So I was getting about 12.5 mpg on my '70 Chevelle SS 396/TH400 when I first got it. Had a major issue with gummed up fuel lines and tank. Replaced all the fuel lines with high quality fuel injection grade hose (in case I decide to go to FI later). Replaced the gas tank, fuel sending unit, fuel pump, fuel filter, inlet washer, and hose from carb to fuel pump. After getting everything dialed in with the idle mix screws and timing to get her running smooth my fuel mileage seems to be down to closer to 7 mpg. There are no leaks anywhere. Vacuum is good. Timing is good. What in the world am I missing here?
One possibility - things were so clogged up before that you were actually starving the engine for fuel and it was running lean. Drinking through a tiny straw.
Now that you have it opened up, you have it drinking through a fire hose and have to tune it appropriately for better gas mileage. I bet it makes more power now though eh?
What carb do you have, what is the jetting, what is your timing set at, how did you adjust the idle mixture screws, etc. If you can, post all that relevant information.
It has the original Quadrajet carb that was professionally rebuilt by the previous owner, so I don't know the details of the jets, etc. The total timing is set at 38, the idle mix screws are at 2 turns each. Those settings were necessery to get a major stumble out of it. Whenever you got on it around 3000 rpm just as it was shifting back to 3rd it would fall on its face. Had to let out of the throttle to get power back. But once I got the timing and idle mix screws set where I have them all of that was gone.
Should I consider smaller diameter hoses for the fuel lines? I'm using 3/8" for the send and 1/4" for the vapor return.
I have two guesses. First, its running better so you've been hammering it more around town and as a result mpg plummeted. Secondly, perhaps how your tallying the mpg is not precise enough (not a large enough sample size, etc). Maybe your initial 12.5 mpg was a little too high to begin with.
I think you have trash in your carburetor.
Do you know anyone you could swap on a 600 vacuum sec Holley or another known good running carb to try ?
7 mpg is really bad so this shouldn't be hard to figure out.
I guess Im not seeing something but of your running hose from the gas tank to the mechanical fuel pump that's not going to work.. You need to run tubing, you are pulling not pushing fluid. You need to have the least amount of hose possible..
If you have a sump with two takeoffs I would run two 3/8" tubing lines to a tee in the fuel pump with a 1/4" return line from the supply side of the fuel pump back to the tank.. I researched this with a few builders who design plumbing for industrial machinery. They told me if you are pulling fuel from a rear mounted tank to the front of the car into a fuel pump, your best bet is two 3/8" lines. They said this would be much better then running one 1/2" steel tubing line.. They said hose is definitely out when it comes to pulling fluid.. In no time the inner hose will be collapsing and you won't see anything different. I sleeve hose with 3/8" tubing when it has any kind of bend in it..
Instead of wasting several hundred dollars worth of high grade fuel injection hose wouldn't it make sense just to use a low pressure electric fuel pump at the tank?
Initial timing: 12 to 14
Vacuum advance limited to 10 maximum (install Crane 99619-1 stop plate, set to 10 degrees)
Total IDLE timing would be 22 to 24 on full manifold vacuum
SET TIMING WITH ANY "DIAL BACK" FEATURE ON THE TIMING LIGHT OFF, OFF, OFF OFF, OFF.
For 14 initial, mechanical would like 22 to 24 degrees
This would give 36 to 38 total
LEAVE ANY DIAL BACK FEATURE OFF, MARK THE BALANCER FOR DEGREES
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