Well that is a larg part of your problem right there. Timing is critical to good perfomance and fuel efficiency. Vacum advance is a must on street driven cars too. Get those things fixed.
Low teens is doable, but you are going to have to spend the time to learn how and adjust your carb and timing correctly.

These topics have been covered many times before and you can use the search function on the wbesite for some great discussions.

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Casually looking for a Clifford hyperpak intake for cheap.

I was very surprised to not see a vacuum can on the distributor or a slot where the visible distributor plate is. I'm on the hunt to access the 7/16 nut under the distributor and just finished reading up on valve adjustment procedure. I may be getting a little better than 10mpg factoring in the 31inch rear tires. My speedo is reading about 7mph slow at 40 showing. That may have me at 12mpg corrected. After I get an idea of my fuel pressure and timing I'll fine tune the carb. I've dialed the idle down to fix the dieseling at shutoff but once it warms up its running well except for the hesitation. I'm also running about 4 1/2 turns on the mix screw. Its fun to be tooling on an slant again. Its been at least 20 years since I've done anymore than basic filter and oil stuff on one and I was a very green tinkerer then.
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vapor genie vaporizer

Sounds like you have a lean burn computer set up on your truck.
10 mpg is not so good......
You might want to switch it over to 74 or so distributor and electronic ignition.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

I did a lean burn search. Yes, its got that with the computer on the fender. All of the canisters and such have been removed from the previous carter carb in favour of the new 1920. The computer is still working and doing timing changes just operating with limited inputs?
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Millechili

Nope...the Lean Burn is an all-or-nothing deal; you will want to finish the job the PO started by installing an advancing distributor. Parts store "reman" units are a poor choice; you can get a nice brand-new one from Old Car Parts Northwest. Connect the vacuum can directly to the carburetor's spark advance port. You will find your truck's performance, driveability, and fuel economy will all go way up.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I told my grandfather what the mileage was on his old truck and he said the dealership told him, emphatically, that if it didn't get 17mpg to bring it back.
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Honda Civic Type R specifications