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Tech Tuning

Fuel Injection/Carburetion Basics

The Basics

An internal combustion engine needs two basic elements to operate, fuel and oxygen. The fuel can be any number of things: alcohol, nitromethane, natural gas or used fryer oil, but most important to a powersports enthusiast is gasoline. The oxygen comes from air, but can obtained from other sources, like nitrous oxide or even be an additive to gasoline in the case of oxygenated fuels. For the sake of time, we're only going to look at the basic gasoline and air engine.

The Power

First and foremost, there is only one way to gain more horsepower with an internal combustion engine, and that is by adding more air & fuel. This is the most misunderstood concept by beginner engine tuners. You can increase the amount of power you are getting to the ground by making the engine or transmission more efficient, but you will not get any additional power, you’ll simply lose less in the process.

An exhaust pipe will never increase horsepower on your bike. At best, an exhaust pipe alone will get you a minor horsepower gain at the wheels. A free-flowing exhaust makes the engine more efficient wasting less horsepower pumping the exhaust gases out the tailpipe.

Tuning, and why you must.

How can we advertise six, eight and ten horsepower gains? The increased power comes from the extra fuel our exhausts allow you to add into your engine. This is why re-jetting, or using a properly mapped fuel controller, is so important. If you take only one thing away from reading this article, that was it. Now we can get into the technical hows and whys of carburetion.

First, it is vitally important when creating horsepower and not destroying your engine that you maintain a proper fuel/air ratio. Air/Fuel Ratio (AFR) is the ratio of the weight of the air to the weight of fuel inside the combustion chamber. To completely burn gasoline in dry, 70 degree sea level air, you would need an AFR of about 14.7/1. By completely burning the fuel, you will release all of the energy it has to offer. Unfortunately, you will never be able to completely burn gasoline inside any internal combustion engine. Due to the melting temperatures of the various metals inside your engine, and the need to burn the fuel instead of detonating the fuel, a much richer AFR is required. When we tune a bike, we aim for an AFR of around 13.0/1 for naturally aprirated engines. It is possible to work with higher AFRs, but nothing will destroy an engine faster than running it excessively lean.

Trying to obtain the proper AFR without an exhaust gas analyzer or O2 sensor on the bike can be difficult, but it is not impossible. We have done extensive tuning on the HMF Dyno, so we can provide you with a baseline jetting spec, or have loaded a baseline map into your HMF Optimizer. With that information, you should be in the ballpark. From there, final tuning of you engine isn't too bad if you know what you are looking and listening for.

What to look for.

The first thing you should listen for is popping or backfiring during deceleration. Ride your bike up to redline with a wide open throttle, then completely close the throttle. Don't use the brakes as the bike idles down to a stop. If you hear any pops or burbles or backfires, note where they were in the RPM range. Popping is a very good indicator of a lean running condition. There have been many engines that have been tuned to an acceptable AFR using this method alone. You want to add one increment of fuel (one jet size up, or one position up on the optimizer) to the RPM area that the popping is occurring. By repeating this process until the popping stops, you should have achieved an acceptable AFR.

You can sometimes diagnose a rich running condition by testing the throttle response of the engine. If you quickly open the throttle from a cruise just above idle, note how much "snap" the engine has. If the motor misses a beat and then accelerates, it could be a sign of a rich AFR. This method isn't 100% effective to diagnose a rich condition, but it is a good indicator.

The truth is in the plugs.

By performing a plug check, you will be able to determine your AFR more accurately. It is a little more time consuming, but it is much more accurate. A spark plug that is being used in a correctly fueled engine will be chocolaty brown on the ceramic insulator. If it is lighter than that, the engine is running lean; if it is darker or even wet with gas, it is rich. When you do a plug check it is important to turn the bike off at the RPM that you want to check. If you want to check the high RPM fueling, you need to ride the bike to that RPM and flip the kill switch. If you let it idle down, when you pull the plug you will only be able to determine the fueling at idle.

From these indicators and the baseline setting HMF provides, AFR tuning should be something you should be able to do yourself in your garage with a minimum amount of tools. 


The last thing I'll cover in this article is how an exhaust pipe can affect the amount of air and fuel coming into the engine. It seems like it is on the wrong side of the engine to do this, but there is one factor that lets an exhaust pipe and muffler do so.

There is a small period of time during the end of the exhaust stroke and the beginning of the intake stroke that both the exhaust valve(s) and the intake valve(s) are open. This is referred to as "valve overlap". You will most often see this in reference to cam shafts, as it is the cam(s) that determine how long the valve overlap and when it occurs. The tuning effects of changing either of these factors could be another whole article on its own.

Posted Saturday, July 26, 2014


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