Carb/EFI tuning tech

This is an archive of any information relating to MC carbs & fuel injection.

Carb/EFI tuning tech

Postby big bad » Sat Aug 17, 2013 3:49 am

Here are some thoughts on tuning, weather its carb of EFI.

On any type of fuel based vehicle it needs a certain amount on fuel and air to run properly. There is a theoretical base number that exists, 12.8 that's the fuel to air mixture across the board for what your trying to achieve. Its called the stoichiometric ratio, this is the ratio of the fuel to air where it dose a complete burn with virtually no unburned air or fuel. For other fuels its different like propane or diesel.

So in a steel 1000 gallon drum you put a teaspoon of gas, and the rest air and light it, you will have plenty of air left inside, and the opposite a 1000 lb drum half full of you favorite octane and the rest air? yeah you'll have a drum still half full of gas (its a thick tank)

So on your bike you have have control of upping the percentage of "available air" to your carb/throttle body by the air cleaner size and the increasing flow of the exhaust to allow a large through put of air. If you change your exhaust the air cleaner will be the largest restriction in the system. Remember your air is pushed into the combustion chamber by atmospheric pressure ( the air above) Regardless if we up the air, to achieve maximum performance we need to up the fuel. This is geared towards that goal.

When a fuel burns no matter if its in a motor or a furnace after the fuel is burnt it will create gasses we call exhaust. The four most common gasses are CO (carbon monoxide), HCs (hydrocarbons), O2 (oxygen), and CO2 (carbon dioxide). A proper dyno tuner will use these gasses to create a AF graph for your motor throughout the rpm range


PART 2 --- The affects of the conditions
A lean condition is a low fuel to air mixture or other words there is unburned air coming out the exhaust. The unburned oxygen is superheated and ready to burn, in the combustion process it is never complete. The minimal amount of raw fuel reaches the hottest point, the exhaust port. Every revolution of the motor delivers super heated air that ignites and you get the bap'b'bapbap on decel, or decel pop. The increased amount of oxygen entering the exhaust would be replaced with the four gasses mentioned above..... A lean motor also has increased exhaust temperatures, the proper fuel actually cools the exhaust. In 2 stroke motors they burn holes in pistons and valves because lean fuel mixtures, ie more heat, melts a motor down. Blued pipes are also a good indication the bike is running lean. Honestly, its not a pipe flaw. People also can have this issue out of the blue with no pipe changes, this will happen by a vaccum leak usually a cracked vacuum line, pair plate not sealing or a exhaust leak.

A rich condition is a high fuel to air mixture, or to much fuel. When the there is to much fuel the vapors will exit the combustion chamber and enter the pipe. There is lack of oxygen so it won't burn without it. The vapor has a to get pushed out under hard throttle and never gets a chance to burn. Until you let off the throttle. The velocity of the air in your pipe decreases substantially and the fuel builds it gets hot enough and mixes with a lil air, (like ehn you start to open the throttle from a rolling idle. And "BOOOOM" I've been shot. A shotgun blast burns all the fuel in one shot. You may also smell raw gas coming from the exhaust.


PART 3 1300 tuning
Carburetion principals are the same, but the designs of the carburetor are different. The 1300 has three ways to control the fuel. If you do pipes and air cleaner id do a AF screw, 1 shim scar, and a 200 main as a base line.
Plane, car RC truck, monster truck a carb is a carb. There are different components in some. This is the same technology you have a float delivery, needle modulating carb



1 The AF screw
The AF screw controls the fuel at idle and off idle. While most run between 2-2.5 turns you can go further, try not to break 3 turns .Start at 2 1/4 turns. The screw cuts off the fuel as its turned in and opens the passage to richen when its screwed out. There are many tools you can get to assist the ease of adjustment. So mileage usually won't be a severely affected from the AF screw.

The scar mod has more of an affect between the off idle through mid/upper range. This is the process of clipping and shimming the needle. This is a link to a copy of the original post by SCAR

3 Main Jet
The main jet controls the "cruising speeds" the peak area of performance from mid to high RPM. There are many jet kits to choose from, factory pro seems to be the preferred.


big bad
Posts: 844
Joined: Sat Jul 05, 2008 10:15 am
Bike: 1800 f vtx 05


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