Service

SERVICE

In search of the Ferrari exhaust

I have a 1985 Ninja 900 and it’s great. But I miss the sound I’d grown to love in the past—that Ferrari-like exhaust note of an MV Agusta.

Two questions arise. If I install a muffler that has no or too little back pressure, will I burn my exhaust valves as my friends warn? And if I use something conservative like a Dunstall, will I need to rejet?

Seriously, these performance cans don’t create that audible satisfaction I need, so what can I do?

James P. Doyle, D.O.

Newton Centre, Massachusetts

Almost any good 4-into-J exhaust will give you the sound you want, though a streetable baffle may take some of the edge from it. But then again, there should be little need to rejet if you use a standard baffle; and even with a competition baffle, burnt exhaust valves are not very likely.

If you want to take absolutely no chances, Kerker offers pipe and jetting kits designed to work together. These generally move motorcycle carburetion from the lean side of a usable mixture (where motorcycle companies set it) to the slightly rich side, where throttle response may be better but fuel economy slightly worse.

Canadian parts in the U.S.

Last summer I purchased an RZV500R, one of the bikes you onced dubbed one of Japan’s secrets. The bike came from Canada, so it is a European model. Flaving and riding the bike is a real joy, but obtaining parts is a real chore. Do you know of any shops that I could mail order parts from, parts that are for the RZV only. Many of the more universal pieces I can get from FJ 1200 and FJ600 parts books.

Chris Flamilton Houghton, Michigan

We've had good luck obtaining Canadian Yamaha parts from Sports Afield IT 310 Patillo Road, Tec u ms eh,

Ontario N8N 2L9; (519) 72 7-396 7.

But there probably are many other Canadian Yamaha dealers who would appreciate your business as well.

Performance confusion

This is about your tests on the

1984 Honda Nighthawk S and the

1985 Yamaha Maxim (both 700s and air-cooled). If a person went by your performance specifications, the Maxim should flat outrun the Nighthawk.

No way! I’ve got a friend with a Maxim, and I’ve got a Nighthawk, and I flat blew him away, even though I let him say go. Are your specs way off, or did I just get one great-running Nighthawk? Also, your top speed on the Nighthawk was wrong. Your top speed said somewhere around 130 mph. Sorry, but my completely stock Nighthawk will run an easy 145 mph.

Why doesn’t my bike fit up with your magazine’s (or any other’s, for that matter) spec chart?

Steve Miller Clinton, Indiana

If a person went by our performance specifications, he couldn 7 compare a 1984 Nighthawk to a 1985 Maxim; we never printed a test on the Maxim. It seems your Maxim information must have come from another magazine. A few years ago, when the major U.S. motorcycle magazines all did acceleration testing at Orange County International Race-

way, comparing times between magazines was somewhat meaningful. Unfortunately, OCIR is no more, and the different magazines have different test sites and different wa ys to measure quarter-mile times, as well as different riders. We at CYCLE WORLD go to considerable effort to make sure our acceleration figures are accurate, and consistent from test to test. But we wouldn 7 suggest comparing our time on one bike to magazine X's time on a different bike; the testing procedures are different enough to make the comparison nearly meaningless for closely matched bikes.

CYCLE WORLD s top-speed measurement has also been affected by the demise of OCIR. Formerly, it was listed as the speed reached in half a mile; actually, it was the speed reached about 4000feet down OCIR 's long dragstrip. While a consistent comparison, 4000feet wasn 7 far enough for most motorcycles to stabilize at their absolute maximum speed. Our current test procedure (listed in the specifications as simply top speed) gives a bike 10,000feet (two miles) to reach maximum speed, a figure that certainly will be higher than that produced by our old test.

In our 1984 test, a Nighthawk 700 reached 125 mph in 4000feet. A retest with our new procedure would produce a higher speed, though almost certainly not 145 mph. Speedometer error can be extreme at high speeds, and we'd suggest you subtract at least 10 to 15 mph to anything you see on your bike's dial beyond the century mark. Our $2000 radar timing equipment taught us that lesson long ago.

Finally, while we never published a test of a 1985 Maxim, we did measure a Maxim 's acceleration for a test that never found room in the magazine.

Our conclusions match yours: The Nighthawk is quicker than the Maxim.

Gearing questions

Last March, I bought a new factory leftover 1981 Honda CB750C and ordered a back issue on said bike. On your test bike the engine speed was 4508 rpm at 60 mph, whereas my bike’s engine speed is 4450 rpm at 55 mph, 4900 at 60 mph, and 6000 rpm at 72 mph. I live in Northeastern Ohio and was wondering whether or not the climate and altitude difference would account for the 400 rpm difference from the test bike, assuming the sprockets and tires are equal in size to the test bike.

If the climate doesn’t account for the difference, what can I do to parallel the rpm of the test bike? Also, are the maximum speeds in gears accurate? Did a rider actually have the 750C tapped out at 126 mph or are these calculated speeds? If so, what is the top speed of the 750C?

Mark A. Ochkie

Salem, Ohio

Your motorcycle and our test bike are probably quite close in their rpm/ speed relationship, but that fact is likely disguised by inaccurate instrumentation. Neither climate nor altitude would cause any discrepancies, though different tire brands might; tires that bear the same size markings but are made by different companies often are not the same in diameter. Our speed-in-gears (and engine-speed-at-60-mph) figures are based on calculations, taking gear ratios and rear-tire diameter into account. Because we can't determine tire growth or wheel slip, there may be some deviation between our figures and actual performance. But as neither wheel slip nor tire growth have very large effects at two-digit speeds, our calculations should be within two or three percent of actual results.

In contrast, motorcycle instruments are notoriously untrustworthy for precise measurements; speedometers often read five to ten percent fast, and tachometers can be equally optimistic. As your observed numbers vary from our calculated engine speeds by about 10 percent, it's entirely possible that the discrepancy can be accounted for solely by instrument error.

As for top speed, we measured that to be 120 mph, given about a 4000foot run.

Radian wobble woes

I own a 1986 Yamaha YX600 Radian and enjoy riding it whenever this midwest climate permits. This cycle was definitely made to handle well at low and medium speeds, however, the front end wobbles and shakes noticeably at speeds of 70 to 100 mph. with the wobble increasing with speed.

With a passenger on board, my cycle handles a bit more smoothly at those speeds, but a passenger slows down acceleration and limits handling.

I do have a small Rifle fairing mounted to the front but its light weight makes no noticeable difference in handling at higher speeds. Do you have any suggestions on how to alleviate my Radian’s wobble?

Don Matejowsky

Champaign, Illinois

A first step would be to distinguish more exactly the nature of your problem. Engineers who study motorcycle dynamics describe two typical motorcycle misbehaviors: weaves and wobbles. By their definition, a wobble is a high-frequency oscillation of the front tire, fork and handlebars only, generally with a frequency of around 6 to 8 cycles per second (fast enough to blur the tips of the handgrips). Many (if not most) motorcycles will exhibit a wobble when ridden hands-off at about 40 mph. A weave is a coupled oscillation of the front wheel and fork and the main motorcycle frame, at a much slower frequency, about l to 2 cycles per second. Weaves often happen in corners, and they feel as if the motorcycle's frame has suddenly grown a hinge right under the seat.

From your description we can 7 be certain exactly how your Radian was misbehaving, but we suspect a weave, and we suspect the handlebar fairing was a contributor. Try running the

bike with the fairing removed and see if it handles better. Also, weaves are very sensitive to rear wheel alignment and rear tire wear; as the tire wears flat in the center, a weave is more likely, and a new tire may improve matters.

If it's a wobble, the fairing still may be involved, but there are some other things that can be checked as well. Your bike's steering-head bearings should be snug, and the front tire and wheel balanced and free of any significant run-out. One possible addition that would help would be a steering damper, a device that can effectively reduce wobbles, but rarely corrects weaves. But once again, if nothing else works, we'd suggest removing the fairing and seeing if that helps.

Honda caliper mounting

I recently purchased a mint 1977 Honda CB550F SS. Its original owner converted it to double front discs, and I’ve noticed that the calipers are mounted in front of the fork legs. Why is this? Would it improve braking to move them behind the fork legs, and could this be done by simply turning the fork legs backwards?

Also, because of the bike’s excellent condition I would like to switch to cast mag wheels to replace the spoked ones. Do you know what Honda model wheels would work with the least effort? If this switch is possible I would like to keep the double disc.

Robert Smith

Falmouth, Massachusetts

Early Honda Fours used a pivoting caliper design that works best when the calipers are placed in front of the fork legs, so that the pivot arm is in tension during braking. It is possible to reverse the fork legs and the caliper mounting but it won 7 improve braking perfor nance.

We don 7 know of any Honda cast wheels that would easily fit on your bike; in days gone by, the Lester Wheel Company offered wheels that would easily fit, including a drum with the rear wheel. But these wheels are no longer available, unless you can find a pair in a salvage yard. That's not as unlikely as it seems, though, as Lester wheels were rugged enough to outlast most motorcycles.

A not-too-expensive alternative to improve both the appearance and the function of your motorcycle would be to have aluminum rims laced to your hubs using stainless steel spokes. The result would be attractive, and surely lighter than any but the most expensive racing mags.

V-Four versus inlineFour

I am 17 and own a 1985 500 Interceptor. I love the vibrationless, smooth power it puts out. Since my bike, many touring bikes, and Formula 1 bikes all use V-Fours, I was wondering which type of engine layout has the best power or power potential? The V-Four or the inlineFour?

Aaron Luethe

Chinook, Washington

Most engine tuners we know prefer working with an inline-Four for racing. An inline-Four's even firing order makes exhaust tuning easier, with best results coming from a conventional 4into-l pipe. Its intake system is generally simpler and easier to work on.

An inline-Four has two camshafts instead of four, and a simpler cam-drive system. With the alternator mounted behind the cylinders, an inline-Four can be as narrow as a V-Four that has its alternator on the crank.

But despite all that, there is probably negligible difference in power output between a V-Four and an inlineFour engine, given the same effort put into each. But the inline engine will be less expensive and simpler, enough so that Honda, the industry's biggest Vengine proponent, has introduced inline-Four engines in its latest sportbikes, the Hurricane 600 and WOO. 0