Can-Am 250 Mx-4

CAN-AM 250 MX-4

CYCLE WORLD TEST

The 250 That Acts Like A 370

Tired of lining up at the starting gate next to all the yellow' bikes that look just like yours? The 1978 Can-Am 250 MX-4 may be just the change of pace you need. More than the paint has been changed, The front end sports 38mm Marzocchi forks, the rear gas Girlings. Petty fenders

and redesigned brakes take care of rider

protection and stopping. Frame geometry

and a quiet pipe are also new.

The orange MX-4 also has a new bore

and stroke configuration. The stroke has

been lengthened from 57.5 to 61mm. The

bore has been changed to 72mm, still over-

square, that is, bore larger than stroke, but

change has successfully broadened the power output. The power now comes on

down low in the rpm range, much like a

Suzuki RMC, but lower and much harder,

Gobs of horsepower are produced almost

off idle and the surge lasts most of the way

through the scale producing a claimed

horsepower of 37 @ 8000 rpm at the rear

wheel. None of the test riders thought

there was any reason to doubt the claim. In

fact, all thought the bike would be corn-

petitive in the open class. The power and

torque are that impressive. Imagine the

strongest 360 or 370 you have ever experi-

enced and add an imaginary 5 bhp to it.

That’s what the MX-4’s motor feels like.

Can-Ams have always been known as fast. Fast on the track and sometimes fast into the pits with an expired ignition. The Bosch ignition received most of the blame (wrongfully) but the real cause of the problem proved to be loose main bearings that let the rotor hit the other components and cause the ignition failure. With a change in bearings this problem has been cured. (Ours caused us no problem during the test period.)

The gross power the motor makes becomes even harder to believe when one takes a look at the carburetor. A small 32mm Bing sits on the end of the rotary valve intake tract.

Unlike most rotary valve powerplants the Can-Am motor’s carburetor is located > behind the cylinder and a cast duct routes the gas through the upper rear of the side case to the valve. This makes the carburetor easily accessible and ends the two most common complaints about rotary valve dirt bike engines: width and dirt leakage around the cables and other access holes needed when the carb mounts on the side.

The kickstart lever is on the left side. It folds out of the way and clears the peg when in use. When starting, the 14:1 compression ratio makes itself known and a healthy kick is required to move the lever. Much like starting an open bike.

The transmission shifts smoothly but the short lever feels awkward and quick shifts are hampered by it. The rubber pad on the lever hits the instep on the rider when

trying to change gears and almost everyone had problems knocking it out of gear on rough ground. It happened several times when entering jumps. Makes the person doing the jumping rather uncomfortable to know he is in the air and the transmission is out of gear. One inch or so added to the lever’s length would cure the problem. It would allow fast shifts with the toe and give the rider’s foot room to move around without bumping the lever out of gear.

The oil injector has been dro4pped this year and mixing is now required. Best for a true racing machine. Fears of cable breakage, the oil reservoir running dry, etc. are eliminated. If you have gas, you have oil— unless you forgot to mix it!

The MX-4 frame is a considerable revision of the MX-3 and looks strong enough to hold a Harley V-Twin. The huge backbone, formerly the oil tank as well, has been shortened and angled down at the back. This lowers the center of gravity and may make up for some of the extra height that comes with longer wheel travel, although at 37 in. the seat is not low. There’s a pair of front downtubes running below the engine and up to the swing arm pivot. Between the pivot and the backbone/seat/ shock mounts are three pairs of tubes. Looks more like a tuned exhaust or maybe a heavy-duty umbrella. Doesn’t matter. What happened is, the Can-Am designer team made the frame lower and stronger to handle the engine. That required added tubes, so more tubes were added.

The normal motocross seat height seems a bit higher than normal, because of the firm seat and firm suspension. The perched-on-top feeling is soon adapted to, if you’re tall, and has been forgotten by the end of the first day’s riding.

The swing arm is long (more than 21 in.) and bent to allow the use of longer shocks. A flat gusset is welded to the top (inside the bend) to brace it and the front of the arm has metal plates welded to it to increase strength.

The handlebar/seat/peg relationship is one where the bars feel a little low (mainly because of the seat’s firmness) but otherwise comfortable. The bars are nicely shaped, capped with soft grips, and the

control levers are dog-legged Maguras with pivot covers.

.Footpegs are bolted to the frame in a clamp-type style and can be moved fore and aft a small amount to fit the rider, a nice idea. The saw-toothed units are mounted at the right height and mud won’t cause any slipping. The brake pedal also has a saw-toothed top that prevents slippage when wet or muddy.

Close to 10 inches of travel at both ends of the MX means you won’t have to buy shocks or forks or kits. New 38mm Marzocchi leading axle forks are used in front. These mount in triple trees with rear-set handlebar mounts. Not a big thing but it sure makes changing fork oil a lot easier. Large body gas-charged Girlings handle

things in the rear. Spring and damping rates are right for a fast rider. Like most Girlings they aren’t rebuildable, but a replacement body is only $37— reasonable for a modern shock. Brake hubs at both ends look like last year’s but the steel liners in the hubs have been changed and a new lining compound is used for the brake shoes. These changes, combined with a full floating backing plate and brake rod at the rear, make the brakes excellent. They are progressive, don’t grab, chatter or fade and are as good as any we’ve tested.

Heavy spokes lace the hubs to Sun rims with rim locks. Dunlop K88 tires 300-21 frönt and 450-18 rear are used. Not the best, not the worst. Most will find them satisfactory and use them until worn out. > The bike we tested was a production prototype, meaning the production model will be slightly different. How different? The gas tank and side number plates on our bike were fiberglass—the production model will use plastic for these items and the tank will be slightly taller at the rear so it will blend into the seat.

CAN-AM

SSO MX-4

$1699

Marzocchi magnesium leading-axle fork Fork travel....................................9.8 in. Engagement................................7.0 in. Spring rate..............................22 lb./in. Compression damping force 8 lb. Rebound damping force..............18 lb. Static seal friction........................12 lb. These 38mm magnesium-slider forks are extremely good units. Long travel with plenty of engagement provides good action with little flex. Spring rate is right, and, for most applications, 5 wt. oil will provide the proper damping rates.

Girling gas shocks Shock travel ................................5.0 in. Wheel Travel................................9.8 in. Spring rate......................83/145 lb./in. Compression/damping force......60 lb. Rebound damping force............100 lb. Excessive compression damping makes the shocks too stiff for all but the all-out MX racer. Control is good, but comfort and fatigue factor can be improved by swapping the stock shocks for the aftermarket pair. Tests performed at Number 1 Products

The seat had a good cover and firm foam

but a strange kicked-up shape at the back. This will be removed and recontoured on the production model.

The only other change will be the pipe. Ours was handmade, welded up from many small parts. The production model will have die-formed cones and is supposed to tuck in tighter. We hope so. The test unit had a tendency to cook the rider’s

leg in a tight right-hand turn. The quiet 86dBA Trapp silencer at its end is marked as a stock production piece. It has been modified by removing the bolt that normally holds all the pieces together and now uses four stout springs instead. This modification works well and allows some movement which keeps the discs carbon free. Neat. At 86dBA the MX-4 is green-> stickie legal. Many manufacturers have trouble getting their enduro bikes quiet enough to be legal and here is a 250 racer that probably produces more horsepower than anything in its class, with a quiet forestry-approved spark arrester/muffler. Let’s hope more manufacturers do the same.

A +^520 Diamond chain was on our bike. A stationary roller just below and behind the swing arm pivot keeps the chain taut. A heavy rubber pad is bolted to the top front part of the swing arm so the chain won’t saw into it. The distance between the centers of the swing arm and sprocket shafts is only three inches, meaning chain deflection won't be much anyway. A steel guard bolts around the countershaft sprocket and makes it almost

impossible to throw the chain into the

cases.

The MX-4 is a little hard to ride until the rider adapts to the big bike horsepower and the light front end.

It starts picking the front end off the ground right off idle if the throttle is turned on rapidly. The front steers as precisely as a Maico. thanks to the strong frame and 38mm forks. Steering is light and precise but unless the rider is careful with his throttle hand, the front tire will be just off the ground enough in the turn to hamper directional control. A riding style like that required on Huskys works best: Dive into the corner, try to sit on the gas cap or as far forward as possible and gas it. Once the light front is adjusted to. slides, berm shots, etc. are done with ease. Off-camber

turns are tricky. A combination of brute power and mediocre tires. The Can-Am is easy to loop overjumps if the throttle is not used judiciously. Some of the test riders thought the MX handled a lot like the new Huskys (probably because of the torque and light front end), others thought it felt much like the new Honda but with more low end horsepower. All of them liked the bike (everything but the shift lever), and thought it would be very competitive. (We normally race our MX bikes but we tested this one before dealer availability, so avoided racing exposure.)

The new Can-Am MX-4 is a finished, competitive, and quiet MX tool. With its dynamite motor, excellent brakes, solid frame and forks, it is sure to find its way into many victory circles. g>]