I've spent the past two weekends trying to work the bugs out of my transmission/carb/valves/timing/etc. Basically I've been trying to tune my setup to work properly.
In case everyone has forgotten here is what I've got. - 400ci engine (212/222 cam) w/ 6X-4 heads - 200R-4 transmission - 2.79 open rear end - REbuilt Quadrajet Carb - 235/45R17 tires
To make a long story short, I am getting a horrendous 10 MPG on the open road.
I don't see how in tarnation that is possible with the highway gears and the overdrive transmission. At 70mph I am going MAYBE 1700rpm.
The transmission works fine, goes into lockup in 4th gear as it should. Rear end works fine. Engine seems to work fine (timing discussed later).
So, I suspect the carb, but the guy that rebuilt my carb tells me its probably not the issue. He says its the combination of my overdrive with the highway rearend that is destroying my gas mileage. He says that the engine isn't getting into it's powerband and therefore has to bog down to get anything done. I understand that, but at highway speeds I am barely hitting the pedal to move forward.
I've been contemplating for a LONG time to upgrade my rear gears, move to a 3.23 open for a minimum, or a 3.55 posi. But, before I go spend $750+ on a new set of gears and/or a posi, I need to make SURE that it's not anything else.
SERIOUSLY, I can't believe that I should be getting 10 MPG, and there isn't anything I can do at this point to make it run better.
My friend here has a '66 389ci tri-power GTO w/ a 200R-4 and 3.23 rear gears and he gets about 21-22 MPG on the open road. Now THATS what I'm talking about.
For those that are carb savy, let me give you the information carb on the Q-Jet he rebuilt for me.
Number: 7043282 Jets:72 Rods:42 Secondary: BE .041 Float: 1/4" drop Enlarged: Idle bleeds, main well orifices, and secondary shot.
Does that sound like too much of jets/rods for a 212/222 400ci engine?
FYI: We re-timed the engine last weekend. It was about 12 degrees (48 total @ 2500) and we moved it back to 3 degrees (36 total @ 2500). FYI: The change in timing didn't help the MPG at all, it was 9-10 MPG before at 11 degrees, same now at 3 degrees.
I would really appreciate any and all help. I love my car, it's a blast to drive. At $3/gal and at 9-10MPG (when I KNOW I should be getting 16-18), it really hurts to drive it.
I spent a lot of time and money designing and building this Firebird to be a 87 octane highway cruiser, not a @#*&%$ gas pig.
