I think I know why 125 two strokes race in the 250 four stroke class and the 250 two strokes must race the 450 four strokes.

[plasmajam]

The following is a "knee-jerk" rant that shouldn't be taken too seriously as I don't know all the particulars:
I am crying "foul" on the AMA rules* that say two strokes can only be raced against four strokes @ half capacity** (ie: 125cc 2-strokes must race 250cc 4-stroke, 250cc 2-strokes must race 450 class 4-strokes). Two strokes are being made the unwanted step-chid of motocross because, I suspect, the manufacturers have a hand in it. A two stroke bike costs a few hundred dollars to rebuild and a four stroke costs thousands. Two strokes are faster, cheaper and lighter than four strokes, but for some odd reason having to do with money, manufacturers want them relegated to oblivion and sell only four strokes that are slower, but more expensive and the factory shops will be called on to repair the majority and that equals rightious bucks!
Get this, their rationale that the manufacturers give is that a two stroke "fires" the piston on every stroke, whereas, due to the valves and cams on a four stroke, the piston is fired "every other" revolution SO THAT is why they say a 125 2-stroke equals a 250 4-stroke..get it? The 125 puts out 250cc of power because it fires every revolution as opposed to a four strokes firing every OTHER revolution. Your thoughts, please.

*I don't know if the AMA made these rules, but they seem to be going along with said rules.

**I don't know what the exact wording is, but I can find out.

[plasmajam]

I was reading an MX online mag (VitalMX?) and they were deflecting any and all attempts to pit a two stroke against a four stroke of the same displacement in their testing and reviews....why?

http://www.vitalmx.com/

[metalspine]

The E.P.A. is why Two strokes have been a dying breed. The mixture of oil in the gas puts off a lot more smoke than plain gasoline in the four stroke. So all the technology has been targeted towards four strokes since like 2006. The E.P.A. has been adding ethanol into gas also which makes the bikes run worse.
I have heard rumors that Honda has a new fuel injected two stroke that puts off less emmissions than even modern four strokes. I've also heard they can add traction control. So they may return someday.
Personally riding a two stroke is a big disadvantage because of traction due to the piston firing every stroke., in turn creating more wheel spin You have to work a lot harder to control the throttle as well as brakes on a two stroke. A four stroke has "engine brake" meaning when you let off the gas it slowws down faster because its only firing every other stroke.

[plasmajam]

Thanks for that info. All good points. All the hoopla of the "Man" trying to keep two strokes down are probably based on what you explained to me (EPA and such)...It all just seems unbelievable, wot with me easily blowing away a TT-500 on my YZ-80..not top end, tho...on an MX track.
I think the Aussies are allowing two strokes to compete again (much to the manufacturer's chagrin), but the two stroke has to be bone stock from the intake to the exhaust manifold.
I still have my reservations that the corporate bean counters are somewhat responsible. This does not, of course discount your excellent points that you explained in your comment. Thanks again!

[OG kott0n]

2 strokes can be clean. See DI technology in watercraft and snowmobiles. Per CC there is no replacement for power every stroke vs not. They are lighter, cheaper and faster per CC.

These are "closed course" bikes which means there is no EPA regulations.