Fuel Consumption

[otis71]

I have a ULH that runs good and seems to be dialed in just fine but fuel consumption is at best 25 mpg. Any knuckle or pan I’ve owned has been way better in comparison. Are flatheads or at least 80” flatheads known for this or is something off?

[T. Cotten]

... or is something off?

Odometer?

....Cotten

[otis71]

Four gallons won’t give me quite 100 miles if that’s what you’re asking. That includes all of the reserve.

[T. Cotten]

That's about what I got, Otis,...

With a fully loaded sidecar!

...Cotten

[otis71]

So back to the question-I should expect better on a solo?

[dukekleman]

Otis,

I don't have the experience of running tanks of gas (long trips) through a Big Twin Side Valve motorcycle...

I have done a lot of long trips on Panhead and Shovelhead's though and they are far more efficient than that!

I'll add this, when consumers and manufacturers check fuel mileage there are several factors involved in the results seen. I'm not sure about the driving speeds and scenarios of your particular situation I guess.

Keep in mind that various throttle positions on the carburetors throttle disc result in different AFR's... either way I would expect better mileage than that.

Duke

[T. Cotten]

So back to the question-I should expect better on a solo?

Plenty of room for improvement, no doubt, Otis!

Let us try to identify variables (other than 'tuning') for now.
My example illustrates payload plus wind resistance are big factors. And we know road conditions and riding habits factor in.

If we dismiss mileage-sucking ethanol fuels and other things we cannot control, we are back to 'tuning'.

Please beware of arbitrary over-enrichening to put color on the plugs.

....Cotten

[JoJo357]

Hello Doug, just for comparison, my stock 1946 model U will average between 29-35 mpg. If I baby her, i've squeezed out 42-45 mpg. I believe the ULH is 5.7:1 ~ and the U is 5:1, so not much difference in the compression ratio. I weight in about 175#, and this is with no additional weight on the motorcycle. Just a thought, I had a small manifold leak once, and it robbed some gas mileage till I found it. Also, with the low compression, you can run straight pump gas, but I burn the recreational *(R90) unleaded fuel. Sometimes if I can't get the R90 octane, i go for the premium or mid blend. Sounds simple, but what tire pressure are you at? And yes, my pans & shovels did better on the gas mileage. *Hope this helps Doug.

*M.A.D.*

[Rubone]

I consistently get 25-28 on my 44UL, it is a 3&R gearbox and has always had the hack attached since I built it up in 1977. Even loaded with a passenger and luggage it is near 25.
1979 and 2021... DCP_4143.JPG RK sidecar 79.jpg

[otis71]

I run the 16” tires on the low side for comfort but with modern tires I will try more air. I run non ethanol 90 octane gas as much as availability allows.

[Steve Slocombe]

I make about 40 miles per Imperial gallon on a solo VL, which is about 33 to a US gallon, with a similar 5 to 1 compression ratio flathead. The UL will be a tad heavier, but check your tyre pressures, wheel alignment and drive sprocket gearing, and of course speed will make a difference too. I run modern tyres around 30 psi.

[Jim]

I make about 40 miles per Imperial gallon on a solo VL, which is about 33 to a US gallon, with a similar 5 to 1 compression ratio flathead. The UL will be a tad heavier, but check your tyre pressures, wheel alignment and drive sprocket gearing, and of course speed will make a difference too. I run modern tyres around 30 psi.

Like Steve, I also get "about" 33 mpg on my '36 VLH. It's kind of hard to tell exactly, due to me not having a speedometer working. I usually figure my gas by starting with an empty tank, adding an exact measurement of the gas I put in, and then I figure my mileage with google maps of the route I rode. When I get home, I siphon the rest of the gas OUT of the tank, measure how much gas is left and figure my MPG from there.

[Jarek]

I have driven 500 miles this year and I have also checked the fuel consumption.VLD 36 solo , 33 - 35 mpg.If I drive more dynamically and fast (60 miles per hour and more) definitely around 33 mpg. Tires 4,50 - 18 , pressure like in the book " Rider's Hand Book " ,honestly a little more because there is low pressure given, tires - Coker

[T. Cotten]

Back to variables, Folks,..

Even though we have little control over it, we must still consider the fuel. Practically all pump gas has ethanol, which takes more volume to produce the same power. Aside from politiks, it is added to increase octane.

The sad irony is that Flatties don't need, or use it; Any extra octane just goes out the exhaust pipes.

As an armchair engineer with no way to test my theories, it is easy imagine the cooler burning fuels may delay Flatties' reaching optimum temperature, a hundred degrees hotter than an OHV. If the machine only sees short runs, poor mileage seems natural.

....Cotten

[chuckthebeatertruck]

My 46UL has ULH cylinders, some hairy .425 lift cams,and is stroked to get to 90 inches. Stock 4 speed. The engine is in a 58-64 duoglide chassis with hydraglide forks and full, stock rear suspension. So, it isn't a "light weight" despite being "bobbed." I do run it on any old fuel, 87, 89, 91 or 93 octane -- the bike literally doesn't care. Oil -- nope, the bike doesn't care -- but it sure does vibrate a LOT less when running 15/50 synthetic (yes, seriously -- Mobil 1 15-50 synthetic; I've used it in all my Harley's for several years now without issue).

When on the oem linkert, I got 35-38 mpg. Switched to a keihin cv and instantly went to 40 mpg. I later switched my ignition system from a magneto to a little experiment I am working on and instantly gained a few more mpg. Standard 6" air cleaner with a K&N filter was used with the Linkert as well as the CV. I simply modified the backing plate for the CV.

A little more tuning and I’m consistent at 45-50mpg cruising and 40-45 mpg when I crank it on. I don't worry about even looking for a fuel station until I've hit about 150 miles on a full tank. I am not lean, there are no air leaks as confirmed by bubble testing, and my plugs look like modern plugs. I ride the bike in Chicago traffic and as mentioned above; fill it on any old gas -- ethanol, no ethanol, is that even gas?

I put 2000 miles on the bike this year, including riding it on the Eagle River National Road Run. I did have hot starting problems on the Road Run - which turned out to be a bit of dirt blocking the pilot jet. Instead of the normal one kick hot start -- it was more like 10. Otherwise, the Keihin CV has been totally and completely fuss free on the bike and make a noticeable (really noticeable) difference in both power and economy.

My bike is, however, geared tall which contributes to my very good cruising economy. Because there is so much low end torque with the stroker -- I can sorta kinda get away with the high gearing without burning my top end too quickly.

I do not baby the bike and my normal cruising speed is 55-70mph verified by GPS.

The 90" UL gets the very best fuel economy of all my bikes -- including much, much newer bikes.

YMMV