2 stoke?

It depends on your personal size. If you a normal size guy like 180-190# a stroked Sudam will work great. If you are like 120-140# a 115cc is very competitive. If you are bigger you might want to look at a 250cc MX conversion like Tim Taft's cases. The 250cc minus the tranny are only like 5# heavier than a Sudam. That is where the 2 strokes are going to shine. If you are in the 120-170# you could go with dual Yamaha's that you would only have to weigh 370#. This makes really nice useable power that really come off the corner hard. You could also go with a pair of 100cc reed engines like TT75 or Reedjets and have to weigh 430# and have all the power you can handle. Just remember double the engines triple the trouble. Many of the 450's are in the 60-70# or more, someone who has one should chime in on that. The 450's make great torque and that is their advantage that off sets the added weight.jmo
Right now im running a nicely built 390 but im a heavy guy at 280# but im trying to figure out what my next step is or what my biggest competition will be next year when we start racing bigger tracks
It depends some on what your used to running. The 250 in my opinion will be the dominate engine in the near future. It run so much like a sudam yet has that big hp that really yanks you off the corner. The 450 is also going to be very hard to beat. It has a completely different feel based off its tremendous torque. Cost per hp the 250 can't be beat. Were very close to having the entire 250 package worked out. Guys that are used to how a sudam performs will love the 250. Guys used to the thumpers will for sure love the 450. Costs to the racer are coming down Thats good for every one and the sport.
AREA 51 UAS WA....2000 Phantom Gauntlet Eddie Sanders cr250r. This is a Micro sprint motor. With Extensive work in the head, custom 8 petal reed block, and a disgustingly large ps11 pipe it really pulls of the corner. Methanol fuel with the dome head and a mnster Big Belle PWK carb fitted with a power jet. We use the 520 chain and rear sproket and just pick the ratio thats fits the track and conditions. Our sproketinventory went from 20 or better to 3. Yhis kart has run on all types of tracks from 1/5 mile to 1/10 mile and at Skagit...think its 1/3 is wat I did the math at. This beast rolls on Buriss rubber and The Kid does the rest. You may notice we do not run a radiator because the Methanol does not allow for the higher engine temps you would see using gasoline. The total lenth of hose used to circulate through the motor is maybe 24" top....just put a temp guage in line and its goood to go......monitor the plug and we havent ever seen water temps over 160*.....Very pleased so far guess we will see how it all plays out.

Salem 11/2/13

Depends on how you want one, some run the tranny and QRC mounts, just drop it in a gear and go. I make conversion cases and kit's to convert them into kart engines, My cases for the 450's are coming, Justin Westerfield builds 450's by removing the gears but uses stock cases to make a jackshaft out of the tranny box. You can check out my process and conversions at "Taft Racing" on facebook.
I started running a cr250 and absolutely love it, so much power and even on a small track, it will pull off the corner just as well as the 4 strokes. Mine is a stock 03' unit, running 520 chain setup w radiator. The kart scales out well and overall weight is right inline with UAS, so not interested in removing anything to lessen it. If. Can manage to hang on to it for a full night of racing I'm sure it'll be up front, my last race out , I have some of the fastest lap times in the field. From what I can tell, it will be much less maintenance than most traditional open motors, just pricy to get it all bolted up for the 1st time, but if you can fabricate parts, you won't be disappointed in the end result.