Service

SERVICE

Paul Dean

No dancing, please

I own a '94 Honda VFR750, which I think is the best all-around motorcycle made. After 10,000 miles, the only complaint I have concerns a front-end wobble between 40 and 55 mph on deceleration when I place little or no pressure on the handlebars. This problem started at about 4000 miles, and when I took the bike back to the dealer, they replaced the stock front tire with a Dunlop Sportmax. That helped a little, but the problem was still there.

After another 2000 miles, the wobble started getting worse, so I took the bike back to the dealer. They were very helpful, and checked and adjusted everything they could think of, including the steering-head bearings; but again, although this helped, the problem was still there. The dealer even called Honda’s tech-support people but didn’t get any useful answers.

The problem now seems to be at its worst. I know that a steering damper would help, but I resist that solution be-

cause I know that this oscillation didn’t exist up until 4000 miles; so, it can’t be a characteristic of the VFR like some people are telling me. Can you help?

Frank Zappetini Jr.

Torrance, California

Actually, Frank, this condition indeed is characteristic of the VFR750, as well as numerous other motorcycles of all makes and sizes. The reasons that wobbles of this type occur are much too complex to discuss in great detail in this confined space, but here ’s the Reader’s Digest version:

As the front tire passes over roadsurface irregularities, even small ones, the center of pressure on the tire ’s contact patch constantly moves from side to side in accordance with the angle and contour of the pavement. Although very slight, this movement of pressure gently tries to steer the front wheel in one direction or the other; but before the wheel can be diverted very far from its straight-ahead position, the steering geometry’s self-centering effect (called “caster”) pulls the wheel back toward center. And here is where the problem begins. The forward motion of the bike gives added energy to these side-to-side movements, enough to cause the front wheel actually to return a little too far past center; and that, in turn, causes the caster to make the wheel move back past center a little too far in the other direction. This series of side-to-side movements continues, with each successive snap back toward-and beyondcenter being more exaggerated than the previous one. The result is a back-andforth shake of the front end.

Most of the time, this waggle either doesn't occur at all or is so insignificant that you can’t feel it. But the aforementioned energizing of the front wheel, in combination with the natural frequency of the entire steering mass, makes many motorcycles very susceptible to front-end oscillations at certain speeds. If you take your hands off the grips when riding one of these bikes in this speed range, the handlebars-and everything that moves with them-will dance back and forth.

Why does this happen only when you let go of the grips? For one thing, any time you ’re holding onto the bars in a normal riding position, the mass of your upper body changes the frequency of the front end enough to prevent it from oscillating. What’s more, your arms and torso act as a damper that helps cancel any waggles before they have a chance to develop.

The front tire also plays a major role in damping out front-wheel oscillations. Some tires are designed with more damping ability than others; and sometimes, the tire that allows a certain motorcycle the best sport handling may provide the least amount of oscillation damping. As tires wear and their tread surfaces become uneven, their tendency to induce this front-end dance usually increases.

I personally own a VFR 750, 1990 vintage, and have ridden every model of that bike from 1986 to present. I also know quite a few people who own 1990-to-1995 models. In all in stances, the aforementioned wobble is present to some degree; it is charac teristic of the breed.

Thankfully, it doesn `t seriously im pair the VFR `s overall performance; it's more of an annoyance than any thing else. Installing a steering damp er is the best cure; but if you can `t abide that remedy for some reason, you can minimize the waggle by using only original-equipment front tires (which were designed to cope with this condition) and adjusting the steering-head bearings until they are almost-but not quite-too tight.

Temporarily Single

I have a 1983 Yamaha XS650 Spe cial with a problem. When it starts, it only runs on one cylinder, but after warming up, the second cylinder kicks in and the bike then runs fine. I've had a reputable, licensed mechan ic clean both of the carburetors and their jets, change the sparkplugs, and check the coil wires and ignition pick up. All were okay. We are at a cross roads about what to do next. I would appreciate any advice you could offer about how to remedy this problem. Anthony K. Potts Elkton, Maryland

In all probability, one of your Ya maha two ignition coils is producing a weak spark. When an engine is cold, the fuel drawn into the cylinders va porizes so poorly that the mixture does not easily ignite. This is why car buretors have chokes or cold-start en riching circuits: to give a cold engine enough extra fuel to support easier ignition and combustion.

But these ultra-rich mixtures also cause a greater electrical resistance for the high-voltage current that must jump the gap across the sparkplug electrodes. And when an ignition coil is producing a marginal spark, that added resistance often is too much for it to overcome. As soon as the engine warms and the mixture is returned to its normal operating ratio, though, the resistance drops, making it easier for the spark to jump the gap.

There's a relatively easy way to de termine if either of your coils is the culprit: Switch them. First, start the engine and determine which cylinder is not firing. Then, turn off the engine and swap the two coils, including all of their wiring. Now, the former right coil will fire the left cylinder and vice versa. Restart the engine and see if the dead cylinder has changed sides. If it has, the coil connected to that cylinder is definitely at fault. If not, at least you ’ll know the coils are okay.

If the problem lies elsewhere, it will most likely be in the carburetor that feeds the dead cylinder, despite your mechanic ’s diagnosis. You can switch the carbs to perform the same kind of test as with the coils; that swap is more complicated, though, because the left and right carbs are mirror-images of one another, which prevents you from connecting the throttle linkage. But after making the swap, you can handoperate each carb ’s butterfly separately to determine if the non-firing cylinder changed sides with the carbs.

If the rest of your engine ’s key components are up to snuff (compression, ignition timing, valve adjustment, etc.), you should find the cause in one of these two areas.

Trumpet tips

Liked your recent test of the Triumph Daytona 1200 (January, 1996, issue). Lve owned one for a year and 10,000 miles, and I have a few tips:

1 ) The dealer can reduce the reach to the handlebars by relocating the bars above the top triple-clamp. Raise them another inch above that on the stanchions if you wish, and rotate the clipons slightly inward to clear the fairing.

2) Cut away the foam on the front lip of the seat and re-staple the seatcover. This puts you two inches closer to the bars. I also cut some foam out of the seat, which lowered me and had the effect of raising the bars.

3) Remove the rubber from the footpegs, which will lower your legs a half-inch. Now you can sport-tour in comfort. I’m 5-foot-9 and have made two cross-country trips like this.

People stop me at every gas station when they see this bike and the Triumph logo. That’s a lot of the enjoyment of owning this bike-being an active part of the resurrection of this long-revered marque. Tom Pease Memphis, Tennessee

Thanks for the practical hints, Tom. Maybe Triumph also can learn something useful from your input. □