Two-stroke KTM off-roader
Are two-strokes too dirty to survive? Not KTM’S clean, clever, fuel-injected engine
JUST WHEN YOU thought the two-stroke engine was dead, those clever chaps at KTM have breathed new life into it. Their secret? Using fuel injection. By doing so they claim to have produced a motor with greater flexibility, improved economy and, biggest achievement of all, one that’s clean enough to meet the latest Euro4 emissions regs.
Fuel injection represents one of the most significant advances in two-stroke engine design. Though the Austrian manufacturer isn’t the first to have had a stab at the task, it’s the first to put it into mass production on a motorcycle. So far only two of its bikes are equipped with the Transfer Port Injection (TPI) arrangement, the 250 and 300 EXC road-legal enduro bikes.
I approached my first trail ride on the 250 version with massive curiosity, if not a fair bit of scepticism. It’s a very tall order to make a fuel-injected two-stroke work effectively. Given the constantly changing and challenging nature of the off-road terrain it has to tackle, my expectations of it fitting the bill perfectly weren’t high.
The day started well, even before the motor was fired up. KTM has made two-stroke life a whole load more convenient by fitting a pump to feed the engine with two-stroke oil, sparing the rider the tiresome chore of accurately pre-mixing the lube with fuel before filling the tank. Starting the little single cylinder stroker brought more happiness. Coming to life readily and without any need to touch the cold-start knob, the motor burbled away quite happily without needing regular blips of the throttle to keep running. Some trademark blue smoke appeared from the exhaust when it was stone cold, but sensors around the engine keep a very lean fuel:oil ratio of just 100:1 at tickover, to keep pollution to a minimum. For a stroker, that’s pretty civilised stuff.
More surprises came along from the moment I got underway. Like all twostroke enduro machines, the KTM felt very light and manoeuvrable, its suspension giving the sort of control and compliance many road riders can only dream of. But it was the engine that raised my eyebrows most. With the fuel and oil being metered so precisely, and with the unleaded injected into the transfer ports much closer to the cylinder where it’s then compressed and burned, the combustion is significantly
“Starting the little single cylinder stroker brought more happiness”
more efficient and effective. In real terms this gives several advantages, a main one being the motor has stronger and reliably cleaner bottom-end drive. I discovered screaming the 250 wasn’t necessary for healthy progress, and my left foot and hand weren’t anywhere near as busy as I’d expected. Top gear will pull cleanly and with purpose from just 20mph.
Of course, trail ride often and there will be times where your pace will be lower. I found the challenge of dealing with the terrain was made a whole lot easier by the engine’s flexibility and its perfect fuelling. It can’t quite match a four-stoke in this respect but it does have the advantage of producing slightly softer initial drive further down the rev range when grip levels are very low. I found the reduced engine braking of the stroker was another big ally when the surface was slippery.
Life became a lot more exciting than it is on a ‘thumper’ when conditions let me spin the stroker’s crank harder. The 250’s maximum of ‘only’ 51bhp might not sound much, but it was enough to get the bike up to 70mph on the roads between trails and was easily sufficient to thrill and make very speedy progress over rougher terrain – though some care was needed to manage this meaty power level on the dirt. Though the 250’s delivery was remarkably linear, the rate at which the thrust increased at higher revs did require respect at times. I found the best policy was to short-shift up through the gears and use them to calm the engine, with just a quick tug back on the bars and a spike of clutch slip to briefly loft the front wheel over stuff like tree roots or serious potholes. If you do think the engine’s a little too lively, there is an electrical plug under the tank that can be quickly removed to alter the ignition and fuelling mapping, reducing the power.
The TPI bike is frugal with its fuel consumption. Over around 350 miles of trail riding the 250 returned a very useful and near four-stoke-equalling average of 50mpg. Giving the bike a total range of 100 miles from its nine-litre tank makes the stroker far more practical to ride in areas where fuel is much harder to find – especially as there’s no need to carry around any engine oil to mix with the refill.
Another big plus of this TPI engine is that there is no need to adjust the fuelling.
“I’d seriously consider owning one of the TPI bikes” “Top gear will pull cleanly from 20mph”
Carb’d bikes have to be re-jetted regularly to give their crispest performance and cope with more major changes in climate and altitude. The KTM’S sensors and ECU manage such affairs automatically. The fact that it doesn’t flood and become extremely difficult to restart after a fall is something I can attest to as well!
Before trying the 250, I’d liked twostrokes but considered them a chore because of the premixing fuel, poor range, and occasional less-than-perfect running. With all those issues now a thing of the past, I’d seriously consider owning one of the TPI bikes. The only drawback for me is the relatively noisy exhaust note. It has a slightly anti-social ring to it, especially at higher revs, giving the impression you’re a teenage hooligan. Even so, that can’t dull my interest in ownership.
I’d probably have to wait for the privilege, though. The high demand for the strokers among off-road fans has created a waiting list. It’s not a cheap bike at around £7430 on the road, which is £350 more than the now-discontinued carb’d version. But given how accomplished it is, that could actually be considered decent value. KTM’S 14 years of R&D has clearly paid off and is well worth paying for.