Gas Mileage woes....

Torque Junkie · September 9, 2001

if I thought 10 mpg est. was bad, so far, I'm getting more like 6 mpg :angry:(70 miles on the guage showing 3/8 of 1 20 gal tank. I shouldn't really count my chickens before the eggs hatch and guestimate until I fill 'er up again, but 70 miles should still be well over 1/2 tank.

Are there any easy tuning things to do to increase mpg? When I had the exhaust put on, the guy at the shop said that it seemed to be running overly rich....

Any other things maybe?

Thanks for any help.

-Mike

Pastmaster · September 9, 2001

Are you running a Q-jet? If so, you might think about running a smaller CFM Carb. My buddy had a 76 C-10, which I sold as a parts truck, that got horrible mileage, and right before we sold it, we put on an old GM 2-bbl and intake off a old 350. The truck didn't have as much power, but it got better mileage right away.

Torque Junkie · September 9, 2001

It's a Carter AFB 4bbl of unknown CFM.

I just checked it w/ the timing light, and about sh*t myself! It was like 25 degrees advanced!! it was well beyond the range of the dial even. So I retarded the timing back to the white mark which as about 8 degrees advanced, so I'm gonna run like that for a while and see how she goes. If I retarded it too much and have a loss of power I'll advance it back some.

If it's still drinking gas, then I"ll adjust the air/fuel mixture and lean it up some.

and if that doesn't work, then I have no idea what to do from there. I may have to step back to a smaller carb if that's what it takes.

As soon as money permits, i'm gonna be swapping the TH-350 for a 700R4. That O/D should increase hwy mileage 15-20% or more.

-Mike

76BBSub · September 10, 2001

You should be just fine with that carb, it's most likely a 600 or 650 CFM. The problem may be the jetting. You can get jets and rods, or a kit with a range of jets and rods, and try to dial it in a little bit. Also, check the float level, if it's too high it can cause a rich condition in some circumstances.

When you checked your timing, you did disconnect and plug the vacuum advance hose, right? With the vacuum advance at idle it's pretty common to see well over 25 degrees of advance. Also, if you're running stock heads your engine may need a little more advance than what Chevy says in order to make optimal power and mileage.

Let us know what you find!

BTW, I usually get 10-11 mpg with the 454 Sub.

Jeff

Torque Junkie · September 11, 2001

Quote: from 76BBSub on 9:32 am on Sep. 10, 2001
You should be just fine with that carb, it's most likely a 600 or 650 CFM. The problem may be the jetting. You can get jets and rods, or a kit with a range of jets and rods, and try to dial it in a little bit. Also, check the float level, if it's too high it can cause a rich condition in some circumstances.

When you checked your timing, you did disconnect and plug the vacuum advance hose, right? With the vacuum advance at idle it's pretty common to see well over 25 degrees of advance. Also, if you're running stock heads your engine may need a little more advance than what Chevy says in order to make optimal power and mileage.

Let us know what you find!

BTW, I usually get 10-11 mpg with the 454 Sub.

Jeff

When I timed it, I had the vacuum advance disconnected and plugged the carb side of it with a screw.

Well, having driving it with the new timing, it seems overly retarded, so I'm going to advance it back about 3- 5 degrees.

My dad drove it too and is certain that the secondaries aren't opening, and since they're most likely vacuum activated, I have no idea how to adjust it's 'sensitivity'

Any suggestions?

Thanks for your help everyone. I just wan't this thing running in optimum condition before it starts getting cold.

-Mike

76BBSub · September 11, 2001

On the right side (opposite the linkage) there's a little tab sticking out from the secondary side of the airhorn. Underneath is a little set-screw, should be an allen-wrench (6-point socket) type. Put a screwdriver into the screw on the end of the "tab" I mentioned earlier (should be a flat-blade type screw), then loosen the set screw. The only problem is right now I can't remember which way to turn the screw to "loosen" the pre-load on the spring, which allows the secondaries to open earlier. I -think- you turn it in to loosen the pre-load, so try turning it in, remember to count the turns in case you need to go back to the original setting, then try turning it out about 1/8 of a turn. If you get the pre-load too loose the secondary air valve (flaps) can swing open too fast, hit the stops, then slam back closed, so you need at least a little pre-load.

Hope this helps, sorry I can't remember the details...

Jeff

Torque Junkie · September 12, 2001

Awesome!!! Thanks Jeff... you rock!

I'll tinker with it tomorrow or Thursday, depending and let you know what I discover.

-Mike

(Edited by Torque Junkie at 10:47 pm on Sep. 11, 2001)

Torque Junkie · September 14, 2001

Argh.. no adjustment screw or anything. I stuck a screwdriver and opened the secondaries like that, and they felt as though they had no resistance to open except the counterweights that closed them.

I'm gettin about ready to dump about ? into an Edelbrock or Holley 600 CFM. Carter AFB seems to be almost an off-brand or something. Frustration is setting in, methinx

-Mike

76BBSub · September 21, 2001

I'm going to try to adjust my secondary spring this weekend, if I do I'll fill ya in on the details while they're fresh on my mind...

On another note, my mileage has dropped off to less than 10 mpg the last two tanks...hopefully it's just a timing adjustment or something, but it's making me a bit nervous!!

Jeff