Story by Andy Wigan Photography by Andy Wigan & Kurt Teague at Transmoto
Having now lived with Sherco's 300SE-R for six months, I've gotten to know the bike pretty well, and I've managed to make it significantly better via relatively few mods and set-up tweaks. I've added practical protection, improved the chassis' balance and trimmed some weight from the machine. And while it has taken some trial and error - through a combination of changes to the power-valve, gearing and jetting - I've finally found an engine setting that delivers user-friendly power for a wider variety of terrain.
It's still got that prodigious grunt, but it now finds rear-wheel traction much more effectively. And that translates into a bike that's less fatiguing - and much more enjoyable - to trailride or race.
Here's an insight into the refinements I've made in recent months to the 300cc flagship of Sherco's two-stroke range.....
At our recent 300cc two-stroke shootout, the Sherco blew its KTM and Beta counterparts away in drag races. But in tight terrain – where a more controlled and progressive surge of power is called for – the Sherco’s aggressive mid-range hit made it harder to get the rear tyre hooking up. That, in turn, affected line selection and energy expenditure. So the first thing I did to address the issue was look at its gearing. The standard 14/49 sprocket combo is a pretty good all-round option. It gives you a top speed of about 155km/h in sixth and works well for cruisy trailriding. But to keep the engine in the meat of its power in tighter bush, it does leave you shifting from second to third, and back again, far too often. So I shortened the gearing by going to a 13-tooth countershaft sprocket (which is what Sherco dealers now recommend their customers do). This 13/49 combo (which is about 7% shorter than standard) relegates first gear to waste-of-time walking pace, but allows you to carry third gear for a greater majority of slower corners.
In the bush, you end up using mainly third and fourth gears, rather than rapid-fire shifting between second, third and fourth. And fewer shifts alone makes the bike a lot easier to ride smoother and more consistently. In reality, though, the gearing change is more of a band-aid solution. It simply masks the fact that the Sherco’s power comes on abruptly in the mid-range. The genuine improvement came when I altered the bike’s jetting – by fitting a kit developed specifically for the Sherco by JDJetting. This creates crisper power across the entire rev range, with the biggest improvement noticeable at lower revs. And that’s important because it means the transition into the mid-range – where the Sherco hits too hard with the standard jetting – is smoothed out substantially. The revised jetting and gearing combine to generate a broader, more user-friendly power curve. These changes transform the 300SER’s ride because, when you’re less preoccupied with gear-shifts, you’ve got more time to focus on where you’re going and on enjoying the ride.
So, what exactly does the JDJetting kit do to the Sherco’s air/fuel ratio to deliver such pronounced gains?
Well, the benefits are generated predominantly by the JDJetting multi-tapered needle. While production dirt bikes simply use the ‘best-fit’ existing Keihin needle, JDJetting fabricates needles specifically to suit the engine characteristics of each make and model of bike. And there’s no doubt the three-taper design of its 300SE-R needle ensures optimal jetting right through the rev range – an improvement confirmed by both dyno and track testing. Specifically, here are the jetting specs we settled on (for sea-level, using Repsol premix oil at a ratio of 40:1):
● We fitted JDJetting’s Blue needle, which is thinner/richer from zero to one-quarter throttle. This helps really smooth out the bike’s power delivery as the revs transition from the bottom-end into the mid-range.
● The needle’s clip is in the third position from the top.
● With the mainjet, we moved from a 168 back to standard 165.
● The standard 38 pilot jet is retained.
● The air-screw is set at 1.5 turns out.
The JDJetting kit for the Sherco includes a custom-made Red (leaner) and Blue (richer) needle, four mainjets (152, 158, 162 and 165) and two pilot jets (38 and 42). The installation instructions also offer indicative settings for various altitudes and temperatures, along with the suggested sequence of steps to make the jetting either richer or leaner. So for $150, the JDJetting kit not only gives you more precise jetting than standard Keihin needles possibly can; it also makes sure you’ve got your jetting covered for a wide variety of riding conditions and power delivery preferences. Click to see the full article