78 T/A gas mileage

It depends on what it has for read end gears and how you're driving it. It should be closed to 12 if it's stock. Things to start with are timing and mixture.

My '74 TA with stock 455 got 11 mpg, no matter how I drove it. I pumped it up a couple of years ago, and now get an extremely fast 8 mpg. ;)

It was getting 13 mpg last year. I'm not driving it hard. I'm not sure what rear end gears it has, but it has posi-traction... perhaps only one gear ratio was available with posi (?). I figured it would be ~12 mpg because the weather is cooler now, not

8.5.

I'll have to check the igntion system over & see if it's OK. Thanks.

I've got a '78 W72 400 manual-trans T/A. On a good highway run, I get

18-19 US mpg. Relatively short drives in winter weather (below freezing) gets me down to 15 mpg.

I'm in Canada where right now it's a dollar a litre, or something like $65 to fill up. Fuel economy is pretty important to me. Mostly I use the car for getting out into the country, so a lot of rural driving. I expect consumption to be about 14 litres/100km in general, which translates to about 17 MP(US)G.

That's pretty bad.

Now I've replaced the 3.42:1 rear gears with 3.08:1 for nicer highway cruising. And I run stock-sized H-rated 225/70-15 tires; I expect that fat mushy 265/60 white-letter items would add a lot of rolling resistance. (The odometer is accurate: the speedo drive gear was replaced, and I've checked it against kilometre posts.)

My Quadrajet is a rebuilt stock item, with the stock very-lean metering rods (about the second-leanest available according to Doug Roe's table of metering rods). The cam is a Crane 204/214@0.050 which makes it a bit larger than the stock W72 item. All emissions items are still there, including working heat riser valve on the driver's side manifold. Exhaust is dual, with dual worn-out catalytic converters. I find I can't run the timing to the max 18 degree advance specified because it pings a lot on my default 87 octane, so I have it backed off to 16 degrees. The advance weights and springs are stock GM, as is the HEI module.

Car doesn't have A/C, but still weighs in at something like 3850 lbs curb weight. And when the T-roofs are out, aerodynamics go right away. But it's fun! (At the strip, the car runs about 15.2 @ 93 MPH; the 60-foot time is a miserable 2.4 with the 2.43:1 first gear and

3.08:1 rear and 27.4" tall tires. Oh, and I don't haul out the spare tire or cool the motor off or anything like that.)

A good bet would be a bad vacuum advance. Gone bad, they are good for about 3-5 mpg drop. I've seen it numerous times. And it'd be cheap to replace. HTH, s

The HEI advances literally weld themselves together with rust due to moisture and the coil firing through the shaft and the advance mechanism.

My 79 TA wouldn't turn a tire on dry pavement when I got it. The advance was stuck, rusted all up. The cat was plugged solid. I bought it because it was in amazing shape and looked almost new, and only had 18K on it. Just whacking the converter off and polishing and lubing the advance cam woke it up quite a bit. The vac advance was fine, it just couldn't move the advance mech, as it was frozen solid. When it was all good, the milage went from 12 to almost 17. When I put the cam, heads, intake, etc, on it, it went down to 14, but it ran great.

The advance problem was never permanently solved until I put an external coil on it.

BDK

Those 'flyweights-and-spring'(centrifugal weights, I guess is more nearly proper) make a tremendous difference, don't they? A point I'll remember--never happened to me, but an acquaintance was having problems w/a customer's 87 Silverado just 2 weeks ago. This guy is known widely as the best carb man in our upstate. Truck wouldn't idle slow enough--& even at high idle, when you put it in gear OR merely switched on ac, it'd cut right off. Off and on, over several days, he'd replaced module, rotor, and pickup coil, and cap was new & looked good. After being ready to give up & return it to cust., unfixed, he decided to try something I'd heard of: the dist.'s main shaft becoming magnetized & throwing the outputs from the pickup coil/module to kilter. Bingo! He grinned when his screwdriver was sucked up to the shaft. He changed dist. with one lying on his bench & it solved the problem. He & I both saw it for our first time. s

I forgot all about that bit! My friend had an older Suburban that didn't run right and after the dealer put all the stuff on it for a huge price, without fixing it, I looked at it, and the shaft was very strongly magnetized. We took it out and whacked it a couple of times, and I ran it through a bulk tape eraser about 10 times and it finally lost 99% of it's pull. The service manager looked at us like we were nuts, but the computer/ignition guy comes over and says, "Yeah, that can do it". They ended up eating all the labor and most of the parts cost, and gave most of his money back.

I always wondered how the shaft could stay so strongly magnetized in a hot motor like it did. One more reason to move the coil away from it.

BDK

Thanks for the good suggestions. Checking the vaccuum advance is a good idea. I never thought to look at the centrifugal weights. Hopefully if this is the problem I can buy new ones at an auto parts store?

I don't know, I took the whole distributor apart, and cleaned and polished everything and was ok. I ended up buying a new distributor after two cleanup sessions, and moved my coil to the fire wall, and it never happened again.

BDK

If yours are too rusty, AutoZone is your friend--in the HELP section. s