The Service Dept
THE SERVICE DEPT
WALT FULTON
Each and every year we find subtle changes and improvements in the performance and handling of the continued models of bikes from all manufacturers. It is certain that they cannot afford to introduce all new motorcycles every year—but a guy isn’t likely to trade his ‘71 in on a ‘75 that’s, identical to it either—so even minute changes are necessary to keep the line selling.
In the March issue of CYCLE WORLD, we tested an improved version of the Yamaha 360, the DT400. As the test points out, the 400 is not only a bigger version of the 360, but it also has some significant engine modifications and small changes in the running gear.
On the whole, we found the 400 to our liking. As with any bike, there is room for improvement. The major fault we found was the front suspension. Over a long stretch of washboard terrain the forks quit working entirely, due to hydraulic lock. According to Yamaha, the forks have not been modified for 1975, and tear-down verifies this.
That malicious hydraulic lock usually takes place on the compression stroke of the forks. This is caused by the valving not being able to respond to the sudden shock of the forks. In the case of the Yamaha, however, the condition was experienced in rebound also. In other words, the forks were seized up completely. It was like riding a bike with a rigid front end. The cause: oil passages in the damping rod that aren’t large enough to respond to the sudden rebound. Whatever the cause, the results are the same, poor handling.
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There is a cure that can be affected by the average back yard mechanic who has a drill motor and the tools necessary to disassemble the front forks. Of course, he’ll also need a “Trickit,” which is designed and built by Number One Products of 1 1509 Bexley Drive, Whittier, CA 90606. For the retail price of $9.95, the do-it-yourselfer gets the modified fork pieces and the 3/32 and 5/32-in. drill bits needed to enlarge the oil flow holes in the fork tubes.
After spending a couple of days on the 400 once the modification was completed, we feel that the $9.95 for this kit is money well spent. Fork action was improved about 100 percent, making any type of riding easier on the body.
