Service

SERVICE

Backwards RD

I own a 1977 RD400. Does this motorcycle have a reverse gear or something? Today I started my bike as usual and put it into first gear as usual. But I let out the clutch fairly quickly to zip out of my driveway and I went backwards into a wall. This is not a joke. It really happened.

Can you tell me what's going on? Is this common with the RD? Kelly E. Crist Kailua. Hawaii

No, your RD isn't fitted with a reverse gear: instead. its engine was

actuali y running backward. This isn 7 a common phenomenon. but neither is it unheard of.

Like most two-strokes. the RD has port timing that is symmetrical. Thus. the engine doesn 7 know whether it 's running forward or backward as far as the ports are concerned. Onl r the ignition timing is directionally dependent. occurring a certain distance before the piston reaches the top of its stroke with the crankshaft turning in the proper direction. If the crank is turning the wrong way. the spark will occur after top dead center.

Either retarded or advanced ignition timing could cause the engine to run backwards. Retarded timing would cause the spark to occur closer to top dead center, meaning that the ignition timing would be very similar whether the engine was running forward or backward. Then, if the engine popped back while you were kickstarting it. it might actualir continue to run backward. Very advanced ignition timing can have a similar effect by increasing the chance of kickback: if the charge fires too early during kickstarling, the pressure of combustion could build up too soon, before the piston has passed through top dead center. As a result. the piston could be driven back downward and cause the crank to rotate in the wrong direction, and again, the engine could run backward.

In either case, proper ignition liming and a firm kick while starting your RD should prevent this from occurring.

Mounting Hi-Sports on spoked wheels

I read with great interest your article in the Service column regarding the use of tubeless tires on spoked wheels. Because I occasionally race my Ducati, I have regularly mounted the best possible racing tires, including Michelin HiSports, Dunlop Elites and slicks, on my Borrani WM-tvpe spoked rims.

It is possible that the tit of tire bead to rim has not been ideal —I have to admit that until I read vour article, I was unaware of all the possible distinctions between tire and rim dimensions—but I have never had any problems under severe racing conditions.

When mounting tires on my spoked rims, I go to some lengths to ensure that the installation will be trouble-free. To begin with, it is necessary to carefully examine the condition of the rim for dents, burrs and various types of crud. Having a true, smooth and clean rim and tire is important. I always use a generous amount of talc (Johnson & Johnson Baby Powder works great) inside the tire; this lubricates the tube for both the purpose of getting the tube properly aligned inside the tire and when actually riding with the tires hot and flexing. When mounting the tire on the rim I use a lubricant of a type that substantially evaporates, such as water and dishwash liquid. This not only eases tire mounting but also bead seating—in fact, in some cases it is indispensable.

Initially, the tire is inflated with whatever pressure is necessary (within reason) to seat the beads on the rim. This occasionally means 80 psi or so. I am aware of the dangers of using excessive pressure—that’s why truck tires are inflated in metal cages to prevent serious injury from explosion. My opinion is that up to about 100 psi is probably reasonably safe. In any case, I rarely have to use more than about 50 psi in the tube to seat the bead. Immediately after the bead is seated, the tire is deflated by removing the valve core, which allows the tube to completely deflate. Then bouncing the tire on the ground while rotating it allows the tube to assume a straight and unstressed position inside the tire. After that the valve stem can be reinstalled and the tire inflated to its proper operating pressure.

It is clear that properly servicing tires and tubes and wheels can be relativelv involved, but the issue at stake is human safety. Some good judgment and experience is necessary to get the best results. Bill Butler Santa Monica, California

“Probably reasonabl y safe, " indeed. We 'll be the ones standing far back in the corner when YOU inflate a tire to 100 psi; that seems pushing your luck a bit far. But the rest of your advice on tire mounting is sound, and, as you point out, Michelin Hi-Sports can be mounted on WM rims if you proceed with care.

Michelin notes, however; that HiSports weren 't intended for that application and that their bead may occasionally appear to seat on a WM rim without being fully nested.

Michelin strongly recommends not using Hi-Sports on WM rims; and unless there were some very important reason to do otherwise (such as winning a race), we'd take their advice.

Yamaha 650 Twin hop-up

I have a 1981 Yamaha 650 Special showing only 3000 miles on the odometer. I realize the 650 isn't a street-burner of a motorcycle, but then, I wasn't looking for a teethgritting bike with a high G-force rating. What I would like is a little extra zip out of my 650. Can you guys tell me what areas could use the most help, and who and where to contact to get the accessories? Frankie Barco Virginia Beach. Virginia

Rather than specific advice from us, we'd suggest you contact the Yamaha 650 Society (J. G finer. Director, Box 1, Princevi/le, Illinois 61559). The Society is devoted to the Yamaha Twin, publishes a newsletter with tips and suggestions, and keeps close track of who still sells and manufactures parts and accessories for that longlived model.