Cycle News is a weekly magazine that covers all aspects of motorcycling including Supercross, Motocross and MotoGP as well as new motorcycles
Issue link: https://magazine.cyclenews.com/i/126424
No-compromise production motocrosser capable of winning Grand Prix By Jim Gianatsis The all-new. KTM 420 motocrosser is one of those no-compromise production racers that only the Europeans know how to build. It's a bike that, in stock form, is capable of winning in World Championship motocross. KTM has been doing it for years in the 250cc Grand Prix class with the likes of twice World 24 Champion Guennady Moiseev on either box-stock or warmed-over production machines. And now they're winning races in the 500cc class as well. On June 24, Frenchman Jean· Jacques Bruno became the youngest man in history to win a 500cc Grand Prix when he took hi~ stock KTM 420 ta the overall win of the West German round with 1-5 mota finishes. Bruno has been racing stock bikes. 1 inspected the bike he raced at the USGP at Carlsbad and again later when it was returned to KTM of America after the Canadian Grand Prix. As a matter of fact. the engine: in my test bike had been carried by Bruno as a spare to both races and was actually used in the second moto at Canada. I was on hand as KTM service manager Rod Bush tore down the hardly broken-in engine for inspection ... looked to see if the cylinder porting or anything else had been reworked (it hadn't) ... and bolted it back together again before re-installing in the new test bike. The history of the 420cc engine from iu initial prototype form dates back nearly two years to the 1977 Trans-AMA Series wlien then factory KTM rider Andre Malherbe rode what appeared ·to be a conventional 400 MC-5 but with the production engine cases reworked to house a longer stroke crankshaft and larger bore cylinder. Then last year in the 500cc World Championship, Malherbe got an allnew prototype bike, predecessor to the MC-SO, with a completely redesigned engine unit using the 420's (actual 419cc) 85mm x 74mm bore and stroke tested in the Trans-AMA Series. Remaining in the KTM lineup will be the old 400 engine (actual 357cc) with iu primary purpose of an enduro motor with its more manageable powerband, six-speed gearbox and external rotor.Motoplat CDI. The new 420 engine is downsized slightly, has a five-speed gearbox, the countershaft sprocket being positioned a little closer to the sw.ingarm pivot for more constant chain tension, and the ignition is provided by a lighter and quicker revving internal rotor Motoplat CDI. Perhaps the most noteworthy change on the new engine is the primary kick starting, since no one is perfect when it comes to stalling an engine during a race. Despite the increase in displacement of the 420 engine with its heavier flywheels, piston, etc., the weight of the powerplant is no more than the older 400 engine. KTM remains with conventional piston/port induction on the 420 engine with two transfer poTU on each side of the cylinder fed from the crankcase and one additional transfer port at the rear of the cylinder fed through a window in the back of the piston. Just conventional five-porting which isn't very radical, particularly at the cylinder base where the transfers are very narrow and there is a lot of aluminum available to be ported out if anyone was foolish enough to want more power, heaven forbid, since the 420 already has more power than me~ mortals will feel safe with. Carburetion is provided by a monstrous 40mm Bing which proved to be easy starting and perfectly jetted throughout the rpm range. There was a problem, though, in that if yol,l lugged the engine below the powerband (would you believe a lowly 2000 rpm ... no. don't ask why I was riding it that slow) in a tight turn or up a hill, the engine might try to bog and