SSC

Same 80457-1 600 (4160 polished). I spent all day trying to fix the bad cold start problem, couldent find a leak or crack anywhere so I replaced all the gaskets. Anyway I'm stll not getting as good of fuel economy as I should be, around 10-12MPG city. Ive never gotten worse than 14mpg with any holley/350 combo. I rejetted it droped from the stock 65's to 61's for high altitude adjustment. I even got a book Supertunning and modifying Holley Carbs, no help. I hooked up the good old vacuum guage and tested my normal driving habbits, opperation 17in, in N, 14in, in D, 8in on takeoff, part throttle cruise 15-17, max deccel 21in. These should be typical reading for a mild street setup. Where's all the gas going? I shoulda checked the PV reading but Im sure its a 6.5 and it is good. Wheres all the gas going? Fresh tune up too. The Qjet I had on there a couple weeks ago was doing good, 17MPG but the thing lacked WOT and tire shreading power that I dont typically use but is good to have in reserve. Help! I dont want to get a honda!

RB83L69

14 in. in D, but a 6.5 power valve... mismatch.

Try dropping your primary jet size in 2 # increments until it just starts to surge at cruise, then go back up 2 sizes. Get a 9.5 PV.

SSC

I dropped down to a matched set of 58's the lowest Ive got, Idle quality or off idlle responce hasent degraded so its a good sign. Should I get some even smaller jets or just leave it?
I was under the impression that the PV needs to be 2 under the lowest point of normal operation vacuum, 8in so a 6.5 should be good?????? If a 9.5 would be better im all for it. What are your thoughts on a two stage PV?

F-BIRD'88

You have to lean out the idle circuit. ( idle feed restriction) leaning out the main jetting will only get you so far. Then you'll be too lean under mid and power throttle settings/load.

You can lean out the idle circuit by putting a "V" shaped short piece of mig welding wire .025/.03" in the ide transfer hole
in the metering block (see your tuning manual) keep adding wire
to restrict fuel till driveability at and near idle goes away.
then go back one.
When driveing in the city you spend about 80% if your time
idleing or just off idle ( transfer slot). This is where your gas is going. (its richer than it needs to be) The main jets don't come into play till higher road speed.
If you have an afr guage you'll see that the idle is richer than 14.7:1 (it needs to be for driveability) Your afr at cruise can be as lean as 17:1 (best mileage) as long as it doesn't surge on ya.
(cylinder to cylinder fuel distribution).
A heated intake manifold goes a long way to helping here. Helps to vapourize the fuel better.
(heat riser).
makes that factory cast iron intake kind of attractive eh...
As you lean out the idle circuit your cold running driveability will degrade the most. You'll see the need to have the cold/ choke and fast idle stuff hooked up and working, like when ya had the q-jet on there. You may have to fiddle with the accelerator pump adjustment too, to retain drivability.

RB83L69

You want the PV to be closed any time there's no particular demand for power, i.e. idle & cruise. But you want it to open as soon as the driver decides he/she/it would like more power. The leaner you can get the main jets without surging or otherwise misbehaving or causing cold running problems, the better off you'll be; but the leaner the mains are, the sooner you'll need power enrichment as you push the throttle.

The same goes for the idle circuit, as described. The main circuit flows fuel in proportion to air flow through the venturi. The idle circuit flows fuel in proportion to vacuum. If your motor produces more vacuum than the carb is set up for, you end up with the fat rich idle.

Holleys almost always come out of the box way too rich; they're set up for max blast on a motor with too much cam. When you put one on a more tame setup, some calibration is usually required.

SSC

Well I thought I would give an update since dropping the jets.
I'm now around a solid 14mpg city so I'm not going to complain anymore. The car actually runs smoother with the #58 jets and I was able to close the throttle blades a little bit more which probably helped save gas on warm up/idle.