Chief gas mileage

[agreeneone]

I'm curious as to what gas mileage I should be getting from my 1947 Chief. It has about 300 miles on a new engine rebuild. Everything is in good running condition. I have been riding at 40-50 mph on roads without much traffic, starts or stops. I seem to only be getting 15-20 mpg. There are no leaks and the carb seems to be adjusted properly. Is this mileage normal?
Thanks,
Aaron

[Skirted]

Should get around 30mpg and up depending on how you ride and geography of area. I often ride two-up on the 48 and can get 60 miles + out the left tank (2 gallon) through the hills and valleys in SW PA. Its gets better riding solo. Other guys have said they can get in the high 30s and up to 40 mpg.
Initially after getting my bike it seemed to be running well but after 45 miles the left tank was about empty. Went back through basics checking timing, points, etc and found some small issues with wear in the distributor and also intake leaks at both gland nuts then reset the carb mixture. Not exactly sure where the main problem was but it made a noticeable difference both in fuel mileage and performance.

[PRG]

Howdy Aaron,

Chiefs can benefit from improvements in efficiency in the order of ignition, fuel and finally compression. For the most part, many are setup too rich these days to balance out a variety of short comings, both inherent and accumulated along the way.

Converting your ignition to the Clyde Crouch style electronic unit as sold by Rocky and others establishes a stable platform for running long reach plugs (assuming your rings are healthy) which project the electrode into the combustion chamber as opposed to the recessed arrangement with a J6. Just remember to turn off half the threads before install if your heads contain the short bronze inserts. You'll now be able to run less advance with more complete burn, especially at road speed, throttle response is also improved slightly as these motors tend to feel a little "flat."

Now with plug exposure/flame propagation arrangement loosely resembling an OHV configuration you can (very carefully) lean your motor out. You'll need a rubber duckey or similar composite float for more consistent metering (brass floats are heavy and sluggish) and this can be run at a touch over 5/16's float level which will decrease some of the fuel surge up the nozzle you get on high vacuum situations in the Venturi such as when abruptly shutting the throttle on upshifts/downshifts, etc.

Finally, for the more adventurous a slight bump in compression - not for for extreme performance mind you - can more fully exploit the benefits of the previous two. We have a gentleman locally who rode his 47 more than 120k miles since reassembling it in 2000 much of that at 70mph interstate speeds and at times using no head gaskets on his hot rod rebuilds. No need to go to that extreme. In lieu of a more scientific approach, measure the crush depth of the fiber head gaskets when removed from one's motor. Order copper replacements and deck a several thou below that figure. I might add, ensure your head bolts do not bottom out before torqueing and with a decent gasket cement there is no need to strain everything involved with 50 ft lbs of torque, my 46 is torqued at 30.

[T. Cotten]

With all due respect to Peter, Aaron,...

Original-design hardware works great, and should give you 30+ mpg, when all things are in order.

....Cotten

[agreeneone]

I appreciate the input and advise. I'm going to re-check that everything is set and adjusted properly and all parts are in good working condition. Hopefully this is just something minor that can be easily corrected to get better gas mileage.
Thanks again,
Aaron

[T. Cotten]

Aaron!

Aside from the obvious things, like timing, valve-adjustment, drivetrain, etc, you may find it useful to bubbletest your manifold (http://virtualindian.org/11techleaktest.html), as any vacuum leaks will naturally require more fuel.
Within the carb are concerns, primarily the float: It should be light and buoyant. Brass productions are known for poor performance.

Let us know what you find,

.....Cotten

[pisten-bully]

Is this mileage normal?

Aaron, I'm not really trying to be a smarty-pants here... really, but are you using your 67 year old odometer to check mileage? If you are then maybe you're not registering the correct mileage on the odometer and it's throwing off your MPG calculations? For example, a speedometer drive gear for 16" tires when used with 18" tires will show fewer miles on the odometer than actually traveled on the ground (I've got that right, don't I?).

I'm getting 30 to 40 MPG here.

[Coolbreeze]

I have not checked my mileage since the '70s, but I seemed to getting ~35 mpg then. My '48 Chief is running a Scout carburetor with a Chief venturi - or at least that is what I'm told.

[T. Cotten]

George!

Today's ethanol blends will never have the volumetric efficiency of yesteryear, but we can try our best, anyway.

Both Sport Scouts and pre-'50 Chiefs came with 1 1/16" venturies standard, but Bonneville models and later 80" Chiefs had 1 1/8" venturies.
Traditionally, smaller venturies give better mileage under normal conditions, but far more mileage would be lost to a poorly-fitted venturi than to a larger one, I believe.

....Cotten

[phillie]

40 on mine always and on good days I have gotten up to 50.