Performance and Efficiency: Gasoline Usage in Harleys

Many readers have been asking questions that nibble around the edges of the subject of gasoline and how our Harley engines use the stuff. Since we seem to be in for high gasoline prices for the indefinite future, this seems like a good time to practically consider gasoline usage in Harleys.

For years I have known that a great majority of Harley owners do not consider the cost of gasoline to be very important. Also, mileage has seldom meant much to this majority. If there has been a concern, it has been about range-how far one can go on a tankful. For a number of very good reasons, including cost, morality, and national security, these attitudes are changing. With that said, let me get practical and perhaps somewhat blunt.

I’ll be first in line to say that stock Harleys could use more power, especially in the middle part of their rpm range. However, it is a myth that reduced mileage and efficiency are inevitable companions to improved performance. Quite the opposite can and should be true.

An engine with an aftermarket exhaust and air cleaner, but with stock motor parts, should produce mileage close to what it delivered when it was still stock. If it does not, the carburetion (or EFI) is out of tune.

A stock Big Twin normally delivers around 40 to 42 mpg at 65 mph on a flat windless road. The same bike with our 88/95 conversion should get at least 10 percent better, while delivering 40 percent more torque at cruise speeds. I have had many Harley owners tell me that they are getting 49 to 51 mpg at steady interstate speeds with this engine conversion.

How can this be, you ask? It’s simple, really-an appropriate cam design, higher compression, and a free-flowing exhaust and air cleaner together with correct air/fuel mixtures.

Most Harleys are modified in ways that affect fuel mileage. Owners normally change to more open (louder) and less restrictive exhaust systems and air cleaners. Both of these modifications have an effect on the air/fuel mixture strength; they make the engine run leaner. Stock bikes are already rather lean for emissions purposes. Air-cleaner and muffler changes further lean the mixtures, so much that correcting this is necessary for the engine to run well.

Unfortunately, these mixture changes are normally done incorrectly. The result is an engine that, although it doesn’t melt down from being too lean, runs rich enough to give less-than-optimum performance and fuel efficiency-especially at higher elevations.

Maybe you own one of these bikes. Does it start to miss and emit black smoke at a few thousand feet above sea level? Is there a hard hesitation when you roll-on the throttle from interstate speeds? Does it deliver less than 40 mpg at 65 mph? None of this has to happen. If the mixture alterations have been made correctly, your bike should run better than stock, not worse.

Stock Harley Big Twin and Sportster engines, both the carbureted and EFI versions, are tuned to be EPA-lean in the lower half of their throttle range. The result, especially with the carbureted engines, is flat mid-throttle acceleration. Very little alteration is required to correct this.

The stock Keihin carburetor is a fine instrument. It is reliable, long-lived and easy to tune-if you know how and use the right parts. Because Harley has used the Keihin for more than 15 years, there are enough alternate tuning parts lurking in The Motor Company’s parts books to make do quite well.

All the Keihin carb needs to deliver proper mixtures for stock and modified engines is a different needle-one that your Harley dealer can easily get from The Factory-and a simple adjustment.

The needle I have used and recommended since 1990 is the needle from the first (1988-89) 1200 Sportster, Part No. 27094-88. This needle is smaller in diameter (richer) in its straight-diameter section, which controls mixture strength in the 1/8-1/2-throttle range. Buy it and install it.

You also need to open the idle mixture screw about one turn (adjust for best idle). You do this by removing the soft aluminum plug covering the screw on the bottom of the carburetor.

The main jet seldom needs changing because the stock jet is about two sizes too large. An open exhaust and air cleaner makes it just about right. This jet only becomes effective at throttle openings of 3/4 and above, so unless you ride at wide-open throttle all the time you needn’t concern yourself with the main jet.

EFI engines are also tuned to be rather lean in the 20 to 40 percent throttle range. They seem to be more accurately adjusted at idle. There are a number of aftermarket EFI tuning kits that allow richening the mixtures in this range. I have experience with the Power Commander and recommend it.

If you take this information to your mechanic, factory or otherwise, there is a reasonable chance that you will meet resistance to my advice. Harley sells carburetor kits and everybody is a carburetor expert. Normally, I would recommend Harley parts because I have some knowledge of how hard they try to get things right. The Motor Company’s jetting kits, however, are an exception: they deliver very rich mixtures and wear out quickly. Many, many of you have one of these kits installed in your carburetor and are getting mileage as low as 26 mpg.

The needle and needle jet in these kits are made of brass; the stock Keihin needle is polished hard-anodized aluminum, and the needle jet is hard brass. The brass-on-brass of the kit wears quickly and further richens an already too-rich mixture. The stock parts hardly wear at all. I have examined stock needles and jets with 45,000 to 70,000 miles of use and could not detect measurable wear.

A majority of mechanics change the slow (idle) jet to a larger size in an effort to richen the mixtures in the lower throttle range. I think they do this mostly because they don’t know about the importance and effect of the needle’s diameter above the taper (the straight part). If you can get your mechanic to try what I have suggested, he can see for himself, and the resulting performance improvement will convert him.

By the way, if you use a Mikuni on your Harley, buy and install the “mileage” or “lean-out” kit. Mikunis are also tuned too rich. Fox Distributing in Illinois (630-513-9700) sells this kit (the parts are genuine Mikuni). It improves mileage, throttle response and engine smoothness. Also, go to and download the tuning manual if you don’t have it.

I have several reasons for going on this long about air/gasoline mixtures. First, I want you to get the most out of your ride, and I know many of you are riding poorly tuned machines. Second, it is time we consider efficiency and not just power. Third, I know and have proven many times that it’s possible to have both power and efficiency.

Finally-I guess I am getting old and crotchety-I no longer feel any obligation to not offend factories, suppliers, dealers or poorly informed mechanics. Gasoline and its efficient use is simply too important for such niceties.


Please enter your comment!
Please enter your name here