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Fuel Management 101

Uncle Loomis - Merchandising


Welcome to Fuel Management 101!

You are here for one of two reasons - you know that you need to modify your fuel delivery system but need some education, or you have no idea why you need to modify your fuel delivery system, and need slightly more education. Sharpen those pencils!

Your Engine As A Pump

It may help to think of your engine (for this class, anyway) as a pump. Depending on your engine’s size, it can move a given amount of air and fuel into itself (in the proper ratio, of course), burn it for power, and expel it.

If you took Exhaust 101, you will remember that we have a few ways of making the engine give up more horsepower:

  • Make the engine go faster - this is what you do every time you twist the throttle. Most engines are limited in speed (their redline) by the valvetrain. Speeding up the engine requires stronger engine internals due to the increased heat, friction, and speed. Typically, this is a pricy and labor-intensive, and some engine styles (like big-inch, long-stroke V-twins) respond poorly to this approach, because their power is not made high up in the rev range, and critical engine parts bear heavy loads.
  • Make the engine bigger - this is what’s done when an engine is bored or stroked. Typically involving stronger internals, this is another labor intensive process that can thin out the bulkiest of wallets.
  • Make the engine hold more air and fuel - this is what a turbo, nitrous oxide, or a supercharger does. Instead of making the engine bigger, forced induction physically shoves more burnable matter into the same size engine by compressing (pressurizing) the air. Again, the enemies here are heat, stress, and cost. All three will be plentiful if this method is chosen.
  • Make the engine more efficient - this is the easy and affordable way to pick up a few extra ponies. In terms of ‘efficiency’, we’re not talking about your fuel mileage, but rather, how good your engine is at turning air and fuel into horsepower without producing waste products like heat. This approach is usually not labor-heavy or too expensive on a factory engine, but it is limited in its scope - you will not be able to pick up a hundred horsepower by bolting stuff on to your engine. The idea here is to unlock just a little more power by making the engine’s intake and exhaust less restrictive, limiting power losses by reducing the work the engine must perform when it draws in air and fuel and expels exhaust gases.

So, as you can see - each of these activities requires the engine to consume more air and fuel in order to produce more power.

When do we want it? We want it now!

So you may be asking yourself why you would want to modify your fuel delivery system. I can only think of two reasons you would want to change up how much fuel goes to your engine.

  • If you have modified your bike from stock in any way in a performance sense
  • If your bike is still stock

See what we did there? Every stock motorcycle’s fuel delivery system can be made to produce more power - and the same holds true for bikes that are being ‘hot-rodded’!

Remember that pump we discussed earlier? Good. Let’s pretend you put a freer-flowing exhaust onto your bike. Or maybe a trick air cleaner. Maybe you added a new head (or heads), or maybe a whopper camshaft(s) - any modification will do. Maybe it’s several of the above - which makes the example even more germane. You have allowed the engine to ‘breathe’ easier. Think about your own lungs - it’s easier to breathe through a snorkel than a soda straw. When you modify the engine to ‘pump’ more easily, you allow it to move more air than the factory did.

For this to be safe for the engine, however, we need to add fuel. Simply moving more air and running the same amount of fuel means you are running your engine lean. (The opposite of this condition - too heavy on fuel - is colloquially called rich.) Engines that run lean have some characteristics that are very detrimental to engine life and performance. They tend to run hotter than an engine receiving the correct amount of fuel. They often hesitate when throttle is applied quickly. Effectively, the hesitation means that your engine could work worse than before you installed your go-fast goodies. Worse still, all that extra heat is bad news - not just for your thighs, but for your bike! Heat cooks and kills. It scorches oil, and deforms parts, and puts gaskets under stress and undue exertion. That means that your engine’s longevity suffers. You don’t want your toy to take an early exit, do you? So, what this means is that power comes with modifications - but the bike needs additional alteration to take advantage of the power-adders.

That mix of fuel and air - often referred to as the air-fuel ratio - is very important in the internal combustion world, and very easy to change slightly, for better or worse. The air-fuel ratio is an interesting concept. The stoichiometric ratio is, numerically, about 14.6 parts air to 1 part fuel - that’s what burns most efficiently. With that ratio, emissions are at their lowest, and you are getting the maximum power per unit of fuel.

However, most gearheads are interested in the absolute power their engine can make. Engines tuned for power are typically inefficient in terms of burning the fuel entering them completely, but they make the most horsepower per cubic inch. (Or centimeter, if that’s your measurement of choice). The air-fuel ratio that is best for power on most naturally aspirated street engines is around 12.5-13:1. So you will have an engine that is less efficient at getting maximum power per unit of fuel in that range, but will be delivering more absolute power, at the expense of more fuel used and additional tailpipe emissions.

Now I know a few of you are saying, “Great, buddy, but my bike is just like it came out of the factory. She’s cherry. All original! I don’t need that jazz.” If you’re on an old Knuck, fine - you’re right. However, after about World War II, the government began requiring manufacturers to meet emissions standards. It turns out that an easy way to make engines run cleaner was also to make them run leaner. So, they came from the factory set up a little light on fuel, and enterprising hot-rodders realized that re-jetting them and drilling out all the anti-tamper plugs installed in carburetors was the hot ticket for getting the bikes to scream - and the same holds true for today’s fuel-injected motorcycles. Fire up your favorite search engine and look up “the Harley tax”. It’s such an ingrained act of ‘fixing’ factory lean-tuned bikes that the pesky process has earned itself a name!

Keep in mind - all this modifying is often in flagrant violation of all sorts of federal and state laws. So, making big power often comes at the expense of being able to get an inspection sticker for your bike (see our 200-level class Moto Law: Keeping The Man Off Your Case).

Shut Up and Take My Money

So we have convinced you, eh? Wonderful. Let’s talk about what you might want to buy. Like any other fairly intensive bike modification, you need to ask yourself a few questions (and answer them honestly) before we can give you some ideas on what products might make you happiest.

  • Is your bike carbed or fuel injected?

There are fuel management products for both style of bike, but they are vastly different. Carbureted bikes modify the fuel delivery through mechanical means, whereas products designed for fuel injected motorcycles modify the existing fuel system electronically. Products for carbureted motorcycles include larger-than-stock jets, adjustable needles, and jet kits. Jet kits are a ‘pre-packaged’ solution for bikes with common modifications, and often work unbelievably well. Individual components like jets and needles can be purchased, but require some tuning homework on the part of the installer. The payoff is often higher output for a specific bike versus a ‘canned’ jet kit, and individual component selection can be the only option for a highly-modified motorcycle. If you’re truly adventurous, the whole delivery system can be swapped for a different carburetor - mechanical carbs can be swapped for constant-velocity units, and it is not uncommon to see more - but smaller - carburetors installed for even more horsepower. Conversely, some folks swap from multiple units to a single for easier tuning or better fit in a custom frame.

For those of you with a fuel injected scooter, you have no fewer a range of options, but they install and tune differently. Options range from units that splice into your existing harness and tune much like a carburetor to boxes that re-configure your factory computer to boxes that sit in-line with your factory ECM (Engine Control Module) and modify incoming signals to it. Some allow you to fine-tune many parameters, and others have simpler functions to keep things easy to work with. Other products allow some bikes to use their oxygen sensors to automatically tune the bike many times per second.

  • Is your bike highly modified?

Let’s consider your goals. If you have a heavy bagger, and you want a little more thump and grunt from the lower end of the rev range, you are likely going to install a new air cleaner and exhaust, and maybe a mild street cam. In this scenario, a “canned map” (a pre-written program that is fairly generic) should be perfect. Something much more capable will likely be wasted money and poorly-invested time. However, if you have a super-stripped cruiser-style bike with lightened flywheels and a crazy compression ratio with an exotic set of heads, you will likely need the capability of tuning beyond something “canned” and easy - many dyno pulls will be necessary, and tiny corners of the fuel profile will need to be altered.

Similarly, for sportbikes, secondary injector control is something to think about. Some I4 sportbikes have two sets of fuel injectors. Not all fuel control products on the market allow control of both sets of injectors. For many stock or close-to-stock applications, the stock secondary injection settings are fine. For highly ‘worked’ or forced induction applications, however, being able to manipulate the second set of injectors can be highly beneficial.

There are also systems you can ‘grow’ with, that start simple but have the option of being manipulated more heavily. This might be the ticket if you want to ride and modify at the same time, or for the rider who has a build in mind, but also is working within the constraints of a budget. It may be more expensive up-front to purchase this type of fuel controller, but if it saves you the purchase of a second unit later, the overall cost may actually be considerably lower.

Finally, for those running stock motorcycles - if you are in a place where you can legally modify your bike, altering the fuel delivery system will almost always yield a cooler running, better-performing motorcycle. Almost all bikes now have some emissions controls built into them, and to simplify production and lower inventory costs, many manufacturers simply build their bikes to the highest applicable emissions standards. (Those standards are usually those of California if your bike was intended for sale in the United States.)

Every stock bike we have seen at RevZilla has benefitted from fuel tuning - dyno results do not lie. Remember, your bike was produced to burn efficiently, not powerfully. If you can legally modify your bike, doing so will yield you lower engine temps and higher output in almost all circumstances.

  • Are you installing this, or is your local tuner/mechanic?

This could be the single-most important question in this class. The reason we ask is that if you are new to light tuning, you will be able to learn most any of the consumer tuning products. However, if you have a tuning guru in mind to work magic on your motorcycle, ask him what he likes to use. This goes double for those of you tuning a carbureted bike. There’s not much point in paying an experienced tuner who is partial to a brand or type of tuner or jet kit try to learn something that might not yield you a bike that runs as well as it could. Especially when you factor in labor costs and the cost of each dyno pull, the monetary cost could be greater than using what he is comfortable with. Don’t forget the non-monetary costs that stem from the tuner himself being frustrated, and bike not performing like it could. We’d rather have you walk away from this class educated and purchase nothing from us than have you buy something that leaves you or your engine guru unhappy.

Now, this is not to say that a backyard wrench can’t do this modification himself. Some tuning products are plug and play, and simply require ‘tweaking’ as you ride the bike to get the best from it. Installation difficulty ranges from a simple plug to units requiring a bit more wiring and possibly welding different oxygen sensor bungs into your exhaust. There are a range of options to be had, so don’t be scared off by potential difficulties. As far as the actual tuning process, if you don’t have access to a dyno, it’s usually not a problem. If you know how to ‘read’ spark plugs, a few reads and a few nice long jaunts on your bike should have you dialed in for all situations except the dragstrip.

That process may involved repeatedly installing or removing your fairings (if your bike is equipped with them), which can be tedious, but most parts of the install process are not particularly difficult for a rookie with a good attitude and a proficiency with wrenches.

  • What do you want to see?

Nope, that’s not a typo. Aesthetics are important to many riders. Some riders want to see buttons, lights, and gauges - there are standalone modules that offer such whizbang gadgetry that can be mounted to the handlebars where it can be seen easily. Other providers offer a ‘skin’ that can be applied to your smartphone (and some even run cordlessly via Bluetooth!). This can be particularly useful for the rider who uses a comm system - your phone, music, and bike performance can all be monitored from one small, slick package mounted to you handlebars.

At the opposite end of the spectrum are folks who want to see nothing at all on their bars. For this crowd, there are units that either hide under the seat, and others still that the user plugs in, modifies the ECM, and unplugs, with nothing at all showing - all you’re left with is a clean, unmodified appearance.


A good ‘tune’ is essential to having a bike that is pumping out its maximum amount of horsepower, from a heavy bagger to a super-fast track bike. The actual hardware - the parts you bolt in - are vastly different between carbureted and fuel injected bikes, but the end result is the same: better power and throttle response from the bike.

There are a number of products available at different price points, ranging from very simple and easy to install and tune, to pieces requiring very careful and laborious installation, component selection, and setup.

No one product is ‘right’ for a bike - the ‘right’ product is determined by the bike, tuner, and riding style. Armed with the information you have just learned, you should be able to start asking some of the questions that should help lead you into the correct product for you. If you still need more information, we’re here to help - don’t be shy about getting in touch with us to narrow down your options!

Some products and brands we would recommend checking out are:

-- Uncle Loomis