2 Stroke and 4 Stroke Jetting

When you "open up" a restrictive air box on a 2 -cycle or 4-cycle you are letting more air into the carborator. In both cases you need to jet richer: provided you really letting more air in. i.e. some air boxs are fine as they come from the factory.

bueller416, you are correct. I'm no engineer, but from my experience in most cases that is right. 4-strokes use the scavenging effect of the exhaust to help pull in the intake charge into the combustion chamber. That's why the intake valve is starting to open before the exhaust valve is closed. If you increase the pulling effect of the exhaust (gases in motion want to stay in motion...creating the 'pull') then a richer charge may be required. It leans out the mixture (or at least has that effect).

I have less of an understanding of the 2 stroke exhaust effect on the engine, but I do know that the temperature of the exhaust changes the speed of the pulse back to the engine from the end cone on the expansion chamber. The scavenging effect of the pipe helps draw the intake charge all the way through the engine and into the exhaust pipe. The pulse coming back then crams the intake charge back in the combustion chamber before the exhaust port is closed for combustion. It's a little more complex (in my mind) than the 4-stroke scenario. There's more involved with the exhaust temperature and pulse speed that comes into play with the jetting than I fully understand at this point. From my experience with jetting the KDX200H, the -35 and -30 FMF pipes as well as the RAD valve had an effect of richening the mixture and required leaning the jetting. Maybe some one who understands this a little better can chime in and clarify some things.

bueller416 said:

I On a 2 stroke when you modify the exhaust and air box, you need to go leaner on the jetting.

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That does not match my observations.

In general, whether 2 stroke or 4 stroke, you need to jet richer when opening things up. More air requires more fuel to keep the A/F ratio in a usable range and to keep you from doing bad things (like melt pistons and burn valves). 2 strokes usually come rich from the factory to keep Joe Schmoe from burning a hole through the piston running wide open for 5 miles down the side of the road in 40 degree weather. So leaner jetting is usually required to get the max performance for the experienced tuner / rider. 4 strokes generally come lean from the factory, usually for emissions reasons. Plus a 4 stroke can run lean much more safely than a 2 stroke. A 4 stroker will start to miss and pop when it gets way too lean. A 2 stroke will seem to "run better than ever" when lean, only to fail with an impressive amount of damage shortly there after.

Some mods let more air in and require leaner jetting. Boring a bike out is a prime example. The engine will have more vacuum on the intake (pulls harder through the carb), and therefore gives more signal or pull at the jets. A jet doesn't give one exact specific flow, it changes with the amount of vacuum the carb is seeing, position of the throttle, etc. Increase this pull and you actually increase the amount of fuel coming through the jet.

But again, generally you'll need to richen jetting when opening things up.