BUILDING A BETTER TWIN
by Joe Minton
Note: This article was originally published in Motorcyclist magazine in 1981. It has become a classic among the XS650 faithful, but due to its age many of the original prices and suppliers are incorrect. The technical information is still relevant and I will not be responsible for any damage caused by implementing these modifications.
The Yamaha 650cc twin has been made for more years than any other Japanese motorcycle. Introduced in 1968, as the XS-1, the 650 has, with many chassis updates and one engine design change, been selling strongly ever since. Variously designated the XS-1, XS-2, TX650, and XS650, Yamahas SOHC 650 sells to first-time riders who do not want to spend the big money for a four, or to those who simply like the way it looks and sounds and feels. Yamaha's first Special was the XS650, and the bike started a styling revolution that is still going. Each year we hear rumors of how "Yamaha isnt going to make any more 650s next year," and each year they make and sell another 10 or 15,000. The recent past has seen them get quieter (EPA), cleaner (EPA) and a bit faster (Yamaha). And each year they shift just as badly as the year before. Owners and magazines testers really love or really hate them, which to some people qualifies the TX/XS650 as a real motorcycle.
The big twins heritage is older and more interesting than you might think. During the early 1950s, Japanese motorcycle manufacturers were frankly copying European designs. It was during this period that the seed was planted which was to become the XS650 of today. Horex, in Germany, designed a 500cc SOHC vertical twin that was thoroughly modern in concept and execution. The impecunious and enthusiastic engineer of Hosk in Japan made an Oriental version of the Horex. The Hosk was rather expensive - and fast. It was the only Japanese bike that could challenge the English singles and twins for performance. Eventually Hosks constant cash-flow difficulties delivered them to the arms of Showa, which continued to produce an improved version of the Hosk (many of the Hosk engineers went to work for Showa when they took over the Hosk factory). Showa sold to Yamaha in 1960 and the old Horex-inspired 500 twin went with it. Before the sale, the same engineers who had copied the Horex designed a 650cc version of the Hosk which incorporated all that they had learned during the ten years of production of the 500cc version. The 650 promised to be simple and cheap to manufacture - and very strong. After a six-year hiatus Yamaha built a similar bike and has been doing so ever since. Your XS650 can be traced, directly, back to a design of the early 1950s, almost 30 years. Since the bike has been around for over a decade and has sold like mechanical hotcakes, it was a natural to include in our update series. It also has enough well-known glitches, which could use some attention, but we were surprised to find that it hasnt received a lot of attention from after-market companies. In fact, we eventually had to design our own pipe and piston kit to get the kind of performance we were looking for. We wanted the modified 650 to have the broadest possible power spread; people who ride twins lug them around, so we wanted to find some lugging power. We also decided to build a 750cc version of the bike. After all, Kenny Roberts used to race them that way. The displacement boost got results. If you build either of our engines, youll get a real power boost, youll lose no reliability, and youll own something that is unusual and fun.
STAGE 1 CHASSIS
BEARINGS: Both the steering and swingarm-pivot bearings are substandard on the XS650 and should be replaced by superior aftermarket bearings. Tapered-roller steering bearings are far better than the cup-and-cone, loose-ball bearings supplied as standard equipment on the Yamaha and most other medium-sized, medium priced motorcycles. Tapered-roller bearings will last the life of your bike (if lubed properly), giving superior feel and, in some cases, better stability. There are currently two tapered roller bearing assemblies available on the market: the Dresda and those available from Racers Supply (Japanese). The Dresda bearings are a little easier to install and have more bearing area, due to the larger rollers and races. However, they are hard to find. But the difference between the Dresda and the Japanese is largely theoretical anyway. We have installed both on many bikes and neither has failed. The Dresdas cost about $35, the Japanese cost around $30. Installation is the same as with standard cup-and-cone type bearings. You must pay particular attention that you do not damage the surface of the races when you install them; there is a thin edge that you can drive against on the outer races. Use a punch with a carefully ground, squared end on it so that when you place it against this thin edge of the roller bearings outer race there will be little tendency for the punch to slip and mar the working surface of the race. Apply some grease to the outside surface of the race to make it slip into the frame more easily. Adjust the steering bearings so that there is no free play detectable when you grab the front fork tubes near the bottom and push them fore and aft; the fork should, however, be free to rotate under the influence of its own weight. After a couple hundred miles of riding, recheck the bearings adjustment; a bit of play may develop after some use and youll have to tighten down on them some more. Lubricate the steering bearings with quality grease such as the moly-filled constant-velocity-joint grease available through Volkswagen dealers. Yamaha fits the now-infamous plastic bushings to the XS650s swingarm pivot. They wear out by just talking about them and, when worn, make the bike feel as though it has a hinge in its middle-because it does. Get rid of them. Pro-Tec and Racers Supply sell bushings that will last a long time. These bearings require no special installation procedure. Smear a thin film of grease on the outside of the bushings to make driving them home easier, and use a large socket or other object as a buffer between the bushings and your hammer. These bushings ride on a ground steel sleeve that has a rather poor finish. Clamp the sleeve (replace it if its galled) in a vise and polish the bearing surfaces with 400 wet-or-dry sandpaper. The original rough surface has small pieces of steel sticking out that embed themselves in the bushing, then act as cutting tools to cause rapid wear of the bearing assembly. Polishing the bearing sleeve removes most of these chips and greatly extends the life of the bearings. Use the same grease recommended above. If you want a real surprise, change both the steering bearings and the swingarm bushings at the same time. The improvement in steering and handling qualities will be remarkable, considering you have only changed from one bearing type to another.
REAR SUSPENSION: Yamahas rear spring-rate selection is pretty good, although there is too much preload and the springs are mounted on dampers that seem to be there just to keep the springs from flying away. The rear suspension has too little travel (three inches), no compression dampening, and too much rebound damping. After trying several different damper/spring brands and configurations, we found one that works very well. Fox Shox has introduced a line of street bike dampers derived from their very successful dirt-oriented units. We found that the part No. 38-0097 (S-3) dampers were perfectly suited to the Yamaha twin. We chose 13-inch dampers to give a substantial increase in ground clearance. If you fancy yourself a canyon racer, you may prefer to do the same, but the longer shock will raise the saddle height and center of gravity a little. If you like the ride height of your 650, buy the 12.5-inch version of the S&W D-series damper. (Fox doesnt make a 12.5 inch model.) If you weigh less than 200 pounds, you will probably prefer 85-115-inch/pound springs. Riders over that weight and those who regularly carry a passenger will need the 95-125-inch/pound springs. Fox also supplies tow-rate springs, but none of them are suitable for the 650. With Fox dampers fitted to the project bike, we could not induce the rear-wheel bounce that was formerly so common when riding the XS650 on bumpy roads. The ride was soft, yet hard riding brought no signs of under-damping or instability. Fox street dampers are of the nitrogen-emulsion type and are pressurized via a Schroeder valve at the top of the shock. They are rebuildable and have proven to be of high quality even though they do not have the window dressing of many of the better-known street-oriented damper lines. I recommend them and feel that they are the best-performing street dampers currently available.
FORK: There are two ways to go in modifying the front fork assembly: air-assist and conventional. Both have advantages and drawbacks, but both improve ride and control. Late XS650 forks are good in most respects but share some of the same faults found in most current fork assemblies. There is excessive seal-drag and the good-looking but inferior-performing dirt-scrapers cause unnecessary sticktion. The XS fork (as with other recent Yamahas) has too much rebound dampening that will result in wallow and rough riding on bumpy roads. The 650 also has too much compression damping, which contributes to its jolting behavior when it encounters a single bump such as an expansion joint in the roadway. To modify the damping, remove the wheel, fender, brake calipers, fork caps, and springs. Make a tool from a bolt with a 17mm head and two nuts to fit in the bolt. Screw the two nuts onto the bolt and tighten them against one another. Fit a socket over the nut(s) and tape it in place so that when you hold the assembly upside down the tape will not allow the bolt to fall out. Now find all the extensions you own, clip them together, and put them on the taped socket. Fully compress the fork leg and insert the extension/socket/bolt assembly into the fork tube and twist it around until the bolt head drops into the top of the damper rod. Fit an 8mm Allen wrench to the damper-fixing bolt in the bottom of the fork leg and loosen it, while preventing the damper rod from turning with the special tool you have made from a bolt and four yards of extensions.
Remove the damper rod from the fork tube. Note: There is a copper washer under the head of each of the damper rod fixing bolts. It may be stuck in the fork leg, which is fine: just be sure it is there when you reassemble the fork. Make sure you have recovered the anti-bottoming pistons that slip over the bottom of the damper rods; they must go back in place when you reassemble the fork. (If you put a small amount of grease on them, they will stay on the damper rod while you insert it into the fork leg.) There are two quarter-inch holes near the bottom end of the damper rods. Drill two more quarter-inch holes through the rods so that there are a total of four around the bottom of each rod. These holes control the compression damping; you have just reduced it by about 40 percent. Near the top of the damper rod there is a small hole. Use a No. 54 drill to slightly enlarge that hole, and then drill all the way through to the other side of the damper rod. You will have two holes when you are finished. De-burr the damper rods and thoroughly clean them. You may wonder why we dont just use thinner fork oil instead of drilling holes. We could, but this gets better results. Besides, 10W oil is a better lubricant than 5W. At this point, you have to decide whether you are going to install air caps. If not, purchase a pair of 1976 Yamaha IT400 fork seals and install them in the fork legs. If you are going for the air-fork option, install 1976 Yamaha YZ250 fork seals. These seals have less friction than any seals weve used. Yes, we know. The XS650 has 35mm fork tubes and the recommended seals are designed for 36mm tubes-but they work. The stock springs are of the correct spring rate and length for the XS and need not be changed. The stock pre-load cam also offers a useful range of adjustments, although the front end will dive excessively under heavy braking. To control front-end dive, we recommend adding more than the normal quantity of fork oil. This will introduce an air-spring effect that will increase the total load capacity of the fork, while decreasing dive. With the fork compressed fully and the spring out of the fork tube, pour Kal-Gard 10W fork oil in until it is six inches from the top of the fork tube. Pump the fork slowly until all the air is out of the damping cavities and add oil, if necessary, until it is six inches from the top of the tube, install the springs, caps, brakes, fender, and wheel. The XS fork will work best as an air fork. The initial travel will be softer, braking dive is well controlled. And, of course, the forks are wonderfully adjustable for different riding conditions or varying rider moods. We have modified several of these forks to air-assist and have had no leakage problems. Make the same damping modifications detailed above and use the same amount of the same weight oil. You will need different springs; S&W No. SP-1530-19 fork springs should be cut to 18.5 inches and installed with S&W air caps. If you wish to use some other brand of fork caps, be sure that the springs are cut so that there is .7 inch of spring preload when the caps are fully seated. We found that 10 psi was a normal working pressure for all-around use. Twisty-road riding was more comfortable at 14 psi, and cushy freeway comfort came at 7.0 psi. Like the steering bearings and swingarm bushing modifications, the fork and rear damper changes are best done at the same time to fully realize just how much the Yamaha can be improved. We think that you will find the difference remarkable. Your bike will ride much better and handle much more securely.
STAGE II CHASSIS
BRAKES: Yamaha brakes are among the best in production today; they are fitted with rigid calipers and have a solid feel that gives the rider excellent control. However, they can be improved. The disks themselves are rather heavy (the heaviest in the industry), and Yamahas disc material leads to galling. Small pieces of the disc material are torn loose and imbed themselves in the brake pad. These imbedded particles then act as cutting tools that gouge grooves in the disc and reduce its effectiveness as a braking surface. There is a simple solution to this problem: drilling the disc. If the disc is drilled in such a manner that the entire surface of the pad is swept by holes, the metal particles are carried away, and galling will never take place. In fact, galled disc drilled in this manner will polish up and work much better than it did before being drilled. The holes should be left somewhat sharp at their edges - not so sharp that you might cut your finger on them, but sharp. Specialists II will drill Yamaha discs with a 168-hole pattern that will do the job for $40. There is nothing you can do that will improve your Yamahas brakes as much as this drilling operation. The holes will not improve wet braking much, if at all, but they will greatly improve the effectiveness of your bikes brakes. Perhaps you ride hard or carry a lot of luggage. If so, you might want to install a second disc on your XS. Europeans buy the XS650 with two discs installed at the factory; those conversion parts are available from your Yamaha dealer. Have him look on the 1978 or 79 parts microfiche; near the bottom is a special section devoted to the part numbers needed to convert the XS to double front discs. The conversion is not cheap. We found a second disc and caliper at a motorcycle salvage yard for a much lower price. You may use one of the calipers from the XS750 but must change the pads to the same ones used on the 650. There are two different pad materials available to fit these calipers: those fitted to the 650 (and XS11) and those furnished with the 750 triple. The 650 pads are softer and require less lever pressure to operate but do not perform as well in the wet. Either pad set will last a long time and wear need not be of any real concern. We recommend -3 stainless steel braided Teflon brake lines for the improved rigidity and feel they give the brake system. You may purchase these lines from Russell Products. When fitting twin discs, you may wish to use the larger master cylinder from the XS750; we did not and prefer the light, two finger brakes that result with two discs being fed by the original 14mm-diameter master cylinder.
BRACING: The Yamahas frame is fully up to the stresses of hard riding and bumpy roads. The fork, however is not, and neither is the swingarm. If you really want to fly on your XS, you will need to fix the fork-flex problem, and if you want the most rigid chassis possible you should stiffen the swingarm. Many current production motorcycles share a shortcoming with the XS; the front axle is not clamped securely on both sides of the front fork. The front axle is clamped by a cap on one fork leg and merely passes through the other leg. Fork rigidity is very much influenced by this. If you have the opportunity, stop by your dealer and compare the fork stiffness of your bike with one of the new Yamahas that has the axle clamped on both sides. To do this, face the bike and put the front wheel between your legs; grasp the handlebars and pull them from side to side as though you were trying to turn the wheel. There will be a lot more flex in the fork of a bike with the axle clamped only on one side than with the two-clamp design.
Okay, so your bikes fork flexes. What can you do about it? The answer is as simple as bolting on a fork brace. We used one of the braces sold by Racers Supply. These braces are made by Racers World, cost $59.95, and work. The brace is made of two pieces of steel tubing bent to fit over and around the bikes fender and welded to steel bars drilled to fit the fender-mount lugs on each fork. Some care should be exercised in fitting this brace to your bikes fork. Individual motorcycle tolerances make it almost impossible for Racers World to make every fork brace fit every bike. It is very important that the fork brace be a slip-fit between the fork legs. If there is any stress between the brace and the legs, there will be an undesirable increase in fork stiction. Usually only a bit of filing is needed to fit one of these braces. Just be sure that the fit between the legs and the brace is snug and does not result in any side pressure on the fork legs. Bumpy, curving roads will flex your bikes swingarm. The heavy cast rear wheel on the Special versions of the X650 particularly affect handling. If you really wish to prepare your XS for the best possible handling and stability, you should seriously consider bracing the swingarm. Racers World has a series of weld-on swingarm braces for popular superbikes. These cost a lot less ($60) than the custom swingarms and are effective and simple to install. (You just take the brace and swingarm to a certified welder and let him do the work.) We talked them into manufacturing weld-on braces for the XS650. The braced swingarm they furnished us fit, was straight, and made a big difference in the stability of our XS650. You can purchase one of these braces (even if you own some other bike) from Racers Supply. We recommend them to anyone who wants to have the best possible chassis performance.
TIRES: Roadrace-quality handling will do nothing if you dont have tires to go with it. We tried several sets of tires before setting on our preferred set: a sport-compound Metzeler on the front and a Pirelli MT28 on the rear. The Metzeler is a round-profile tire and complements the steering geometry of the Yamaha twin perfectly. The sport compound version of this tire allows the rider to take advantage of the excellent brakes that result from our modifications. The Pirelli is also a round tire that retains the same feel at any lean angle. Its traction is above reproach and it lasts. But there are other tire combinations that perform well on the XS, depending upon how you use your twin. There is no better touring setup than the Conti twins. These tires will give the best ride, last at least as long as anything else, and will give more traction than the stock chassis can handle. One rear tire (16-inch) gave one staffers XS650 11,000 miles before needed replacement. If you must ride in the wet, we strongly recommend Dunlop K181s. These tires are magical in the rain. They probably wont last as long as the Contis, but they will make you last longer in the rain. I have used ACP balancing fluid for four years and have never had a balance problem during that time. Tires wear evenly, and even at 130 mph I have a smooth ride.
Part II Engine
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