Service

SERVICE

Virago carb-launcher

I own a 1984 Yamaha Virago 1000 with about 18,000 almost trouble-free miles on it. I say “almost” because the bike has a peculiar problem that is slowly becoming an expensive one. Since the bike was new, I’ve had problems with it backfiring and popping through the rear carburetor. Over time, this popping turned into small explosions that eventually blew the carb completely off its rubber manifold at around 10,000 miles, and in so doing ripped the manifold under the clamp that secures it to the carb. My dealer replaced the manifold (at my expense) and tuned the engine, and told me to start using octane booster. I did. but it didn’t help. Since then, the carb has blown off countless times, and I've now replaced a total of six manifolds at a cost of $13 each. I’ve even tried using new or entirely different clamps, all of which have failed to cure the problem. My friendly dealer still can't explain why my bike does this. Can you? AI Bohr Winnetka, California

Yamaha issued a service bulletin (No. M85-021) in October of 1985 that addressed backfiring and afterfiring problems with early lOOOcc Yamaha Virago V-Twins. These bikes are fitted with two air-bleed systems that help the engine run acceptably while also meeting emissions requirements. One is an air-injection system that meters fresh air into the exhaust, thereby promoting a more-complete burning of the escaping gases; and the other is a mixture-control valve that leans out the mixture during trailing throttle by bleeding fresh air into the intake tracts when the bike is decelerating. A minor malfunction in either of these systems can cause backfiring (spitting back through the carbs) or afterfiring (popping through the exhaust). The bulletin explains an easy test to determine whether or not either air system is causing the problem. The test involves simply blocking off the vacuum hoses that control the systems.

In all likelihood, though, the problem you describe sounds like one caused by a malfunction in the mixture-control valve—either a weak spring or a leaky valve itself—resulting in a too-lean mixture at the wrong time. And because differences in intake-tract resonance between front and rear cylinders (caused by the V-Twin engine's unevenly spaced firing impulses) tend to make the rear run leaner than the front, the backfiring always occurs in the rear cylinder.

On your 1000 Virago, both airbleed systems have their control valves located under the little chromed cover (the one that looks like an air-filter housing) on the left side of the bike.

You can try the test yourself and if the popping ceases after the vacuum lines have been pinched off, you've found the source of the problem. But you should give a Yamaha dealer a shot at it first, reminding them, naturally, of the existence of the service bulletin. Who knows—maybe you'll be able to get the problem cured permanently at no charge.

Drag-race dilemma

I own a 1984 Honda V65 Sabre, which was featured in your May,

1984 issue. I have compared its performance to that of the Yamaha FZR1000 featured in your August,

1987 issue. According to your published specifications, the Honda weighs 594 pounds and claims 121 bhp. The Yamaha weighs 497 pounds and claims 130 bhp. Correct me if I am wrong, but shouldn’t the Yamaha outperform the Honda by a more respectable margin than .25 seconds and 2.95 mph in the quarter-mile? Their terminal speeds are not that far apart, and neither are their time-to-speed numbers. To top it off. the Honda does not have all those fancy “advantages,” such as an aluminum frame, a full racing fairing, five valves per cylinder, chain final drive, sticky radial tires, etc.

My point is. how far has engine technology really advanced in the past three years? I am not comparing apples to oranges: I am comparing drivelines to drivelines. I certainly would not sell my Sabre for this Yamaha.

Gary Gaboriault

Fort Washington. Pennsylvania

You might change v our mind if your interests were more in roadracing than in drag racing. Essentially, the Yamaha was configured specifically to excel on a roadraee course, so quite a few of its crucial elements—its gear ratios and powerhand shape being two of the more important ones—were designed for optimum performance in that environment. On the other hand, the V65 Sabre (which, incidentally, weighs 576 pounds dry; the 594-pound figure is with a half-tank of gas) has a l()8cc displacement advantage over the FZR 1000, and was designed more with stoplight-to-stoplight and quartermile racing in mind; thus, its powcrband (incredible mid-range wallop) and gear ratios (including a relatively low first gear) were so decided.

Consequently, the Yamaha launches off the line (which is by far the most critical part of any drag race) comparatively slowly, whereas the Honda bolts away like a jet. The Yamaha ultimately catches and passes the Honda, although not by much; but the fact that the FZR can catch it at all is a measure of its superior performance, for the only way to make up for time lost at the start of a drag race is with sheer power.

As far as the FZR 's “advantages " are concerned, they are advantageous for roadracing and serious sport riding, not for straight-line racing. Its higher top speed ( 155 mph vs. the Sabre's 139) is largely a result of its full fairing; and its aluminum frame, sticky tires and chain drive enhance its lap times around a road course. Thus, in a drag race, the FZR is just barely quicker and faster than the Sabre, but a road race between the two would be a joke, an outright embarrassment for the Sabre. So, you see, in the end, you actually are comparing oranges and apples.

Dain bramaged

I want to widen the powerband on my CX500 Custom from what it is to at least another inch wider. But my friend says that my centrifugal rubber cam will over-durate and allow the high-speed exhaust bearings to slow', causing the valves to burn.

Is this a common problem in the CX family or is it a matter of proper tuning afterwards?

Brian Porpealia

Niagara Falls. Ontario. Canada

This is a family problem, all right, but it's in yours, not the CX's. Everyone knows that when cams overdurate. the high-speed exhaust bearings speed up, not slow down.

Wake up and smell the exhaust fumes.