Mixing It Up: Properly Jetting Your Motorcycle

Proper jetting not only provides more power, but also delivers a smoother running engine

Installing a jet kit on a motorcycle
Salvisburg in his office. With the sole exception of the computer terminal where he does his analysis of the dyno’s data, the dyno can be run from the seat of a motorcycle.Photography by Dean Groover

The first performance accessory most cruisers purchase is an aftermarket pipe. Consequently, the first performance items we compared in Motorcycle Cruiser were exhaust systems. In every instance, the pipe manufacturer recommended a jet kit be installed for optimum power. However, our experience is that only about half of the cruisers who have fitted freer-flowing exhaust systems to their bikes, have also installed jet kits. The people who refrained from installing jet kits usually gave one or both of the following statements: "The jet kit was too expensive." and/or "The pipe improved my bike's power so much I didn't need a jet kit."

Installing a jet kit on a motorcycle disassemble carburetor
The first step in creating a jet kit is the complete disassembly of the carburetor to be jetted. Every part is measured to 0.001 inch accuracy. Here the top diameter of the needle has been measured. Next, the bottom diameter and distance from the top of the needle to the beginning of the taper will be recorded.Photography by Dean Groover

To check the veracity of these statements we contacted a tuner who makes his living jetting motorcycles. Marc Salvisberg, owner of Factory Pro Tuning, builds performance parts for motorcycles. His specialty? The art of carburetion. Factory’s reputation for developing jet kits explains why race teams like Team Kinkos send their bikes to Salvisberg for performance tuning.

Salvisberg feels that all riders, not just racers, benefit from having a properly carbureted bike. While an aftermarket pipe may make a bike more powerful, making sure the carbs are properly jetted will make an engine produce more power and improve the quality of how the power is delivered—whether the rider has a hot pipe bolted on or not. For an example of the feel of the better quality of acceleration which comes from a correctly carbureted machine, he pulled out his notes from developing the Factory jet kit for the Royal Star. Getting optimum power and smoothness out of the V-four requires three different needle settings for the four carbs (two of the carbs have the same setting while the other two get their own individual setting). Once he had the needles set correctly for all the carbs, he said the bike woke up, producing two more horsepower more in the midrange (about a four percent improvement) and revving more quickly.

Installing a jet kit on a motorcycle measure air-jet hole of carb
Factory uses micro drill bits to check the internal diameter of every orifice in the carb. Here the air-jet hole is measured.Photography by Dean Groover

When we asked Salvisberg about the perceived expense of a jet kit, he agreed that, when you open the box, a few brass jets (one per carburetor) and stainless steel or titanium needles don't look like much for $60 to $100. He explained that the customer is not only paying for the machining of jet kit's little pieces of metal, but also for the development time. Sometimes, a kit may take several weeks of tweaking to reach its final form. For example, the aforementioned Royal Star jet kit took one week to reach its finished form while Team Kinkos' ZX-6R took six weeks.

Salvisberg follows three steps to find the optimum jetting for a particular motorcycle. First, he measures the performance of the stock set up to establish a baseline. Then he gets to know the character of the motor by testing for the optimum fuel mixture for each rpm throughout its range. Finally, he creates and installs main jets and needles to fit the optimum settings discovered in the second stage.

To see first hand a jet kit under development, we spent two days at Factory while our long-term Vulcan Classic (starting with Kawasaki factory jetting and Vance and Hines 2-into-2 pipes) was strapped to Factory's dynamometer.

Installing a jet kit on a motorcycle check engine case temperatures
Salvisburg makes sure the engine cases are up to operating temperature (at least 145 degrees) before performing a series of dyno runs. He also makes sure the engine does not get too hot, allowing short cool-down periods at lower rpm after each high rpm run.Photography by Dean Groover

Baseline Testing
To be certain how much progress has been made, you must carefully establish an accurate picture of where you were before the process began. Salvisberg breaks baseline testing into two parts. First, he puts the bike on the dyno for a series of tests to see how the bike performs. Second, when the carb (or carbs on multi-carburetor bikes) is removed for the first time, he measures every important feature of the carb, including the main-jet size, the number of turns on the mixture screw, the size of every orifice, the diameter of the needle at its top, and at the end of the tapers, where the needle begins and ends each of its tapers (Salvisberg has seen needles with five separate tapers), and the float height. Salvisberg's reason for recording this information is simple: often the OEM's original published specifications are different from what arrives in showrooms after mid-year changes, and he needs to be able to trace where he started.

Next, the results of the dyno runs give direct feedback about what works and what doesn’t. Although some riders may worry that putting their bike on a dyno is excessively stressful, modern dynos with their improved data acquisition systems require only very brief periods under heavy load. After a series of dyno runs, the engine should be no hotter than after a spirited run down a road. The only problem we encountered in our trip to Factory was discoloration of the rear pipe’s canister, a problem easily avoided by blowing high speed air across the pipes. In the close space of the dyno room, a light stand used for the photography in this article was placed where one of the fans would normally reside, reducing the air flow over the pipe.

Installing a jet kit on a motorcycle major parts of jet kit development
This picture shows the major players in jet kit development. The tire transfers the load to the roller for measurement with an accuracy of 0.01 percent at 100 mph while the tube hanging from the exhaust canister measures the CO level exiting the pipe.Photography by Dean Groover

There are two types of dynos commonly used for motorcycles: inertial dynos and eddy current dynos. Many motorcycle shops use inertial dynos. The bike’s rear tire drives a large weight, and the bike’s horsepower output is measured by how quickly the mass accelerates when the engine runs in top gear at full-throttle from low rpm to redline. While inertial dynos are good tools for the type of work most motorcycle dealerships perform, they have one drawback. Inertial dynos run sweep (or acceleration) tests where the readings are taken as the engine makes a run through its rpm range. An entire run through the rpm range on an inertial dyno can take as little as 10 seconds once the bike is in top gear.

Eddy current dynos still place the bike’s rear wheel on a large rotating drum, but the dyno gathers information in steps as the engine moves through its rpm range, pausing at regular intervals for several seconds to have the power output measured. The engine is still running at full throttle, but engine speed is controlled by the dyno’s electromagnetic brake acting on a vented disc that looks like it belongs on a very large, heavy auto. The power output, as determined by the amount of force required to hold the engine speed constant, is then measured and recorded up to 30 times per second, providing a very accurate picture of what is happening. In the case of our Vulcan Classic, Salvisberg took five-second readings every 500 rpm from 1500 rpm to the 5500-rpm redline. Aside from gathering extremely accurate data at each step, holding the engine at each rpm step allows the carbon monoxide (CO) output to be measured, giving the tuner one more piece of data hinting at why the engine does or doesn’t perform well at a particular rpm. Salvisberg, who manufactures and sells both kinds of dynos, cautions that the comparison between eddy current and inertial dynos is not an issue about which are superior. Instead, they are different tools designed to fulfill different needs. Similarly, since different manufacturers’ dynos generate slightly different results due to different manufacturing and calibration specifications, simply comparing dyno numbers without knowing their origin is less revealing than comparing baseline and subsequent runs on the same dyno.

Installing a jet kit on a motorcycle changing main jet
After every series of dyno runs, the main jet was changed. Fortunately, the single carb on the Classic does not need to be completely removed to do so.Photography by Dean Groover

Getting to Know the Engine
With the baseline data collected, the development process begins with repeated dyno runs after the carb undergoes incremental, duplicable changes. Every change in jetting creates a change in output, giving some new knowledge about the engine (even if the change made the engine run horribly). Salvisberg meticulously records all information, including if it misses or backfires. The resulting data is then compared with the baseline and previous iterations to determine what the next adjustment should be. Salvisberg stresses that what looks like fooling around—changing the main jet, running the bike on the dyno, and changing the main jet again—is really the way he learns the character of the engine. By concentrating on only the main jet, Salvisberg makes changes with broad strokes (literally moving the fuel delivery curve, the rate at which fuel is delivered to the engine, up and down to see what happens) that gradually narrow down on what main jet produces the best peak horsepower. Since he's recorded what produced the best power in the midrange, he usually has a good idea what the initial needle settings should be.

Salvisberg says that he approaches each development project with an open mind, trying to intuit from what the data tells him about the engine instead of assuming what needs to be done and setting out to prove it. His analysis is designed to reveal the maximum horsepower that can be produced at each rpm step and the coinciding CO value resulting from the combustion of the fuel mixture. Once he has found the optimum CO reading at each rpm, Salvisberg knows which main jet produces peak horsepower and then uses the needle to force the fuel delivery curve to give the engine the optimum mixture at the other speeds. Piecing together the results of several dyno runs reveals an ideal fuel-delivery curve.

Installing a jet kit on a motorcycle exhaust gas analyzer
The all important exhaust gas analyzer. Without this tool, developing a jet kit for the Classic would take much longer, requiring Salvisburg to tune by the seat of his pants instead of through exact reproducible methodology.Photography by Dean Groover

Knowing basic carburetor concepts will illustrate the role the needle plays in carburation. Air flowing through the carburetor’s venturi flows over the discharge nozzle whose bottom opening rests below the surface of the fuel contained in the float bowls. The low pressure created by this airflow across the discharge nozzle draws the fuel up through the main jet’s orifice to be vaporized by the high-speed air. The needle is attached to the throttle slide which moves up and down, depending on the volume of air flow into the engine to keep the airspeed high across the discharge nozzle. As the slide moves up and down, the needle regulates the amount of fuel allowed to vaporize into the airflow. The main jet limits the maximum amount of fuel to enter the airflow at wide-open throttle at high rpm, the only place where the slide will be raised to its highest position out of the airflow—i.e. where the engine’s maximum power output will be generated. At all other engine speeds, the needle plays an important part in metering the fuel flow. At moderate engine speeds, the slide is not raised very far, making the diameter of the needle of primary importance. A wider needle diameter in the same size jet allows less fuel out of the discharge nozzle than a narrower needle. The needle’s shape and where the needle’s tapers begin, can help to richen or lean the mixture at midrange rpm. Knowing the optimum CO reading for this midrange rpm allows Salvisberg a level of precision in adjusting the needle to tune the midrange with accuracy that would be almost impossible without the ability to measure CO.

Installing a jet kit on a motorcycle jetting changes on software
Mission Control: Factory’s software keeps track of all the jetting changes and how they effect performance. If you look closely, the graph depicting the baseline run and the final run with the completed jet kit installed can be seen on the screen.Photography by Dean Groover

The Completed Jet Kit
Thanks to the single carb that didn't need to be completely removed for each change, testing on the Vulcan Classic went quickly. Salvisberg completed the baseline dyno runs, and over half the testing runs, in the first day. The second day's testing took longer. As he began to close in on the final jetting, the Classic began to spend more time off the dyno in street tests to verify the dyno's numbers. Usually the rider, who didn't know the dyno numbers, concurred with Salvisberg's interpretation of the dyno results after sampling the bike in both highway and city riding situations. The testing pointed out that the original jetting of the bike was too rich for optimum power. After two days, the main jet shrunk from the stock No.140 size to No.132, completely changing the Classic's character.

After the changes, the Classic’s low-end power was noticeably smoother, and neutral throttle in first gear was much easier to maintain. Before, the carburetion only allowed the narrowest margin between acceleration and deceleration. The occasional backfires when decelerating with a fully closed throttle had been completely eliminated. Midrange and top-end acceleration had undergone an impressive change, not only reflecting the new 52.0 peak horsepower versus 50.3 horsepower baseline, but also revealing an engine that revved more quickly, as if the flywheel had been lightened. As evening turned into night and Interstate 5 plowed through sparsely populated areas, the Classic’s new found roll-on prowess delivered some delightful acceleration: “Honest officer, I wasn’t really speeding; I was just testing the bike’s carburetion during a ten-second full-throttle run from 65 mph.” Even with the gas-guzzling shenanigans, the Classic maintained a respectable 33.5 mpg on the trip home (only 1.2 mpg less than our more sedately ridden Classic in our February 1997 big twin comparison).

Installing a jet kit on a motorcycle wiring and air box space
Since manufacturers know that cruisers don’t want a lot of wires and hoses swinging in the breeze and blocking the view of the engine, space around cruiser air boxes is usually in short supply. The Vulcan Classic’s layout shows how incorrectly routed hoses or wires could disrupt the air flowing to the velocity stack at the airbox’s entrance, negating some of the power gained through proper jetting. Always make sure everything is nicely tucked away before refastening the tank to the bike.Photography by Dean Groover

After watching Salvisberg at work, and after our ride back to LA enjoying the fruits of his labor, we decided that the Factory Vulcan Classic Jet Kit delivers more than its $65 worth of performance. We can’t imagine not installing this jet kit on an aftermarket equipped bike to raise it up to its fullest potential. Before we left Factory, Salvisberg started development on the Factory Air Box Plus kit for the Vulcan Classic. In its finished form, the $130 kit will allow removal of much of the air box plumbing, and the carb will breath through a cotton weave filter. The first dyno run unleashed 57.7 peak horsepower. Although the additional power came at the cost of more intake noise, we’re certain that horsepower junkies will bolt the kit on their Classics as soon as it is available.

Installing a jet kit on a motorcycle dyno
While the difference in peak horsepower (52.0 hp versus 50.3 hp) is only around 2 hp, the Factory jet kit delivers more power from 1800 rpm to red line. Peak horsepower comes sooner and doesn’t fall off as quickly. Riding the Classic reveals a quicker revving, smoother accelerating engine. Torque is also improved slightly. The properly jetted carburetor also emits less CO.Photography by Dean Groover

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