Ktm Lc4 612
KTM LC4 612
SOUND OF THUNDER CHAMPION
FIRE-BREATHING, ROOST-SLINGING, ear-splitting four-strokes have got to be the best spectator show in motocross. At least that's the theory behind the Sound of Thunder Series, a five-event West Coast program sponsored by Maxxis Tires and Cellular One.
With one round remaining at presstime, KTM’s Greg Zitterkopf has a virtual lock on the title riding a facto-
ry-backed, 612cc LC4. Zitterkopf has only to finish the final race to take home the number-one plate.
His bike started life as an 1991 LC4 600, but has been thoroughly tweaked by factory mechanic Tom Moen. The frame is basically stock, but was shorn of all extra tabs and mounts. The standard swingarm and shock linkage is retained, but no expense was spared on the suspension, which consists of works WP
units front and rear. The fork is an IBS (Internal Bleed System) model with .44-kg. springs. The shock has 1/2-inch more shaft travel than a stocker. A 6.0-kg. rear spring is used.
Wheels are off a KTM 250 MX model, with turned-down hubs and Excell rims. The rear is a 2.50 x 19, slightly larger than stock to spread out the tire for more grip. In something of a fashion statement, Renthal sprockets are driven by a purple EK chain that matches the team graphics. The seat comes from the company’s 500cc two-stroke. It is custom-fitted to the fuel tank, which had to be heated and slimmed down for a smooth seat/tank transition. Zitterkopf prefers Renthal 242-bend handlebars. The bike’s claimed weight is 245 pounds dry, not superlight, but no heavier than stock Honda CR500 motocrossers.
The heart of the bike is its 612cc Austrianmade motor, which, with the exception of its black paint being beadblasted off, appears to be quite stock. It is far from that. The transmission is only a fourspeed. Because it has no use in motocross, first gear is removed from the standard, fivespeed gearbox. Power is transmitted through a clutch that uses a 250 MX-style inner basket and pressure plates off the factory 500cc twostroke GP bike. The standard rod and stroke are used, but the crank is lightened by 26
ounces and balanced to the piston. The 100mm-bore Nikasil-lined cylinder is milled .028-inch from the base and fitted with an Arias piston made especially for the KTM. Displacement jumps by 60cc.
The head is ported, but retains standard valves. The cam, taken off the company’s European dual-purpose model, has more duration than stock and is cut to obtain more lift. Moen uses several sets of cam sprockets to adjust the cam timing for different tracks. The timing averages 4 to 6 degrees advanced from stock. Carburation is handled by a 42mm Dell’Orto, and exhaust flows through a specially made megaphone with a short-and gloriously ineffectivesilencing core.
As with most big-bore Thumpers, starting procedure was something of a mystery, and I never fully figured it out. Moen tells stories of not being able to start the bike the entire day before a race, and says that the only fool-proof method is to let Zitterkopf, at 6-foot, 1-inch and 190 pounds,
swing away at the awkward left-side kickstarter. I found that holding the throttle halfway open and giving an extremely valiant boot seemed to work best. Once fired up, the motor idled down to an easy tick-over and never hinted at stalling.
Click the tranny into what really is second gear, and take-offs aren’t a problem as the motor sweats torque from every pore. “Greg likes his motor to pull without having to clutch it,” says Moen. This translates into an extremely ridable machine. Point the bike at the loose outside line, whack the throttle, and make like Ricky Graham at the Springfield
Mile. Or take the hardpacked inside line and offcorner acceleration will match any two-stroke.
Because Zitterkopf is a large rider, I expected a stiff ride. Surprise. The 612 was very supple and hooked up without kicking, even over square-edged bumps. Okay, so it’s got to bottom at speed in the rough stuff, right? Wrong, again. Despite being plush and very progressive, the KTM’s suspension bottomed only once a lap, this after landing from the biggest jump on Southern
California’s Sunrise Motorsports Circuit. Such is the advantage of factory suspension components.
As proven by Zitterkopf’s results,
the KTM 612 is a winner. But more impressive is how easy this bike is to ride, how compliant the suspension is. Average off-road Joes would do well to steer clear of the Baja KX and McGrath’s CR, but hard-starting aside, this is a Thumper that anyone could come to terms with. Unfortunately, it’s not for sale. Not yet, at least. U
