ebert -
not bad, but let me add a few things...
kx_guy86 -
99.44% of 2 strokes are "pipey" to one extent or another. normally, the smaller the engine, the pipeyer it is. i wish i could show you a graph, but i'll just try 'splainin it with words.
say a typical MX125 has...i dunno 35 HP and say that same engine has an rpm range of 0-13,000. now, obviously that engine isn't making 35 ponies at an idle of 500 RPMs. its making like...5. and at 13,000 its making like...30. a typical MX tuned 125 2 stroke will make like ...5 at idle, maybe 6 at 1000 rpms, maybe 7 at 2000 rpms, 8 at 3000 rpms and so forth up to say 8-9000 rpms when it jumps from the 10-12 HP to all 35 HP till it gets to, say...12,000 rpms and starts to fall off.
the point when all the power comes on, all at once like, is what we call the 2 stroke "hit". the power comes on, literally, like a switch. its a bit hard to explain, but 30 seconds in the saddle will make a believer out of you. (all these numbers are just my own guesses...they may be off, but the principal holds true)
on the track or in open spaces this "hit" is no problem. in fact, its fun. for trails however, when one must negotiate roots, logs, rocks, or whatever, this "hit" is a severe liability, because the bike is very easy to kill if you are not in the powerband. and on a tricky section of trail, the only thing worse than killing it is goosing it and having the bike fly away on you (or with you).
wereas the power delivery of a 4 stroke is linear and predictable, and much, much easier to control.
like i said before, for a beggining trail rider, i can't imagine a worse bike than a MX 125. if you want 125 type power and MX suspension, yamaha makes the yzf250, a 4 stroke MX bike made to compete against the 125's, but with butter smooth 4 stroke power delivery. i believe KTM makes some high perfomance 250 4 strokes too, and honda will shortly debut its CRF250. even these bikes aren't ideal for beginners, but they are much much better than their 2 stroke counter parts.
jeremiah