Am I really reading this correctly?

[rustytruck50]

Ok...I've been doing lots of reading here about various Quadrajet & timing issues. I have been getting what I would consider lousy mpg(4-5) and driving at highway speed seems to result in a bit of struggling to maintain 60 mph. In park, it sounds like it has a bit of a miss and doesn't run terribly smooth in gear.
I have replaced a bad fuel pump, put new cap, rotor & wires on, switched out the fuel filter& pcv valve...only because it had sat for a long time before I bought it last year.
Could I be having timing issues, collapsed fuel sock, maybe even a bad fuel pump? I've been trying to adjust the idle mixture screws because I was thought maybe I had some problems with running rich, but it really hasn't helped. It's been a while since I've worked with this stuff & I'm a but on the rusty side. So, feel free be a straight shooter---I can take it

[Tom]

Fuel pump is easy to check- check fuel pressure. Check timing. Then check compression. Then worry about tuning.

[RotaryRocketeer]

Idle screw adjustment isn't going to affect anything other than how the truck runs at idle. Start with the basics. As stated above, check fuel pressure and set base timing in the acceptable range. Fine tuning comes after.

[rustytruck50]

Thanks guys...that was what I was thinking about but just needed to hear for sure before I did anything else that would divert me to way, far left field....I do learn a lot here!

[Dead Parrot]

4~5 MPG is pretty bad for these trucks. Mine gets 12~14 @ 70mph.

Things to check not yet mentioned:
Stuck or poorly adjusted choke.
carb float level too high.
stuck heat riser valve on exhaust.(if stuck closed, can lead to damage to carb and a too rich condition)
If exhaust OEM, collapsed inner wall on a dual wall exhaust pipe. (found this on my 75 after my dad purchased the truck in 78. It struggled to hit 60 and the exhaust looked like a diesel.)

Good place to start with timing is 8* btc with the vacuum advance disconnected. Stock motors benefit from more advance but 8 is a good place to start when troubleshooting. You can add more later when you have fixed the problem(s).

[geezer#99]

Save time and go to 14 initial right away. Hook the vac advance to manifold vac. Turn down your idle so you’re back idling on the idle circuit and adjust your mix screws.
Your timing is the problem. Fix that and everything else should cure itself.

[cadillac_al]

What Geezer said. A bad fuel pump would probably give you better mileage.

[Gixx]

Spark plugs/wires ? Sounds like timing as well.

[rustytruck50]

Working on the plugs & wires

[RotaryRocketeer]

While new plugs and wires are never a bad thing, you may be wasting your time with that. Get a good ballpark base timing setting and get your advance setup right man. It's gonna make all the difference.

[rustytruck50]

Thanks....going to hopefully work on that this weekend

[andyh1956]

As stated previously fuel pressure check but equally important is fuel SUCTION! If you have over 5 in vacuum on the suction you need to find the restriction.
Then- check to see if the vacuum advance is working. Hook your Mighty Vac to the advance canister & apply vacuum w/ dist cap removed so you can see breaker / module plate move & the canister holds vacuum.
Then- check to make sure your mechanical advance is free & not stuck to the dist shaft. This runs in an ionized atmosphere & can rust up & stick. Make sure fly weights & springs are in good shape & are functioning properly. There are two little nylon plugs that act as bearings for the advance mech that get hard & fall out & can cause sticking.
And check your manifold vacuum at idle & see if it fluctuates when the engine is running. If it does could be a leaky valve or a bad cam lobe.
All the above good advice should get you a better running engine!
Good Luck!

[rustytruck50]

I will check on this as well....thanks everyone for your help! It's improved 10 fold since I replaced all of the vacuum hoses!

[Gixx]

Yeah, vacuum leaks will kill you, too. Biggest improvement will probably be the timing. You can usually go 5-10 degrees advanced past whatever the "stock" recommended timing is supposed to be. Just listen for "pinging" or "rattle" and back it off (retard it) if you get any pinging. A couple of degrees makes a pretty big difference.

As stated earlier, you might check the heat riser and make sure it isn't stuck shut. It is located in the passenger side "down pipe" and has a spring loaded and weighted flapper that opens when the exhaust gets warmed up. It is supposed to be closed when cold which helps it run better when the engine is cold. I have removed mine and have noticed no ill effects. I live in Colorado and often start mine in near zero weather.