First, I presume you are looking for an increase of performance. Remember this; increasing the fuel an engine uses decreases performance. The more air available for combustion increases the power output. That’s why nitrous-oxide injection is popular. It supplies more oxygen to support combustion. Having the right amount of fuel for the amount of air an engine can “gulp” produces power. Drag racers say it best; “LEAN IS MEAN”! Run the least amount of fuel necessary to get the most power. Just don’t run it so lean that it burns the piston.
Jetting is not as simple as taking one out and putting another in. I always like it when someone gives a “cut and dry” answer as to what jets to use. A stock engine might require jet changes depending on air characteristics. My Honda CR is jetted fine for the midwest, but would have to be re-jetted to run in Colorado or Florida.
There are 3 jetting circuits in the typical carb. Low speed (for idle), mid-range (for steady throttle) and high speed (for wide open). Each circuit needs to be changed as necessary. You could completely remove the main jet, but it would idle fine. Also, the circuits overlap. A change to one affects them all to some degree.
Jetting is dependent on the velocity and volume of the air passing through the venturi of the carb. The velocity of the air passing through the venturi is dependent on the volume of air needed. Changes that affect the air velocity have a greater effect on the jetting than the volume of air passing through the venturi.
Ponder this (I’ll give you the answer later); if you have a carb that has a venturi size of 35mm and you bore the carb to 36mm. Will you have to increase or decrease the size of the jets and which jet should be changed by how much?
Regardless of the changes to the engine, intake, or exhaust, the low speed jet seldom needs to be changed and if it is changed, it will almost never be changed by more than one size. Most needed changes to low speed jetting can be accomplished by adjusting the fuel mixture screw.
So, take the top off the air box, install a less restrictive filter element (reducing resistance to air flow), or change the exhaust (less restriction of back-pressure) and you might have to change the low speed jet up one size or simply back out the fuel mixture screw for a smooth idle.
Now, take the vehicle out for a test drive. On a fully warmed engine with the throttle set at about ½ open, run at a steady speed. If the engine surges it’s probably a little lean. If it blubbers (like the choke is on), it’s probably too rich. This is changed by raising or lowering the needle in the slide. Lower the needle for too rich and raise it for too lean. If the needle is not adjustable, it can be shimmed up for too lean or you can get a different needle if it’s too rich.
Finally you jet for high speed. The main jet is the only control over fuel flow at the top of the RPM range. Take the vehicle out (again fully warmed), put it in 2nd or 3rd gear (dependent on the area you have to ride on) and hold the throttle wide open. When you get near the top of the engine’s rpm, if it surges it too lean and if it blubbers it’s too rich. Jet it up to the point it blubbers, then back the main jet down 2 sizes.
Bogging is a different story and needs to be addressed differently depending on the type and design of the carb. Bogging is caused by a lack of fuel. The engine is getting all air and no fuel. Some carbs have an accelerator pump to shoot gas into the venturi to help get it past the vacuum drop caused by sudden opening of the throttle. The problem is aggravated by installing larger carbs (lower velocity) than the engine is designed for.
As to removing the choke plate; why? The only time the choke plate might be a restriction is at the extreme top end of the RPM range and leaving it in place can even help performance at lower RPMs. In or out, it will have absolutely no effect on performance from idle to ¾ full RPMs, but starting it in cold weather without it could be a bitch.
Answer to question: If you bore out the venturi of a carb, it will cause a decrease in velocity, but the engine has the same volume requirement. Now the decrease in velocity will allow more time for the fuel available to mix with the air resulting in a need for smaller jets. Jetting is more dependent on the carb design than the engine it is feeding.
