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Why Re-Jet

by Mike "Morphd" Montgomery

Why re-jet a bike? This is a question that comes up often in social biker pages and gatherings, especially when one is looking for some better pipes and more performance. In fact, the simple act of changing out the pipe, air cleaner or air box, and jetting the carbs (or remapping the computer chip that controls air and gas flow in injected bikes) is what is referred to by HD as stage one performance upgrade. It is essential to modifying the sound, throttle response, and general performance of any bike. In fact for some bikes this is the only performance mod you can reliably make in hopes of have added performance. But, what if your goal is not performance? What if you just want a different look on the pipe side of the bike and or a bit louder exhaust to give you that attitude you are looking for on your bike? Keep reading, because you will see that even the cosmetic change of the pipe should at the very least have you thinking about optimum decisions for your bike.

When a bike has a change to the air intake (K&N filter or different air box entirely), and a change to the spent combustion (freer flowing exhaust from the restrictive stock pipes) then most people would agree (myself certainly) that a recalibrating of the fuel and air metering system is needed. The increased air flow, both in and out, will lean out the gas that the carbs or injectors allow into the air mix. Re-jetting or re-mapping the fuel curve, means changing out the jets (re-jetting) or increasing the delivery and timing of the delivery of the gas (injected bikes) to increase the gas flow to compliment the increase in air flow.

Your engine is a pump, at a high very high level, that pulls in air and gas (through the carbs or the computer chip monitoring the mix if you have injection), and explodes that mixture to produce the power by pushing the piston down rotating a crank which is converted to the back wheel providing forward propulsion. The higher, or more explosive the ignition of that gas and air mixture, the faster the piston travels down and the greater force is applied to the rotation through the rear tire. Everything is controlled by the carb or computer chip to allow a greater, or a lesser explosive mixture in the cylinder.

I am being very high level and breaking down just the reason why we jet or not or change the fuel mapping on an injected bike. In fact, the injected bikes have much more to consider in that computer chip and a more direct control over not only the mixture but the way it is detonated. I think there is more work to be done in perfecting the computer chip to think about this as currently a properly set up carbureted bike will out perform most (I cannot say all any more) injected bikes.

When you add the ability to the pump (engine) to pull in or push out more air into the mix, you must compensate with an equal ratio of fuel. If you do not, you end up with more air than the proper amount fuel in the mixture which can created a higher level explosion, and or hotter explosion which and can if "lean" enough create damage to the cylinder, piston, head, or all of the above due to the explosion being higher/hotter at the flash point than the materials were designed to handle.

Same is true of the gas in/out. If you restrict the air flow (by changing to a more restrictive pipe, or air filter i.e. stock parts), you need less fuel in the mixture if you once added fuel due to a freer flowing exhaust or air box (or both). If the fuel delivery is not changed in this scenario, there is less probability of damage since the gas will create a wetter explosion and therefore less force and heat accordingly. However, this will cause you to use more gas than you need and degrade your performance.

A leaner bike always is the quicker bike (to a point), but also more wearing to the effected components due to the higher rate of ignition. This is why, when a bike or car is WOT, a shot of nitrous will increase performance, because the properties of the nitrous burns at a higher ignition point, creating more power. This is also true of higher octane gas, all little tricks to produce the highest explosion as possible in attempts to drive the piston with more force. But I digress a bit.

Changing the jetting or fuel map keeps your air and gas mixture ratio within optimum range (if properly balanced) when you change out a component designed to increase or reduce the air the engine will pump in or out. Just changing the jets or fuel map does not make a perfect ratio. You have to replace the jets with the correctly sized jets and needle setting in order to make sure you are at optimum performance with your current setup. This is a whole new discussion really as their are specific components within the carb or fuel map that allow more gas or air in, or restrict each depending on the throttle setting.

A carbureted engine does not monitor the throttle, it is all done by vacuum. As the throttle opens, the pump will create vacuum opening up the gas flow by displacing the "jet needle" from the main jet. The main jet is very much larger than the primary jet comparatively and is the one jet that most will focus on to increase the gas flow as the throttle opens up. It is a beautiful thing when you get the right rate of gas flow based on the throttle position with the right jet settings.

An injected bike may or may not have the ability to sense the throttle position. It depends on how the computer and the underlying program was set. It does monitor air flow, and therefore calls for more or less gas flow depending on the metered reading and current program (map) of the chip. It also has the ability to create the proper timing of the spark. This has the potential to give full control the mixture and level of detonation. However I have not fully studied the systems out there in this depth to understand how they do what they do and how they could do it better. : ) I am a tinkerer at heart.

A personal case study of the effects of air ratio change: My first trip to Colorado was an eye opener, and my bike was fully stock. In the higher passes, I could not get the bike to go faster than 55 - 60 mph no matter what I did, 4th gear whatever and I would hit the rev limiter in all gears much sooner than I was used to. Lots of scientific reasons for this, but the main thing was the bike was not set up to run with less air (oxygen) with that level of fuel I had going. The next time I went, I had new pipe, K&N filter and good jetting for sea level. My bike ran much better, but would still stall at 75-ish at the higher elevations. To me this was the perfect example test of what I am talking about. The change was air in the atmosphere, but still a metered ratio effecting change that degraded the performance of the bike. Interestingly enough, I got better gas mileage but I am not sure if this was the oxygen levels or the up and down grades of mountain riding, could have been either and really a study unto itself.

Bottom line, if you introduce a change to your bike in the way it breaths in or out, you need to visit the idea of jetting or re-mapping if you want your bike to be in optimum running condition. This said, if you change your pipes and you do not want to re-jet or change the fuel mapping for whatever your reason; most of the time you do not have to. To be sure, you should take your bike to a bike garage with a Dyno available and have them place your bike on the Dyno and check its ratios, performance, and torque/HP curves. If something is out of line, you should re-jet or re-map the fuel for your bike. Even if you decide to re-jet or re-map, having a Dyno confirm the changes will help you know your settings are right for your bike and location.

Disclaimer: Any person who decides to perform any of the above listed modifications, does so at their own risk.
The Sabre Group does not claim any responsibility for damage to your motorcycle or individual.