The absolute best I can get is 9 MPG without flooring it with the setup listed below. I have base timing at 11 degrees (37 total). I know you guys can't diagnose this over the net, but what are some things to check? I mean, how can I tell if it's running rich or lean? I haven't a clue because the car runs great except for at idle, and if I floor it in drive. I get about 8" of vacuum in drive. I've tried adjusting the idle mixture screws. When it tries to downshift, I hear like a sucking in of air, but the car goes absolutely nowhere. After a second or so, it kicks in and goes. Any help would be greatly appreciated. I know I'm not going to get awesome gas mileage out of a carb, but 9 MPG is just nuts. Thanks.

 

Maybe it's time for a carb rebuild?

 

Some basics first:

Type of distributor?

How are you setting the timing?

 

The distributor is the MSD Pro Billet adjustable with mechanical advance. I am setting the timing with a timing light with advance. I just adjusted the idle mixture screws and got the vacuum to about 9" in drive, which in affect raised the idle 100 RPM or so in drive, and about 300 RPM in park, without touching the idle speed screw. I didn't want to lower the idle at all because it sits at about 700 in drive. Now I also noticed that whenever I would come to a decently hard stop, the motor would lose RPMs and would die unless I revved it in neutral. Then a friend gave me a suggestion to check the holes on the side of each bowl (sorry I forgot the name), and the fuel should sit right at the bottom of the hole. Well, mine was way off, especially on the secondary side. So I made an adjustment, and the car doesn't even drop RPMs at all when I come to a hard stop. Could this adjustment I made to the bowls help my gas mileage? One last thing...since I screwed with the mixture screws and got the vacuum up a little bit, it seems to hesitate pretty bad if I punch it from a stop, but it is fine under normal acceleration, and fine if I floor it when the RPMs start at about 2500. I have a 4.5" power valve in there. I assume I should throw the 6.5" valve that I have in, right? Would this get rid of the hesitation, and does the power valve affect gas mileage? ANY help would be greatly appreciated. Thanks for the replies.

 

Mechanical only advance? If so, that's not a good street distributor. You need both vacuum and mechanical advance.

If it does have vacuum advance, I hope you disconnected it when you adjusted your timing.

Float level is the first step in any carb tuning. Too high will cause problems, including excess fuel usage.

 

get your timing thing corrected. You want to adjust your float levels too otherwise you'll flood your engine. If you have a hesitation or bog under part throttle accleration you need to either lean out or richen the acclerator circuit which means new acclerator pump springs. You also don't want to open the throttle blades too much when you adjust the idle screw, you'll uncover the transfer ports inside the carb bore also leading to hesitation. The Hotrod website is good for getting tuning ideas

 

Yep, vacuum advance help a bunch at cruise rpm. Correct float level sure won't hurt mileage. Mixture screws are for idle only, they should be adjusted as lean as possible with the engine still running smooth. What jets are installed now? Running smaller in the front and fatter in the rear can help mileage and not kill the power.
The hesitation problem:
"Sounds" like you aren't getting enough fuel from the accel pump's. Adjust them with pump cam's and squirters. If the hesitation is immediate, when you hit the throttle it's not the powervalve causing you problems. With your engine a 6.5 is probably what you need. Yes it affects mileage(it just adds more fuel). Also, it's a double pump carb on an automatic--it's possible to get too much air too easily(not enough velocity) causing a bog.

 

Yeah it's a mechanical advance only distributor. The primary jets are 72, I forgot what the secondary ones are. Thanks.

 

Try testing Throttle Positioning sensor

 

The carb is not computer controlled, so it doesn't have a throttle position sensor. Thanks for the replies.

 

maybe your spark advance is way too low.
What's your initial timing at idle?
then what is it at your cruise RPM?

It's probably your best bet to invest in a good vacuum advance distributor.

That way you can have 30* or more degrees timing at idle and cruise..... much more efficient.