Not a lot to go one, but
I can guess with the best of them.
My questions are;
>at what speed were you driving to get that 6mpg, or by tach, what rpm? or was it combined city/hiway?
Then
>At what elevation are you operating at; cuz 67s at sealevel are way too lean.
>What's your Cylinder Pressure? Cuz that combo wants at least 10/1 true Scr, /or 160/165psi..
At 140 psi, the fuel-economy will tank.
>iron heads?
>Who told you the power peak on that cam was 6800? My guess is more like 5300.
>With 3.73s, a manual trans, and 24" tires; 65mph calculates to 3400rpm. For cruising and to get optimum fuel economy your timing is gonna want to be about 56* advanced.
with an auto-trans your rpm
might be/should be, within +/- 3 %.
What I imagine; and/or am afraid of;
> is that cylinder pressure is lower than optimum, and the
> for cruising;
> the cruise timing is way short of the mark, and
the idle-mixture screws are set way too rich, and 67 MJs are way too lean
and the the throttles are way too far open, in an attempt to compensate for all those short-comings.
> and I'll bet you are trying to run a 160* thermostat, with the carb sucking hot underhood air.
I mean your cam is not that far from mine, and my car comes in at 3650/me in it, runs 3.55s, with a 360 even!, burns 87E10 with PMJs in the 70/72 window, and in my wildest dreams I could never get it down to 6mpgs on the hiway. notta chance. If it wasn't at least 3 or more times that, I'd worry.
Oh sorry, I run at 65=2240 rpm in overdrive lol.
And BTW
Your car requires a certain amount of fuel to make a specific amount of cruising horsepower. Lets say your particular car requires 40 hp to maintain 65 mph. Doing the math on that, I get 16.7mpg. Because your engine is spinning at about double the rpm it would require to make 40 hp, a bunch of fuel is getting burned up and/or wasted in just making the rpm, and the cylinder pressure, and fighting ring-drag, etc. So while I agree with everyone that 6mpg is insane, lol, Yur combo is never gonna be that great, and I can tell you, that one of the reasons, is the very short extraction period of that cam.
By my guess, that comes to around 106*@ zero lash, probably less, which means that your pistons are down just a tic over halfway, when the exhaust valve opens, allowing the still expanding hot exhaust gasses to escape the cylinders and heat up whatever they come into contact with. Are you having overheat issues? this could be part of it.
And another thing;
By my guess, that cam of yours has around 67* of overlap. Which, with headers, and screaming down the hiway at 3400 rpm, is gonna put a major yank on the plenum, pulling raw gas right across the piston-tops and into the headers. Imagine what that does to your fuel-economy.
BTW-2
The 750 double-pumper is the right carb for that combo.
1) the accelerator pumps only operate while the throttle valves are moving, and after the shot is delivered, you won't get another shot until you close the throttle valves first. So; at a steady speed, neither of the pumps are doing anything.
And you can adjust those pumps about three ways, to work on your combo. Not every combo needs them to run at max.
BTW-3
I did not read every post, so by now, surely somebody talked about synchronizing the Transfer slots to the Idle-mixture screws.
If that carb does NOT have a 4-corner idle system,
then IMO, the primary throttle valves will need to be drilled, to give the engine the air that it wants without jamming extra gas into it. If this is not done, then the mixture screws will end up wrong, usually too rich at off-idle but low throttle openings. ie a rich cruise condition.
BTW-4
>you are running a PCV right? this is NOT an option.
> I've never had an unheated O2-sensor read right at idle.
Crap, I'm falling asleep, I gotta go lie down
