Carburetor and Fuel Injection Tuning Basics
by Jim Gianatsis

Any performance changes you make to a stock sportbike, from the addition of a Low Restriction Air Filter or opening up the Airbox to increase airflow, to adding a Slip-On Performance Silencer in replacement of the stock muffler or the addition of a Complete Race Exhaust System, these changes to increase air flow will cause the bike to make less power and run slower until the carburetors are rejetted. That's how closely tuned today's sportbikes are to the ideal lean condition to make maxium power while passing EPA smog requirements. Just taking the cover of a stock airbox will make a bike overly lean and run poorly, making less power

Unfortunately today's sportbikes have stock CV carburetors or Electronic Fuel Injection systems which aren't tuneable as is. For Fuel Injected bikes like the current Suzuki GSXR750 the company Dynojet sells a plug-in adjustable tuning module called Power Commander to modify the bike's fuel injection curve. Ducati 916/996 owners can purchase optional injection computer peformace chips from their Ducati dealer, while more experienced Ducati dealers like Fast by Ferraci can actually reprogram (burn) a chip to match your particular bike modifications based on their testing and race track experience.

Dynojet's Power Commander is available for all popular model fuel injected sportbikes. It plugs into the bike's stock wiring harness and lets you tune the stock bike's fuel injection computer to adjust for exhaust system and engine modifications. The Power Commander is a must have for any performance upgrades. Your local motorcycle dealer can order it for you from any of the major parts distributors. And Its avaialble from our FastDates.com Calendar distributors including White Brothers and Lockhart-Phillips.

If you have a CV carbureted bike you'll need to purchase a rejetting kit for your particular model bike from Dyno Jet or Factory Tuning, company's that have done all the work for you in testing and selecting or manufacturing the right range of adjustable jetting components to make your bike perform to its best for whatever modifications you have done to it. While both companys make great kits, we are partial to Factory Tuning because they back up racetrack and dyno testing with feedback from sportbike riders in the real world to amass a great data base of what works best for any specific application. And they are easily accessable by phone to help you with any particular tunign questions or problems. A Factory Tuning kit normally runs right the first time you install it at the recommended settings without having to do additional rejetting to fine tune it.

For motorcycles running non CV smoothbore carburetors like the flatslide Mikuni RS Series and Keihin FCRs, Sudco International carries and complete range of tuning components for these racing carburetors and can offer advice on tuning. Flatslide racing carburetors are fairly expense, but they do offer a significant performance advantage in any high performance racing application. But for streetbike and SuperStrock racing applications the stock CVs are the way to go. Sudco also sells the Factory Tuning brand Jet Kits for CVs.

Basic Carburetor Tuning    by Marc Salvisburg, Factory Tuning
Should you have to rejet any type of motorcycle carburetor, here are the steps to take, in order:

1. Full Throttle / Peak RPM 8,000 and higher:
Select the correct Main Jet To be sure the engine gets enough fuel throughout the RPM range it is necessary to select the correct main jet first. The Main Jet is located inside the carburetor's float bowl, screwed into the base of the long brass Needle Jet. To get the best, most even top end power (after 7500 rpm and 3/4+ throttle, select the Main Jet that produces the highest top speed. If the bike doesn't pull well at high rpm when cold and gets only slightly better when fully warmed up, the main jet is too large. If the bike doesn't pull well. If the bike pulls harder at high rpm when cold and less hard when full warmed up, the Main Jet is too large.

Once the bike is fully warmed up you can also move the cold start Fuel Enrichner Lever to see if more fuel makes the bike run better or worse, indicating which way to go on the Main Jet size (bigger Main Jet/more fuel, smaller/less fuel). The Main Jet must first be properly selected after 10-15 minutes of hard use. Checking the spark plug tip/exhaust pipe tip for the proper light brown Tan Volor is another good indication of the correct Main Jet. If the color is too light the Main Jet is lean, too dark or Black and the Main Jet is too rich.

2. Full Throttle / Midrange RPM 5,000-8,000: Set the Jet Needle height Set the Needle height using the removeable Needle Circlip and Spacers. Set the Needle height so that the engine will accept full throttle in 2nd gear from 3,500 - 4,000 rpm. The Needle notches for the Circlip are always counted from the top beginning with groove #1. Note: When installing the Needle in the Throttle Slide under it's plastic retiner/spring guide, use the kit supplied Needle Spacers on top of the Circlip, as many as needed, to keep the needle fitted snug so it can't bounce up and down. A too loose Needle might jet well on a dyno with a constant throttle, but with a moving throttle on the road or track the bike will be impossible to jet as the Needle keeps moving up on down.

3 Full Throttle / RPM 4,000:
Set the Jet Needle height again To get the best driveability, set the Needle clip height so that the engine will accept full throttle in 2nd gear from 3,500 to 4,000 rpm. If the engine has a "wet" rythmatic burbling, soggy spot at 4,000 rom which gets worse as the engine heats up, lower the Needle. This will start the "point of origination of taper" later, on the needle, and lean out that area. If the engine is left with the Jet Needle set too high, the engine may foul plugs on the street and will be "soft" coming off low speed corners at the race track.

If the engine has a "dry" flat spot a approximately 3,500 -4,000 rpm, raise the Needle (lower the Circlip position, i.e. go from #3 to #4 in the grooves), to start the taper on the needle earlier and to richen the midrange.

If the engine has a "wet" rythmatic burbling, soggy spot a 4,000 rpm which gets worse as the engine warms up, the Needle is too rich. Lower the Needle (i.e. go from #4 to #3 in the grooves) to start the taper on the needle later and to lean the midrange.

4. Full Throttle / RPM 2,000: Check Float height (fuel level):
Th e fuel level height in the Float Bowl affects full throttle/low rpm and, also, richness or leaness at cruise/low rpm. For reference, a bike that runs cleanly at small throttle openings when cold, but starts to show signs of richness as it heats up to full operating temperature, will usually be leaned out enough to be correct if the fuel level is lowered just 1 mm. Float bowl fuel level is very important. Start by checking the manufacturer's recommend level setting in the Service manula, and work from there if there seems to be a problem.

5. Synchronize Carbs / For the smoothest idle
and 2nd gear/4,000 RPM steady state cruise.
To start, set the Pilot Fuel Mixture Screws (also called Air Screws) usually located outside on the carburetor body, to the recommended settings.

6. Part Throttle Cruise /For smoothest Idle,
4,000 RPM Steady State Cruise, and 1/8th Throttle high RPM operation:
Pilot Fuel Mixture Screw, Float Level and Pilot Jet size (Pilot Jet is located in the Float Bowl roof) are all contributing to mixture delivery during 4,000 RPM steady state cruise operation. If lean surging is encountered, richen the Fuel Mixture Screws by turning out in 1/2 turn increments. Alternative Pilot jets are supplied in the jetting kits if the kit manufacturer has determined there is not enough adjustability avilable with the Screw, alone. Pilot Fuel Mixture Screw settings, float level and Pilot Jet size all affect high RPM 0-1/8th Throttle settings.

Too lean, will cause surging problems when the engine is operated at high rpm at small Throttle openings. Opening the Mixture Screws and/or increasing the Pilot Jet size will usually cure the problem. A lean problem decreases as the engine warms up.

If the throttle is lightly "blipped" at idle, and the RPM "hangs up" before dropping back down to the set idle speed (and there are no intake leaks and the idle speed is set below 1,000 RPM), the Pilot Air Mixture Screws are probably set too lean. Try1/2 turns out to richen mixture. Be sure there are no air leaks and the idle is set below 1,000. After carburetor tuning you can raise the idle speed back up to a safer 1,200 RPM for engine oiling protection.

Too rich, if the Throttle is "blipped' at idle and the idle drops below the idle set speed, then rises back up to the idle set speed. The Pilot Mixture Fuel Screws are probably set too rich: try 1/2 turn increments in to lean the mixture. A rich problems gets worse as the engine heats up.

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Carburetor tuning is a combination of science, art, intuition, perserverance and wizzardry!