Jetting a bike means changing the size of the apertures the gas flows through.
Doing this changes the ratio of fuel-to-air mixture. Generally, 15 to 1 is a start for gasoline engines. From there, a LOT
of variables creep in. The art of "jetting" means to adjust this ratio so your engine pulls well from idle through full throttle.
Simple way is move the taper up, or down, making the mixture slightly richer, or leaner, using washers under the carb needle.
If the jet (hole) stays the same, moving the taper up or down will either restrict the hole(leaner), or open up the hole(richer)
for a given slider(air) setting. Thus, the ratio changes. Pulling the plugs proves this. A light brown color is best. If plugs
are whitish, or gray, the mixture is lean. If plugs are dark brown, or blackish, rich.
Here are few reasons to rejet your bike even if it is stock. Stock bikes come jetted
lean because of emission standards. By correcting the stock jetting of your bike, the combustion should be more efficient.
The increase in power, although slight, means your bike does not have to work as hard. More importantly, a richer mixture
causes your engine to burn cooler, extending engine life. It should also crank easier because the initial coolness of the
cylinders displaces some of the fuel in the combustion chamber. This is reason choke is needed until the bike warms up. Rejetting
a stock bike requires minimal increase in jet sizes so do not expect noticeable power unless you change your filter and or
exhaust. You will then, of course, require more jetting.
US bikes are bad at warming up. They had 37.5 size pilot jets while the rest of the
world got 40, hence the leanness and awful warm up/start up situation. The remedy is to put 40's in place of the 37.5's. You can order the pilot from a Suzuki dealer near you. You should also install 127.5
large round Mikuni mains, go 2 1/2 turns out on the idle mixture (adjust to taste).
Part Name - JET, pilot
Part Number - 09420-40014