350 fuel mileage

[sermis]

I swapped a 350 into my M725 when I got it. It only gets 6.5 mph and maybe 7 mph on a very good day. Don't see many good days. It is not a radical motor, better pistons and a RV cam. Fresh build when I put it in. Has about 8,000 miles on it now. I am running a mechanical fuel pump. Carb is a marine Edelbrock. Carb might be part of the problem. Marine carbs don’t have any vacuum lines. Running manifold vacuum for HEI advance and brake booster. T98 / NP200 w/stock gears and 11.00 X 16 tires. Why is mileage so bad? Does the fuel need a return to the fuel tank?

[brute4c]

A lot of stock trucks get around 10...the 350 is roughly 1/3 larger than the 230...and if you take 1/3 off the 10 MPG you get about 6 2/3 MPG....

Sounds like it is in the ballpark for a stock geared, stock size tire, non overdrive equipped truck.


Lockouts on the front axle will hep some...if you dont have them already...otherwise, gearing changes of some kind wil yield the best gains...axles to 4.56 or so...increasethe tires to 38 inch or larger or add a 15% or higher overdrive.

[gimpyrobb]

You got to put in the OD trans! Since your deuce has one, might as well put one in the 725.

[fasteddie]

I have a 400 SBC, carbed in the 725, Th400 trans, 4:10 gears, no OD. It gets 10.5 in Chicago city driving, have not been on a long enough highway run to check that mileage. Truck came with a 305 and when I drove it back from Michigan it got 10 on the highway.
This same 400 SBC engine set up in a 76 C-10 Chevy[which is where I took it from] got 16 in town, 20 highway. Just has to do with the weight and a little steeper gearing. Thought about OD, just not worth the investment for the gain at this time.

[sermis]

I have lockouts on the front and a locker in the rear. The parts M35 is going to be the donor for the OD as soon as Barrman prefects his mounts. If I can find a divorced 205 it will go in at that time. I am really thinking about going to fuel injected on the motor. Looking for a computer and wiring harness.

[randyscycle]

You didn't mention your driving habits. Not to sound condescending, but a leadfoot doesn't help! What CFM is your carb? Too much can be a bad thing. Is the engine coming up to full temp? Is the choke fully opening when warm? Is the exhaust unrestricted? (old heat riser stuck shut?)

I'd expect a bit better than 6MPG. I have seen huge motor homes that get around 10 or so, and I had a Chevy 1 ton service truck with 4.56 gears, a TH400, and a 454 carbureted, that still got 10-12 fully loaded with all my tools, an air compressor, a generator and a welder.

Of course the 725 does push a lot of wind with its high front and square shape, that may make it worse than a 715.....

[moosejr]

You may be onto something on the carburetor also. Your initial timing needs to be set without vacuum to the distributor, then use PORTED vacuum to the distributor. Using manifold vacuum to the vacuum advance is going to throw off your fuel mixture at the time of ignition. Either you will have too much advance at low RPM, wasting fuel because you aren't burning it all, or not enough at higher RPM, causing detonation, . .either way, not good. I would seriously look at either changing the carb or pressing on with the fuel injection. I made the mistake of putting manifold vacuum to the advance on my old 305 v-6, . .got about 7mpg in a 3/4 ton pickup, . .moving it to ported and readjusting timing and carb, got it up to 16-17. . .good luck with it. . .

[pistolnut]

I made the mistake of putting manifold vacuum to the advance on my old 305 v-6, . .got about 7mpg in a 3/4 ton pickup, . .moving it to ported and readjusting timing and carb, got it up to 16-17. . .good luck with it. . .

Ported vacuum from where? I ran manifold vacuum to the civvy dist. on my 230 and over time got a lot of exhaust fumes, exhaust really smelled bad. I plugged the vacuum line and it cleared out and runs fine except with a stumble off idle. What was I doing wrong?[/thread hijack off]

[foodstamp]

i have a whole wireing harness/computer/tbi set up with the intec mani too if anyone is interested

[sermis]

i have a whole wireing harness/computer/tbi set up with the intec mani too if anyone is interested

I am looking for computer and harness for a true fuel injected not the TBI.