Well here I go...

My 340 has a 4 barrel Carter AFB carburetor.
Sits there a purrs like a kitten, at idle you would wonder what
in the world more could you ask for.

Run it around town, and I'm getting around 6 miles to the gallon
just driving easy no problem with how it's running.

On the highway step on the gas like to pass someone and blaaaaaaaaaa
lift my foot off the gas and all is fine. Accelerate slowly all is fine, step down fast and blaaaaaaaa I'm going nowhere fast.

I read a little on the net about adjusting the idle mixture screws, more or less a general rule of thumb procedure (here is the link) and all went well.
But still the same trouble.

A friend told me to look inside the tail pipes (black would mean to much gas) wiping my finger in there reveals black soot, although the exhaust itself you can hardly see.

Given the fact the fuel mileage is so poor I would have to agree to much gas, butI'm at a loss as what to do about it.

Any idea's would be greatly appreciated.
Thank you
Joe.

 

I'll be watching this too, noticed my pipes are pretty black but have not really laid into the throttle yet. Did get a backfire when I did push it a little.

 

Unfortunately, you can't expect too great of gas mileage.

You could play with squirters, jets, ect..

Or put on a Holley and go with that.

 

You might want to make sure you are not leaking gas under high load from a split in a fuel line that seals up when the pressure is low. This would explain the poor performance load as well as the poor gas mileage. I had it happen to me once.

 

accelerator pump. sounds like to me.

 

What should the fuel feed pressure be?
Also it does have a electric fuel pump and a pressure regulator.

 

I think a better place to check rich/lean would be to look at your plugs.

 

Plugs are clean and dry. But they also look brand new.
I have not had it very long.

 

7-9 should be plenty

 

Thanks guys for all the comments.

I talked to a friend at work today and he also pointed to the accelerator pump and mentioned the float, his reasoning being the motor runs well while not under load and only pukes when the load is applied. He recommended a product called Sea Foam, it's a gas treatment and the parts store agreed it's the best. I dumped in half a can and went for a drive. About 3 miles down the road the engine started coughing and back firing and then just died. It would not restart, at all.

I ended up turning on the key so the fuel pump would run. With the engine off I got my ear next to the carb and pushed down on the accererator linkage, I could not hear any gas being sprayed into the carb. I removed the gas line from the carb, no gas.

As it seems that either the fuel pressure regulator was the culprit all along or the Sea Foam broke loose some junk and plugged it off, or maybe a combination of both. I took the regulator apart, fiddled with it and got the truck back on the road.

I have the regulator adjusted to what I believe must be just above the low limit of 3 lbs. The truck runs better than it ever has, but still is not right. I will be getting a pressure gauge and set it up correctly tomorrow, 8 lbs.

The search for the timing pointer / scale continues........

Oh, by the way, don't bypass the regulator

 

bypassed the regulator and flooded it did you?

 

Looked like a bathtub over flowing

 

4-6 with that carb
the soot is somewhat common with these engines
what gears are in the rear end?
does it have a heavy cam?
my car has gotten upwards of 12 mpg, so 6 isn't good

 

I don't know about the camshaft. By the looks of the rest of the engine I would not be surprised to find out it's something other than stock.

We have found out tonight the fuel regulator is either just shot or needs a rebuild if that's possible. It will adjust from 0 to 4 lbs and that's it. It's supposed to adjust from 3 to 12. I found a direct replacement on ebay for $49 new. Which near as I have found is a pretty good deal. I'm going to check the local parts house tomorrow.

Thank you for the insight.

Joe.