SmartCarb Fuel Systems has recently had the good fortune of evaluating a new 2016 KTM Freeride 250R while installing a 28mm SmartCarb for one of our customers. Strangely, purchasing or even getting our hands on a new Freeride has been a particular challenge in the Midwest even though the area is rich in both Enduro and Trials riding areas. So we enlisted great people like Kim Nakashima, Clinton Lemon III, as well as others in different regions to help us develop a SmartCarb calibration baseline for the Freeride. Understanding the nuances of this unique little bike has been of interest to us for some time and in particular understanding the infamous "stutter" that has been widely reported.

The crossover nature of the bike seems to have created an almost cult-like following already and I suspect riders from both Enduro and Trials disciplines worldwide will begin to embrace this bike much more vigorously. Much speculation and forum chatter has been dedicated to the subject so we felt we would add our thoughts here to the already exhaustive amount of discussion. Delving into this subject recognizing that the very notion of "crossover" typically instills thoughts of dread to any purist, no matter what the product is; I tend to hold these same sentiments. And for good reason as there are many, many examples over the years from GasGas's Pampera attempt at this same niche the Freeride is trying to fill, to amphibious boats, flying cars and just about everything in between that makes it easy to feel negativity toward some crossover products that come along trying to answer a question nobody is really asking. But as Freeride sales seem to indicate, the promise of a product that does several things really well is always held in high regard and presents intrinsic value to people holding those interests.

The particular bike that we evaluated had 20.6 miles on the clock and would not hardly run with the stock carburetor and jetting. We knew the dealership was struggling getting it to run right and when they found out that we were coming to get it they left it where it was set. The bike was very hard to start with the stock carb and needed the choke open and a lot of cranking before it finally fired. Once we got it running we could tell immediately that the carb had an air-leak somewhere, because as soon as we even touched the idle mixture screw the thing lit off like you had throttled it and it would not idle down until you put it in gear and let the clutch out slightly.

The dealership had just completely cleaned and inspected the carburetor and informed us it had been set to factory specs so I didn't feel the need to go back through it. I can say though that it acted just like what we know to be indicative of an internal air leak around the slide, making the idle settings impossible to set correctly and this can affect the whole range of throttling. This was somewhat verified with a simple scope inspection and several small gaps were noted between the slide and the back wall of the carburetor within the slide cavity. Also the very poor quality of the Chinese castings in the PWK 28mm speaks volumes to what may very well be one of the main issues related to the stutter. I don't believe it is the whole issue however but is certainly a part of the problem.

We hold that the very nature of a crossover bike and the inherent compromises involved, in this case specifically the Freeride 250 engine, are responsible for the running quirkiness and prevalent stutter being reported. KTM has made it pretty clear they will not discontinue this engine anytime soon and all literature released from the factory seems to indicate that they have no intention of ever installing a power valve system on this engine. Their claims are improved engine durability as the biggest reason and the ability to run very lean oil mixtures as the next, thus adding value to what is actually a pretty expensive bike.

I'm not saying it can't happen, but there simply is no basis to believe that KTM will install a hybrid SX engine with power valve, or some variant of an Enduro engine, like those continually being mentioned in the forum discussions. People who want that are doing it anyway. Here's why; KTM offers more than enough models to cover any type of riding one could possibly want with a full size bike: everything from the discontinued 110 – on to the current 125s, 150s, 200s, 250s and on up to the 300/350f, EXC 500 Enduro and 450f MX bikes. They have zero motivation to make a fast revving, high output two stroke engine to put in the Freeride and immediately overshadow the steep rake and oversteer handling characteristics of what is technically a slow-speed, tight terrain platform and negate the whole notion of the crossover experience they are striving to create. I would not want to ride this bike with a motocross 250 engine in it, that's for sure. A 150 or 200 Enduro engine, well then things might be a little more fun but still not necessarily what this bike needs, or wants.

The Freeride 250 engine is clearly a trials engine trying to cover a little more ground RPM-wise by the use of a small expansion chamber and a little less "sudden" porting. It is not an Enduro engine trying to be a stumpy, explosive trials engine. The most commonly perceived problem with the Freeride engine is that the low exhaust port of a trials engine poses a problem for a full-on expansion chamber. While I would not call the expansion chamber on the Freeride a full-on expansion chamber, it is still an expansion chamber nonetheless. I agree with this by the way, and just like the banter on the forums this is the first and most obvious place to look for an explanation to the stutter on the Freeride.

A two stroke engine must be considered for what it is: a fully resonant system whether it is utilizing the benefits of an expansion chamber and intake tuning or not. What is not widely understood by the masses is that the primary benefit of a power valve system is what it does in terms of the ability to cleanly carburet the engine. Back before power valves were widely used it was very difficult to precisely tune the carburetor to follow along with sudden changes in scavenging efficiencies provided by a piston port engine with a well-tuned expansion chamber, especially when the compression was high and exhaust porting was a little wild. Today the primary benefit as mentioned above is still the same, the end result though is the more widely recognized benefits of broadening the torque curve and vastly improving charge trapping ability, which makes for substantial fuel economy gains and lower emissions.

I believe the stutter is simply related to the transitory nature of sonic supercharging using an expansion chamber exhaust system, and that the 28mm PWK cannot adequately "read" what this hybrid engine is asking of it below and above the pipe hit, forcing it to go through wild swings of air/fuel ratio depending on where the engine is running at in RPM's. All this is due to a mismatch between the low exhaust porting of a trials engine and the semi-expansion chamber exhaust system.