Sgl or dbl brl carb?

[Poppateez]

Getting 10mpg; would love 15mpg; which way do I go? straight 6 / 250.

[jocko]

Welcome to the board Poppa. Could you provide a little driveline info? Trans/ratios - rear end ratio, tire size, etc? That might help. Personally, I would use a 2 barrel on a 6. 1 probably not enough, 4 too much.

[Wrenchbender Ret]

If the Monojet carb & ign. are all tuned good it should get better MPG then that. Like Jocko says , a lot depends on the gear ratio.

[TheSeeker]

I have the 250 as well, and a 3 speed M/T. I'm not positive on the rear end gear ratio, but I think it is 4.11. With HEI and a rebuilt monojet, I have never gotten more than 11mpg on good day.

[68gmsee]

Originally Posted by Poppateez

Getting 10mpg; would love 15mpg; which way do I go? straight 6 / 250.

Changing the carb to a 2 barrel may not get you better gas mileage unless yours is bad. There are a lot of other mechanical and ignition problems that can cause low gas mileage. As mentioned above, one thing is rear axle ratio but engine condition like worn rings, valves, cam can also affect mileage.

It could be one or several things involved. What I would do is to start with the simplest thing first. Replace all the ignition components like distributor cap and rotor, points, condenser, plug wires and plugs themselves. Maybe even the coil. After that get a kit and rebuild the carb. They're very easy to work on.

If that doesn't help, you're looking at engine rebuild or rear axle swap. I get about 15-16 mpg on my GMC 250 with single barrel and 3.54 rear axle.

[pritch]

Originally Posted by 68gmsee

I get about 15-16 mpg on my GMC 250 with single barrel and 3.54 rear axle.

Yeah, that's about right where I am. 250/4 speed. Not real sure about my gears, either, I used to think 3.73, but I think maybe 3.54 seems more realistic with my mileage and the fact that I can do 70+ all day long on the flat.

[Poppateez]

Pritch thanks for your input.

[Poppateez]

12 Bolt 373 gears 235 75r15 tires brand new

[jocko]

I'm assuming you have a grann4 or 3 ott? If that's the case, the one thing I would focus on, rather than the carb, is the trans. 1:1 final trans ratio will kill highway mileage. If you drive it above 55 mph much at all, you'd probably see more gains by swapping in an o/d trans, such as a T5 if you prefer manual or 2004R or 700R4 if you prefer autos. Might be more than you want to spend - but since you're looking for a fairly BIG improvement in gas mileage, I'm afraid you won't get it just by changing to a 2 bbl (or from a 2bbl to a 1 bbl, vice versa).

Only other realistic option (but it would cost you bottom end acceleration (I'm not talking drag racing here, but basic bottom end torque "feel") would be to swap out the 3.73's for something lower numerically.

50% mileage increase is a lofty goal - and, as you've seen, you're getting roughly what everyone else is getting with your combo, so it would take a major change to get what you're looking for.

Great question - I'd still like to hear from others that may have swapped 1bbl vs 2 bbl on a similar setup just to learn.

[Wrenchbender Ret]

Its obvious you need a higher gear but some tuning & adjustments might help. Make sure the vacuum advance is working. It is good for 2-3 MPG. You could try hooking it up to full manifold vacuum as generally they are hooked to ported vacuum. Check the metering rod adjustment. You might lower it a little if it is at the factory spec or higher. Make sure the float level is correct & the float is not heavy. Try setting the initial timing a little more advanced then specs. If it pings after that you will need to set it back.