekend. It carburates very nicely - no stumbles or bogging. I'm getting 15" of vaccum. I wonder if the PV needs to be changed cause I think there was a 4.5 in there.

My 950HP has 6.5 powervalves in it,front and back. I'd put the powervalve back in the back and jet accordingly. No carburetor will get good mileage with your foot in it. I probably average 10-12 mpg IF I keep my foot out of it,stall converter don't help,soft tires don't help,3.73's arent what I'd call a good gear for fuel mileage.

A carb tuning trick I was told about entails jetting for best gas mileage at cruise, then enlarging as needed the PVCR to give the best WOT mixture. Most of us change jets for WOT and let the cruise mixture fall where it may. Having the right PV helps, too. If it open too soon, mileage suffers, too late and the engine suffers.

If I have 15" of vaccum and a 4.5 PV installed, does that mean PV would open late?

Check your vacume at cruize speed then install a power valve that is just below that. You can also open up the power valve distribution holes leading into the main wells in the metering block for extra power valve flow if you jet it down low on the main jets. There is a formula to calculate this and it deals with the jet opening area decrease and the needed increase in the power valve circuit holes. Maybe someone here can provide the formula to calculate this. Jet the primary side down till a surge is felt then go up two sizes or until it runs clean, then have the power valve circuit flow the extra fuel needed when the pedal is mashed. A double pumper and vacume secondary should provide the same fuel mileage if setup similar. Double pumpers are more performance orientated and are jetted rich on the main circuit and less on the enrichment circuit than a vacume secondary, if mods are made to enlarge the enrichment circuit and decrease the main jet then mileage should increase. JMO here, no expert

yes Vince,,,its opening late or not at all

FYI,

Vince has a well prepped 850DP on the car set up originally for a lumpy 396 motor. With the 4.5 and the high vacuum, the new 454HO motor is probably sucking the power valve right open I assume.

With that set up and the tip in is clean it's probably too rich on the pri side. Try leaning it down until you get a stumble at tip in and then start playing with the power valve. Unless the motor is in a dump truck it shouldn't need anything close to a 4.5 power valve but a really tall rear axle ratio can make it require less power valve than you would think.

It is a 750DP (4779). I think I'm going to try to return the carb to factory specs. I will take the advise and put the PV back in the back.

Do the secondaries need to be cracked open w/this? I think this car could/should idle a little better. If I back out the idle screw it still idles and won't die. However, I can gently force the secondaries shut by hand and then the car will die like it should.

GRN69...Vacuum is what holds the power valve closed,,,his power valve will not begin to open until it drops to 4.5"...the vacuum drops at WOT and the power valve opens to allow more fuel as needed,I tried a 4.5 power valve in my car and it went lean, I had to put the 6.5 back in.