they are still legal, except maybe in california. i know they banned like 2 stroke mowers and stuff. honda is discontinuing its run of cr based motocross bikes at the end of this model year. you cant buy a 2 stroke outboard anymore either i dont think. there are general rules for small displacement stuff like weedwhakers and chainsaws and stuff like that. they are still street legal for now. but i have a feeling thats on its way out in the next 10 or 15 years. itll be interesting to see how it all affects 125 and 250 gp.
Prior to 1976 two strokes were unabashedly allowed in the us, and it led to such legendary street bikes as the 750cc Kawasaki H2, 500cc Kawasaki H1, Rd350 and RD400, T500, GT750, GT550, etc...well you get the idea. Starting in 1976 the EPA began focusing on 2 stroke emissions for new vehicles and size limitations began to come into play. For a time the EPA choose to focus on the larger two strokes so utility equipment was unaffected. Also 2 strokes began to waine in popularity as 4 stroke street bikes began to get faster and more easy to ride.
By the 1980's in the US there was very little market for street going two stroke motorcycles. The companies that produced 2 stroke race replicas didn't import very many into the US (if at all). Smaller two strokes (250cc and under) stil flew under the radar, but the EPA was insistent that two strokes as a whole be cleaner, and pressured manufacturers of 2 stroke engines for cleaner designs.
Today 2 stroke engines are still legal in the US but they are not as common. For road going vehicles 150cc is the maximum and a catalytic converter is usually fitted to those bikes that still use them. until 2006 vespa still sold a 150cc two stroke, and Stella continues to sell them. The chinese scooters are a mix of two and four strokes, mostly 2 strokes for the smaller engines and four for the bigger.
With lawn equipment, the decision to build four stroke engines was a cost issue more than enviornmental, although the EPA does have restrictions on those smaller 2 strokes as well. With four stroke utility engines, the costs of repair, espically warranty repair are down. Traditionally, utility engines were two stroke so that they would be easier to fix in the field and not require as many specalized tools. But as technology improved the engines got more reliable to the point where a four stroke will often outlive its service life without any significant problem as long as it is well cared for. It is common to see a four stroke weedwacker and lawnmower these days, but if you really want two strokes can still be had.
I own a two year old two stroke snow blower. I have a closet two stroke fetish and where reliable I prefer my vehicles to have blue smoke trails behind them, although my 2 smoke snowblower rarely does due to its design.
Anyway, there is plenty of material on the EPA and regulating vehcile emissions on the web and in the library - this was just a summary off the top of my head from the research I did a while back on the economics of the hybrid car (specifically tax incentives). It was very boring.
I just had my T20 signed off at CHP last Thursday (been out of CA DMV system forever). The guy knew it was a two stroke and didn't care. All the auto parts store still sell two stroke oil, so I doubt they're illegal, even in Kalifornia.
Now new two strokes would probably be a different story. And you can't bring in the Royal Enfields here unless the OD is over 7500 miles.
Wonder what's going to happen with the direct injected 2 strokes make it to the US. Aprilia has a direct injected 50cc scoot that supposedly pollutes less than a four stroke. We are probably only 5 years away from the capability to produce full size motorcycles with direct injected two strokes.
Direct injected two strokes have a crankcase oil sump etc, just like a four stroke...fuel is injected directly into the combustion chamber in only the exact amount needed....so, no raw fuel dumping out the exhaust. Supposedly as pollution free as almost any four stroke...and quite efficient when weight is taken into consideration.
I THINK the CA laws are based on HC emissions which kills them because of the burnt oil. Assuming they don't burn oil then they SHOULD be OK - but I wouldn't put it past the politicos in Sactown and Washington to blanket ban "two stroke engines". That would be retarded.