Well, I had the time today to swap out the 670cfm SA for a 770cfm SA. It has the stock 72 primary jets and stock plain silver secondary spring. I changed the discharge nozzle from a .025 to a .031 and changed the PV from a 6.5 to a 8.5 because as stated before, it has 15 to 19 inches of vacuum at cruise. Also, I changed pump cam to black and am using the #2 hole. It's running real nice except for a slight hesitation when stomping the pedal in drive from a stop. As for fuel mileage goes, I'll have to check it another day and let you know the results. Thanks again for your advice and the advice from others.
You do know if your looking to save fuel, you need to lower your power valve right. The power valve is design as an fuel enrichment circuit to assist the primary jets. So if your trying to save on gas, you want to run one as low as possible without hesitation. So your cruising down the road and step into the pedal just a little and vacuum falls to say 7.5 your 8.5 opened up and gave more fuel to your engine when you may not need it yet. Now if you had the 6.5 power valve it would have stayed shut and you would save fuel.
You just can't willy-nilly changes. Like taking the .025 nozzle off for .031 ( so let's dump more fuel down the carb) and throwing on the smallest black cam ( well let's try and reduce the fuel I'm dumping down the carb). what the hell are you doing?. These are all willy-nilly moves.
I understand you switched carbs because of one person saying so. But understand this, you exchanged velocity for a bigger carb that has a slower velocity speed which requires more fuel to cover the stumble issue. You have a bigger hole, now it requires more fuel to cover.
I understand the guys thinking to get the blades to close some to get closer to idle circuit, in this case it really don't matter, both carbs will be running on main jets at 3200 rpms. carb with smallest jets uses less fuel ( hello this is easiest to understand)
You would be best served going back to the smaller carb, your already ahead of the game running small primary jets, you lean out the idle as lean as possible you can live with. Run the smallest nozzle and smallest accel. cam and the lowest power valve.
Did you know when your rpm are below 2200 to 2300 rpm your just on your idle circuit, so your gas mileage is influenced by your idle mixture screws, so you want those as lean as possible.
Did you also know when your on your idle circuit which is below the 2200 to 2300 rpm mark, your power valve has no function what so ever. Your power valve can be open or shut at idle, don't matter, your not drawing any fuel from PVCR until the main jets are pulling fuel. So in simple terms, when your main jets are pulling fuel from the fuel bowl only then the power valve has any function.
SOAP BOX TIME
All of you in the hotrod world still believe that power valve works during idle. Example- my engine idles with 4.5 inches of vacuum I must run a power valve below that like a 2.5, what B.S.!, Your power valve channel restrictors can only draw fuel when the main jets are drawing fuel. Are your main jets drawing fuel during idle or just driving around under 2300 rpm- NO!
The power valve is nothing more then another set of holes to add to the main jets. So instead of running a 90 jet in the primary you get to run a 75 jet and when vacuum falls under the power valve setting during ACCELERATION it will become active to boost the main jets. You don't want your extra fuel to come on at 2.5 inches of vacuum, what the hell, when you step on the gas hard you want the extra fuel now, not near zero vacuum, this causes lean spots and dead spots everyone trys to cover up with more squirter. OMG!
So if your big azz engine idles with 4.5"hg and it cruises with 12"hg you probably want like a 10.5" power valve, so as soon as you step on the loud pedal you got all the fuel needed now. Maybe thinking about gas mileage maybe an 6.5"hg power valve.
But why in the hell would you want to wait til your vacuum dropped down to 2.5" before assisting your jets? So many of you think this way because you all don't understand what's going on.
And if you still can't wrap your mind around all this, ever heard of the folks blocking off the power valve? What happens when you do this? You have to up the jets about 10 numbers right? Has a light bulb come on yet? Block the power valve you have to jet up- you jet up because the power valve assist the jets. You block the power valve off and this takes away the pvcr holes that aid the jets. Getting it yet? Power valve and jets work together.
Power valve has nothing to do with idle circuit guys, never did never will, argue all you want, you block off the power valve, are you *ucking with your your idle circuit? NO!
So when it comes to power valve, you pick it based on what your trying to do, mileage as late as possible without stumble, horsepower soon as possible, a drag car you pick it based off your starting line rpm launch vacuum- you stage your car say up against the brake and it read 12"hg at 3000 rpm, then you probably want the 10.5" power valve. Maybe same car you got 8.5" during stage and you have a 10.5 in your carb, that's fine, power valve is open and you getting all the fuel your carb can give at launch. but if you had a 2.5 you probably have a dead spot and your waiting for that fuel on launch.
OFF MY SOAP BOX
With the smaller carb you have a smaller hole to cover under rapid pedal changes ( you just figured this out, your bigger carbs has a stumble) , so this should allow the black cam and a .025 squirter to reduce fuel usage yet.
Do what you want, but please understand the changes you make and what it effects.