The last few years has seen the major Japanese manufacturers all but abandon production of 2 stroke
motocross bikes, in favour of 4 stroke 'thumpers'. Nowdays, the secondhand market is seeing more and more
of these 4 strokes becoming available, and buyers have to make a choice between 2 strokes and 4 strokes. I
know what I prefer, but I will try and be fair to guide those of you that may be on the fence as to which bike to
The two bikes in the pictures above are roughly the same age, have approximately the same capacity and
were made by the same company. I'm sure you'd agree they look fairly similar at first glance too. These bikes
are very different beasts, however.
By far the most noticable difference between the two is the power they produce. Because of the way a 2 stroke
engine works, they produce roughly twice the power of the same capacity 4 stroke engine. This means that the
2 stroke KX250 has actually been replaced by a KX450F 4 stroke, and the KX250F shown above is a
replacement for the KX125 2 stroke.
Hand-in-hand with the amount of power they produce is the way they produce it. 4 strokes tend to deliver the
power in a smooth curve, with plenty of torque accross the rpm range, while 2 strokes have a much narrower
'power band'. On a powerful bike like the KX250, the power band can be a vicious creature, seemingly intent
on throwing the rider off the back of the bike. The majority of riders who have lots of experiance on both bikes
claim that 4 strokes don't take as much work to get around a track on than a 2 stroke. On the other hand, most
recreational riders feel that the 'kick' of the power band is one of the major benefits of a 2 stroke.
Because of the different ways both engines funtion (which I'll deal with in a different article), the bikes sound
very different. Personal preference comes into play here, and I personally adore the ting-ting of 2 stroke
exhaust note compared to the dull thump of a 4 stroke.
When it comes to motocross bikes, the 4 stroke engine is a lot more complex, with a lot more parts, than it's 2
stroke equivalent. This translates into a bulkier package, that weighs more than a 2 stroke. For example, the
2007 KX250 2 stroke weighed 16lb less than the equivalent 2007 KX450F 4 stroke. While the power to
weight ratios are roughly the same for each bike, there's no escaping the fact the the 2 stroke is a lighter bike
and in my experience, the lighter the bike the easier it is to traverse tight and twisty trails.
Perhaps the most important difference between the two bikes, at least for me, is the costs to run and maintain.
The 2 stroke requires oil to be mixed in the fuel, and this oil is not cheap. I pay around $28 per litre for my oil,
which makes up 33 litres of mixed fuel. 4 strokes don't require oil in the fuel, so they have a slight cost
advantage here. 2 strokes can also have a tendency to foul spark plugs, especially if not tuned correctly, so
again the 4 stroke has a small advantage.
When it comes to maintenance though, the advantage definitely swings in favour of the 2 stroke. Every 20-30
hours of riding, I replace the piston rings in my trusty KX, and every second set of rings I replace the piston as
well (known as a top-end rebuild). The full top-end rebuild costs in the region of $200, provided I do the work
myself (of course I do). This is common practice for a 2 stroke MX bike and keeps the motor in tip-top
Similar maintenance on a 4 stroke is a much different story however. Beacause of the increased complexity,
there is a much more rigorous maintenance schedule, incorporating the valves, cam, etc, which need periodic
adjustments to continue functioning properly. When it comes to rebuilding, many of those same complex
components require replacement at frequent intervals. Some cynical people claim that the only reason the
manufacturers have switched to producing 4 strokes is simply to make extra money on maintenace and repair
costs. While I don't have any precise figures as I have never rebuilt a 4 stroke MX bike, I have spoken to
people who estimate that the maintenance and repair costs of a 4 stroke can be as much as 10 times the
costs of a 2 stroke for the same period. For me, that's the biggest deal-breaker for a 4 stroke.
I'll admit I'm biased towards 2 stroke MX bikes, but considering they're lighter, more powerful, more fun, sound
better and cost far less to maintain than a 4 stroke, aren't I justified?
Whatever you decide, have fun out there and stay safe!