Trick Honda Xl250

FEW OF THE current motocross enthusiasts and competitors remember the days of the lusty BSA, Matchless, Ariel and Velocette 500cc single-cylinder motocross machines of better than a decade ago. These big Singles dominated the motocross picture in Europe and were highly developed pieces of equipment by the time they were finally superseded by the much lighter and smaller two-strokes.

Many of us at CYCLE WORLD can distinctly remember the throbbing pulses of the big Singles, a sonority which is rarely heard these days. The sound and sight of a four-stroke Single clawing its way up a hill is beautiful.

When we did our preview test of the Honda XL250 in the April issue, few of us suspected that the machine would become the basis for winning the Baja 1000. And, yet another non-factory machine, prepared by Bill Bell at Long Beach Honda, has beaten the 250 twostrokes at local motocross tracks in the Southern California area. No doubt other speed tuners have been experimenting, too, but Bill’s XL250 was one of the first to be successfully modified.

Los Angeles riders Bob Hosford and Jim McDonald have been riding the XL250 with great success and Bob won the hotly contested expert 250 main event at Ascot Park recently, much to the joy of four-stroke lovers. The little Single is surprisingly close to standard, too!

With any motocross machine which is being modified for competition, the reduction of weight is a primary consideration. The early production test machine we tested tipped the scales at 278 lb. with a half-tank of fuel. Of course, this weight figure included all road equipment, a tool kit and a tiny bit of testing dirt.

Bill stripped the machine completely, ground off all unnecessary lugs and fittings on the frame and re-installed the Honda plastic fenders and the gas tank. A special seat, using an aluminum base pan, saved almost 5 lb. and the total weight of the machine is now 240 lb. with a half-tank of fuel. This figure compares very favorably with twostrokes of the same size.

Standard Honda wheel rims were used because they are as light and strong as Akronts but they don’t fill up with mud and debris. The only change to the braking system was to float the rear backing plate.

Suspension modifications were held to a minimum. Various quantities and weights of front fork oil were experimented with, but the internals of the forks remained unaltered. At present, Curnutt rear shock absorbers are being used and seem to control what most riders would feel to be an excess of rear wheel hop, especially when braking for a corner. Steering geometry is standard, as is the wheelbase length.

TRICK HONDA XL250

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A standard Honda engine is a beautiful thing—quiet, reliable and capable of withstanding a lot of abuse. The first of these attributes has been lost on Bill’s XL250, but the latter two are still there. Even though special racing parts are not yet available from the factory, the XL250 has responded well to tuning efforts. The most obvious modifications were performed on the intake, exhaust and compression areas.

Judicious porting enabled the installation of a 32mm Mikuni carburetor, which gives extremely smooth throttle response except at the very lowest point in the power range. A Weber 40B camshaft is providing good results and the only weak point in the entire cylinder head is the valve springs, as no racing ones are yet available. This makes it necessary to pack (shim) the valve springs to 0.015 in. before coil bind at maximum valve lift to prevent floating the valves. Maximum power is developed at 8500 rpm.

Because motocross races require a muffled exhaust system, several types of mufflers are being tried on the dyno and have shown to give very little, if any, power loss at the rear wheel.

Raising the compression ratio has also proven to be a problem, but was solved by heli-arc welding up the top of the piston and machining to obtain proper valve clearance. Weight was then removed from the inside of the piston crown to get the piston’s weight back down to the weight of a standard item.

The remainder of the engine was left exactly as it came from the factory. The gearbox and clutch assemblies have proven to be very strong and so far, after about 20 races, only two weak points have shown up in addition to the valve springs. These are the piston skirt area, which is very short and shows slightly abnormal wear, and the magnesium alloy outer engine covers, which must be protected from flying stones to avoid cracking.

Long Beach Honda has recently installed a motorcycle chassis dynamometer, manufactured by Patraco, to aid in diagnosing and repairing customer’s machines. Charlie Schott, enthusiastic owner of Long Beach Honda, doesn’t seem to mind one bit that the dyno is being used primarily to test the company’s racing project, Bill Bell’s XL250. In fact, he’s even planning to market special racing goodies for the XL250, which is destined to become one of the most popular off-road motorcycles ever. |0