With all the talk of how great four strokes are, I think both, the two strokes and four strokes have their strengths and weaknesses. I think that riding style dictates speed, some people are faster on two strokes and others on four strokes, if you put the two head to head, the rider makes the difference not the bike.
Ok, lets say that you are in the market for a new dirt bike. You know that you want a Yamaha and that it is going to have to be a 125, an 85 is too small and a 250 is too large. What you can't decide though is if you are interested in buying a motorcycle with a two-stroke engine or a four-stroke engine. Here are some helpful comparisons to help you make your decision.
When buying a motorcycle with a two-stroke engine, you will be buying a machine that has a lot of ability, force, and power.
One of the chief advantages of a two-stroke engine is the quick acceleration and rapid power delivery. This engine is generally less expensive to purchase, maintain, and modify. On a two-stroke, you can usually get a large increase in power by simply adding an aftermarket pipe or carb. However, these engines have some negative aspects to them as well. On the down side, you have to mix your gas, which could be categorized as a good thing if you are a gear head and love the smell of premix, but it means it takes more out of your riding time. Moreover, the sound of a two-stroke engine is higher pitched and can cause your neighbors to complain. In addition, currently manufactured two-strokes do emit more pollutants than their modern four-stroke rivals. That is why you see more and more manufacturers switching to four-stroke power.
When buying a motorcycle with a four-stroke engine, you will be buying a machine that has a lot of torque. That is it. The word pretty much explains itself. A four-stroke is so good in the mud it's ridiculous. They plow right through muddy ruts like they are not even there. They make excellent trail bikes considering they can go through much more rugged terrain than most two strokes, they have a lower pitched sound and as stated above do not emit a lot of pollutants. They are generally easy to gas all you have to do is fill them up and off you go. The trade off is the four-stroke falls off the power a lot quicker when you are revving out. It revs a little slower, it's like a very fast tractor compared to a racecar. It also feels a lot heavier off the jumps. Four strokes can be expensive to purchase, maintain, and modify. It is no secret that you have to spend big bucks to extract more performance from a four-stroke, with cams, valves, and carbs increasing the costs dramatically.
Overall, both engines have their ups and their downs. It's up to you, the buyer to decide what is best for you.