Service

SERVICE

Paul Dean

Mile-high leak

The good news is that I live in Hawaii. The bad news is that when I take my ’88 Suzuki Intruder 750 from sea level to 5000-foot elevations, the final-drive case leaks. And I mean it really leaks. I can do this change in elevation in about 30 minutes at 45 mph. When I come back down, the leak stops. Riding at or near sea level, there are no problems. I’ve had the dealer change the oil seals twice, and they checked the vent and found that it is not clogged; we’ve even tried heavier oil-all to no avail. The dealer doesn’t know what to do at this point. He claims that the new models don’t have this problem, but says he has sold three other bikes of the same year and model that do. Bobby Cooper

Hilo, Hawaii

The final-drive case on the Intruder incorporates an elaborate breathing system that allows air to flow freely in and out of the case to compensate for normal fluctuations in atmospheric pressure. To do this, the system uses a labyrinthine series of passages that separate any droplets of gear oil from that air and route them back to the oil reservoir in the case. Although your dealer has checked the vent and claims it is not clogged, the problem nevertheless is due to a blockage in the venting system. When you ride your Intruder up to 5000feet, the atmospheric pressure there is, ob-

viously, much lower than at sea level; but because the venting system is malfunctioning-and because your ride up the mountain only takes a short period of time—sea-level pressure is still captured within the final-drive case. The higher pressure inside the case forces gear oil into passages that normally would only flow air, and it then leaks out of the case through the vent.

I’m making an educated guess here, but it’s most likely that when the leak occurred for the very first time, it was caused by debris blocking the vent. You then took the bike to the dealer, who disassembled the case, cleaned all the components (inadvertently removing whatever was blocking the vent) and replaced the main drive seal—which, I believe, is the cause of the present leak. The location of the seal in its mounting boss is critical; if the seal is pushed in just the slightest bit too far, it will bottom on the inside of the drive case and block one of the venting system ’s critical passages, rendering the system inoperative. Suzuki makes a special installation tool that prevents the seal from being pushed too far into the boss. If the mechanic did not use this tool, he likely drove the seal in too far, blocking the hole.

Have your dealer install a new seal using the special tool, and the high-altitude leakage problem should go away.

The packing principle

I’m fairly new to motorcycling, having owned my first bike, a ’96 Yamaha Seca II, for just under two years, and I have a question that’s been bugging me for a while. On several occasions, I’ve heard some of the more experienced riders around here talking about a suspension “packing down,” and I don’t understand what that means. I’ve asked them to explain, but the answers they’ve given have left me even more confused. Can you explain what they’re talking about? Benny Esteban

Pompano Beach, Florida

They are referring to a problem that can occur when a suspension has too much rebound damping. It happens most often on dirtbikes, which have a lot of suspension travel, but it can occur on almost any motorcycle.

Every time a bike hits a bump, its suspension compresses as it absorbs the impact, then returns back to or >

near its original length. The purpose of rebound damping is to control the rate of that return. If the rebound is too light, the suspension will return too quickly; if the rebound is too firm, the suspension will return too slowly.

Packing down occurs when a bike that has too much rebound damping passes over a rapid succession of bumps. The first bump compresses the suspension, but before it can fully extend, the second bump compresses it again. And before it can fully return from the second bump, the third bump compresses it again, and so on until the bike has passed over all the bumps in the series. Because the suspension is never able to extend to its normal length after hitting the first bump, every bump thereafter is encountered with the suspension already partially compressed. This causes the ride to be noticeably choppier over all the bumps but the first. And in many instances, each bump is encountered with the suspension a little more compressed than the one before it, progressively making the ride more and more harsh, even though all the bumps are essentially the same size and shape. That is why this behavior has come to be called “packing down.”

The plane truth

Why do wide, low-profile tires provide better performance? It seems to me that with today’s lighter bikes and wider tires, there would be less force on the contact patch (with the bike going in a straight line), promoting hydroplaning. Also, when a bike has tires that have lower sidewalls, how can it be leaned over farther than with tires that have a 90or 100-percent aspect ratio? Steve Wolf

Herself, Belgium

If the road surface is wet, your observation is correct: A wider tire will hydroplane more easily than a narrow one, especially on a lighter bike. That’s one of the compromises brought about by the quest for ever-better traction on dry surfaces. The tire engineers try to mitigate the wet-weather shortcomings of wide tires by using ultra-sticky compounds and designing tread patterns that efficiently channel water away from the contact patch. But in the end, wider tires still are not as resistant to hydroplaning as narrower ones. On dry surfaces, however, the larger contact patch of wide tires

provides superior grip, whether accelerating, braking or turning.

In itself, sidewall height has nothing to do with a bike ’s ability to be leaned over. It may have an effect on the bike’s feel and stability when leaned over, but not the degree to which it can be leaned. The bike corners on the tire s tread, not on its sidewalls.

Of course, if you replace any given bike ’s tall-sidewall tires with shortsidewall tires, you will reduce that bike ’s cornering clearance and thus inhibit its lean angle. That is just one of the reasons bike and tire manufacturers strongly recommend against such practice. Each motorcycle is designed around tires of a certain width and aspect ratio, and rarely is it advisable to stray very far from those specifications.

Unchained malady

I’ve exhausted all my resources and am in desperate need of expert advice concerning a problem with my ’89 Yamaha FZR1000. I need this help because my local dealer employs only second-rate mechanics governed by disinterested service managers. The problem is that during acceleration between 4000 and 8000 rpm, the FZR does this clunking, skipping, hesitation sort of thing that appears to be coming from the drivetrain. The aforementioned dealer supposedly checked the clutch, transmission and electrical system (coils) to no avail. Any help would be appreciated. Kevin Spielmann Ellington, Connecticut

As I’ve said in many previous Service letters, Kevin, you’ve provided barely enough information about the symptoms to allow a decent appraisal of the problem. But a logical process of elimination suggests that the cause is somewhere in the final drive. Your letter implies that this happens in all gears, but most transmission problems would limit these symptoms to just one or two gears; and there’s little that can go wrong with the clutch that would produce such behavior.

So, look for a stretched drive chain and/or rounded-off teeth on the sprockets, or possibly excessive slack in the chain. In either case, the “clunking, skipping, hesitation ’’ you ’re feeling would result from the chain jumping over the sprocket teeth. It occurs between 4000 and 8000 rpm simply because that is the range in which the engine makes its best torque. □