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.

Also, if anyone else is running a 400ci with a 200R-4, would you please mind posting your specs and MPG? Thanks.

Just a thought. Put the tranny into 3rd and run on the highway again. If the MPG improves, then lower your rear gearing. If not, look into the engine.

I put a 2004R into my car with wonderful results. I have a 3.73 rear though.

Is your speedometer/odometer correct? I had a 400 and put in a 2004r, it almost doubled my highway MPG My rear end was a 355. I've gone to a 428 and lost some but I'am still do around 16-17 MPG on the highway.

A common problem with a quadrajet is that the secondaries stick open a little and this destroys the mpg.

Is your new transmission calibrated properly for your tire size and axle ratio? Did you change the drive/driven gears? If not, your odometer may be grossly off and messing up your calculations.

Use my calculator to check: http://thefirstgensite.com/html/speedcalc.htm


If the Quadrajet's air valve spring is properly set, the secondary system will not open until the throttle is well advanced. So you are looking for issues in the primary fuel system, in the speedometer/odometer calibration, and in the timing.

3 degrees initial advance seems a bit low. I would set it to 9 degrees. What is the calibration on your distributor? HEI or points or ?? What is the amount of ported vacuum available at cruise for the distributor? Perhaps your vacuum canister is looking for more or less vacuum signal than it is seeing at cruise.

Rob's third gear suggestion is excellent.

There are a lot of variables. After your next cruise, shut down the engine from idle and immediately look down into the manifold below the throttle blades and see if any fuel puddling is present. There should not be any fuel puddling. If so, the carb has issues.

My odometer isn't exactly correct, but it is within 5 mph @ 70mph.

My wife followed me yesterday to the car show that was about 40 miles south of town. When I was going 70mph by my speedometer, we were actually going about 65. So, I used her car to determine the total miles traveled (since we mirrored) but used the total gallons consumed from my car.

Still 10 MPG.

Now, I've taken the odometer out of the equation. Next!

The ignition is a Mallory Unilite system.

I will take a vacuum gauge and get a reading of the total vacuum at 2500 rpm from the ported side of the carb (where it is hooked up now). After that, I will reset the timing back to 9 degrees, and get another reading on the vacuum.

The 3rd gear suggestion is good, but I don't think it's necessary. This car sucks gas just driving from light to light, not even getting on the highway. Plus, For the last 6 month I drove it without a working 4th gear lockup, so my RPM's in 4th were higher than normal due to lockup not working.

I still suspect the carb. I still get black exhaust soot on my paint on the back of the car below the rear bumper after a full tank of city driving.

I would love to just slap on another carb, but I can't without a lot of work. The throttle linkage, return springs, and TV cable/bracket have all been custom made for THIS carb. I would need another exact 7043282 Q-Jet to try again. I would love to try this.

There is an excellent article in July 2006 Hi Per Pontiac about this exact subject. It's titled, "Performing With Efficiency" on page 62. There are many tips and tricks to tune your quadrajet and also many reasons outlined for poor performance of a quadrajet. They ended up with nearly 19 mpg with the stock transmission.

Black exh soot , indicates too "rich" in my book....
so Jims suggestion abv sounds very likely...
</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif"> A common problem with a quadrajet is that the secondaries stick open a little and this destroys the mpg.
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Yes, I have had one that sticks. Another symptom of this would be a that the idle keeps getting out of adjustment. You will have a fast idle when it sticks.

I don't really have any idle problems. I don't think the secondaries are sticking.

They used to though. The secondary butterflies were rubbing on the intake manifold and getting stuck, and that was rasing the idle after a goot hard WOT run. But we fixed the allignment issue on the intake, and the secondaries seem to respond beautifully after WOT now, and the idle returns to normal.

How can I confirm that when at rest the engine is completely on the idle circuit and the primaries are not engaged at all?

I don't have access to the July 06 issue of HPP. I'll go see if I can find it on the newsstand, but last time I tried that, it was futile. Would anyone be willing to scan the article for me?

The HPP article(s) can be found here:
http://www.highperformancepontiac.com/

http://highperformancepontiac.com/tech/hppp_0606_pontiac_fuel_efficiency/

http://highperformancepontiac.com/tech/hppp_0607_pontiac_fuel_economy/

I'll mail you a copy of the article if the above links do not work.

Scott's link worked perfectly for me. It's a great article for anyone with a quadrajunk. OOps jet.

It could be a combination of things. The engine operating at too low of a speed my contribute. Unlike today's engines that are tuned to opearate efficiently at low rpm od speeds, the firebreather wasn't designed to operate at 1.7k.

I have a 3:23 rear that has collected around 400k miles in 2 birds. I don't have a tack, and I have never payed attention to mileage. It seemed that the best mileage was around 45 to 60. I don't know the exact sweet spot was, but over 65 and under 45, the gas poured through the car like it had a leak in the tank. Over 65 notwitstanding, it seemes that about a 30 mph seady cruising speed, was the real gas suck.

There could be other issues too, but someone might wish to see if they can find a formula to determine engine rpm @ 30 mph. 3:23 rear through a 350 slush box. Wouldn't it be interesting if the rpm was about 1.7k?

I just modified http://halftx.org/speedo.php to also show the RPM in the next to last and final gears at 30mph.

Well, gee the carb guy says the carb is okay. What a surprise.

But seriously, without a wideband O2 sensor you'll have a tough time figuring out if you're too rich. Maybe, try a coast down and see what the plugs look like.

As for the timing, how people state things varies.

What is your initial timing with vacuum advance plugged. What is your vacuum reading at this point.

What is your centrifugal timing and where does it start and end.

How much does your vacuum advance add.

Do you have it hooked to ported vacuum or manifold vacuum?

Does the engine feel sluggish on the bottom end?

Well, I went out and worked on the car last night for a bit.

When I got under the hood and started working on it, the first thing I noticed was the timing was WAY off. It was set about 4 degrees retarded. So I placed a vacuum gauge on the ported side of the carb and it indicated it was pulling 5" vacuum (I assume this means that my carb was not in the idle circuit when at idle, but it was actually in the primaries).

So, I went about retiming the engine. I disconnected the vacuum to the distributor and set it at about 11 degrees advanced. I then lowered the idle which brought the ported vacuum reading back to esentially 0 (maybe it was .2). Then I dialed my timing light for 36 degrees, and ran the engine up to about 2000rpm and watched the mark come back to 0 on the crank. Finally, I hooked up the vacuum advance, set the timing light for 48 total degrees timing, and reved the engine back to 2000-2500 rpm. Again, the crank reading came right back to 0 degrees.

At 1500-2500rpm my carb ported vacuum port is pulling 15-20".

I didn't get a chance to drive it last night after the changes. I can't believe I was almost 15-16 degrees off in my timing. I guess the distributor wasn't tightened down properly. I'll try to drive it around tonight or tomorrow to see if the MPG goes up.

It sounds like you are on the right track now.