AJS Stormer 370cc. Y50. 1970.


36. AJS Stormer 370cc Y50

AJS Stormer 370 Y50 (US. Y50 1970, Y51 1971)


Will the new AJS Stormer put Britain back again, as the top producer of Motocross iron?

A few years ago moto cross competition reached a fever pitch in Europe, just as interest in this sport was beginning to blossom in this country.

On the level the wins and championships were falling to the superior lightweight two-strokes of the Swedes, Czechs, Spanish and others. Initially known as scrambling in Europe, moto cross domination had been taken away overnight from the British machinery.

The four-stroke scramblers of AJS, Husqvarna, Triumph, Matchless, BSA, Norton and others had fallen into the background, lost in the dust of the domineering ring ding. Gone were those beautiful sounding four-strokers that were music to the ears of millions of European spectators for over a decade.

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Then one day in 1967 one of the British motorcycle weekly papers made mention of the secret development work being done by the AJS factory on a new motocrosser.

Without reading further, reminiscent thoughts of the old AJS singles came to mind.

But these were big heavy AJS and Matchless powered four-strokes, most of us had raced one of this at some time or other. Upon reading the content of the article we discovered that the British AJS works was developing a 250cc two-stroke for an attack on the international moto cross title.

In a little over a year from then an AJS Stormer 250cc, in the hands of Malcolm Davis had won the 1968 British Championship. No minor feat, considering the machine wasn’t even in production. With the British 250 Championship as a foundation, production of the Y-40 250cc Stormer commenced.

Now to expand their interests, the Norton/AJS factory has released the Y-50 370cc Stormer, for an attack on the 500 class. Having already tested the Y-40 version of the Stormer, we were in for a few surprises with the 370cc Y-50. Externally the Y-50 is identical to the 250 Stormer, except for the yellow, rather the orange gas tank.

As the photos show, the Y-50 Stormer is strictly business, and not one item is on the machine that doesn’t have a distinct purpose.

The only brightly painted item is the gas tank. Here we would of liked to have seen a little more effort put into applying the paint, but for a moto cross machine, that’s minor.

The overall quality and finish of the chasis, wheels, engine and suspension components, is very good. The welding of the tubular frame was apparently done with the close eye of the quality control department nearby.

The unusual construction of the frame with a large tapered top tube joining up to the double cradle design and 531 Reynolds tubing makes for a very strong structure.

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The engine and the bolt-together integral gearbox are identical to the 250cc unit, with the exception of the cylinder, piston, head and carburetor. The Y-50 engine has the same 68mm stroke as the 250cc, but the bore diameter has been increased from 68mm to 83mm, therefore bringing the displacement up to 368cc.

The cylinder head also has a larger combustion area, but retains the same compression ratio as the 250 at 11-1. To supply the increased thirst of the bigger Y-50 engine a 34mm Amal feeds the fuel and air to the 370cc unit.

The four-speed Villiers gearbox is also the same as used on the 250 mounts. Judging by the performance of the clutch and transmission, they both seem to be up to the demands of the Y-50 engine.

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As mentioned before, the engine/gearbox combination, are integral by benefit of bolting together rather than being cast together as one unit. The clutch is an, all metal plate unit that runs in a separate oil bath, and is driven from the engine via a chain.

The wheels and front suspension units, also both front and rear hubs, are cast and polished aluminum. They apparently provide minimum weight and maximum strength.

One thing that adds to the strength of the wheels is that they both use 40 spoke hubs and rims rather than the conventional 36-hole versions. Surprisingly, the AJS factory decided to design and build their own forks, this gave them full latitude in controlling the handling characteristics by being able to build the fork length and triple clamps to their own specifications.

Upon arriving at our favorite testing area, we ran into our first problem, before we even got the engine fired up. As it turned out it was to plague us for the rest of the day.

You can see in a profile view of the right side of the machine, the kick-start arm sits angled forward and tucked in close to the engine in its normal position.

First you have to reach down, pull out the footrest, then pull in the clutch, to free it from engine compression, and move it back far enough to get your foot on it to allow you to push it down.

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Then with a sharp downward thrust we took our first swipe at the kick arm, and then the trouble started. The kick arm only goes down a quarter of a revolution and stops very abruptly on the foot peg, this then lets your foot slip off the kick start and continue on down to the ground while the spring loaded arm comes back up the front side of your shin.

The worst part is trying to start the engine in this manner in the mud hole or on the side of a hill. Redesigning the kick arm or being able to move the foot peg out of the way would definitely be in order.

(Clearly no one had told them, that you could fold up the foot peg!)

We did finally get the engine started and discovered that it would fire very readily if you could only get more of a stroke on the kick arm. After taking the Y-50 for a short ride to clean it out, we found that it will sit and idle when warm like a well-mannered trail bike.

Yet the throttle response is quick, and brings the engine up to full revs almost immediately. When the machine is cold the clutch drags quite a bit, but as soon as it reaches operating temperature it’s smooth and easy to use, when necessary.

The throw of the shift arm is relatively short and the Villiers transmission pops into gear with a very positive action. Unless the rider makes a gross error it’s very difficult to ever miss a shift and go into a false neutral.

After the unit was warmed up we seldom found it necessary to use the clutch, except when you’re starting from a stop. You have to be firm and deliberate about shifting from one gear to another, for many a race has been lost by just missing one gear change.

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 AJS Stormer 370cc Y50 / 1970

The Y-50 engine is a very potent unit and feels to be every bit as strong as any other machine in its class. At first the power feels a little on the weak side, but this is due to the very broad power band. The engine pulls from about 2,500 rpm, but really runs best in the range from 3,500 to 7,000 rpm.

The power is smooth and almost absent of any sudden surge of speed. This in turn makes the use of the power very predictable and choosing the shifting sequence more positive for maximum acceleration.

There was one characteristic about the engine that was extremely bothersome, the vibration. The engine would pick up this vibration at about 4,500 and carry on going at an increasing rate to around 6,000 rpm.

The direction of the vibration seemed to be straight up and centred in the main tube under the seat and gas tank. The gas tank vibrated very severely. This also made it impossible to grasp the tank with your knees while riding. The seat also echoed the same resonance and made it extremely difficult to rest there for more than a few moments while accelerating.

The handlebars also transmit a good deal of the vibration to the riders hands, and the total combination makes a very difficult to stay aboard for more then twenty or thirty minutes without having to get off and rest. We would suspect that the combination of balance factor and the need for additional engine mounts lead to this vibration problem.

Regardless, it is a very irritating problem that should be corrected for a machine of this type.

The low saddle height, 30 inches, allows the rider, short or tall, to keep themselves in a position of absolute control whether standing or sitting. The location of the handlebars is just right for moto cross and most other types of off-road riding.

The location of the foot pegs is fine for most riders, though a taller man might want to lower them some. They are adjustable so this is no problem.

The handling of the Stormer is probably one of its finest attributes. Impeccable would be an appropriate term. The new Y-50 Stormer has superb handling qualities and it’s stable in even the most demanding of situations, regardless of the conditions.

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With the amount of power available the machine is extremely predictable and never once put us in a position of doubt.

We could come off a jump completely crossed up in mid air, yet each time the machine would land and track exactly where we wanted it to go. The only thing that deterred from the machines excellent handling qualities was the suspension units.

For some strange reason the Girling units on the rear came equipped with 110-pound springs.

These would be fine on one of the old 380 pound four-stroke Ajays, but 60/90 progressive or 88 pound straight wound items would be much more suitable for the lightweight Stormer.

Just about any time the rider desires, the front wheel can be lofted. Balance and weight distribution are excellent. Up front we found that the front forks were another item that could use some changes.

For the first thirty minutes of riding, until the forks had warmed up, the front felt like it was welded together and as rigid as a water pipe.

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When cold we could only get about two inches of travel action from the forks, but as the units warmed they loosened up and traveled their full stroke.

But strangely enough, after the forks warmed the damping action all but disappeared. Playing around with some other spring weights and different oils would be the answer here. A couple of nice features about the Stormer are the exhaust system and rear chain adjustment. The expansion chamber is very neatly tucked up and over the engine in a crossover fashion.

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This keeps the pipe from being in the vulnerable position under the engine and clear of the rider’s legs. The chain adjuster is a simple and sanitary excentric cam on each side of the swing arm bolt. Adjustment of the chain merely requires loosening a few nuts and moving the plate for proper adjustment.

The overall appearance and finishes are good. In appearance the new Y-50 is identical to the 250 Stormer except for the yellow gas tank. Traction at the rear is good, braking and stability, are very good.

The AJS factory has done an admirable job on the 370 Y-50 Stormer, in a short period of time. There are a few areas that could use a bit more tweaking to get all the performance and handling available, but these aren’t serious problems.

There has been a lot of testing and development work put into this machine and it could wellprove to be, Britain’s best moto crosser yet.

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Competition Shop / 1970

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