Cycle World Service
CYCLE WORLD SERVICE
XR wheel swap
If an 18 inch rear wheel were put on a Honda XR350R or XL600R would the fork travel have to be extended one inch to compensate and retain the stock steering characteristics? A buddy told me it wouldn’t change the way they handle enough to notice. Wouldn’t it quicken the steering?
Ted Beers
Arnold, Mo
A taller tire would quicken steering slightly, but you probably can’t install a big enough tire to drastically change things without having it hit the fender when you bottom the suspension. There’s considerable variation in tire height for a given wheel size, so the critical factor is the overall tire diameter. Unfortunately for your intended swap, the 17 inch tires used on the Hondas are low-profile types, and are fairly short. There might be enough room to fit a low profile 18 inch tire and wheel, but you would have to check carefully on the fender clearance.
Find out the installed diameter of the 18 inch tire you would like to fit (tire companies supply this information to their dealers), and compare it with the diameter of the 17 inch tire you’d like to replace: 24.9 inches for the XL600 rear tire, and about 26.3 inches for the XR350 tire. If your new tire is within an inch, it will probably fit with no Modifications needed for clearance or handling. If it’s bigger by more than about an inch, check the fender clearance carefully, and lower the forks in the triple clamps to compensate for the raised hack of the bike.
Engine transplant
I currently own a Kawasaki KZ400 and a KZ650. Is it feasible for a person owning torches and an arc welder and some cycle sense to perform a transplant of a 650cc motor to a 400cc frame?
Cort Blanton
New York, N.Y.
It’s certainly possible, but a better question would be: Is it a reasonable thing to do? We’d suggest not. The swap will take three times more effort than you’d expect (all projects like this do), and the resulting motorcycle probably won’t live up to your expectations. If you simply want to make a lightweight hot rod, you’d be better off modifying the existing 650.
Front end wobble
I recently bought a 1983 Honda CB1100F and it has a handling problem that is very bothersome. It has a wobble in the front end between 40 and 50 mph. It is most noticable when decelerating with my hands loose on the bars. I have had the wheels balanced and trued and the front forks, rear shocks, and the swing arm bushing checked, but to no avail. The dealer says all bikes do this and to not ride with my hands loose on the bars. This isn’t a serious handling defect, but it doesn’t inspire confidence.
Chris Hanley
Calgary, Alta. Canada
Not every bike wobbles hands-off around 40 mph, but it’s more common than not. The motorcycle dynamics people explain that the front end’s natural wobble frequency matches up with front tire rotation frequency over that speed range, exciting the wobble.
The easiest cure is to hold onto the handlebars; that adds enough damping to eliminate the wobble. Other than that, you can try to increase the natural damping of the wobble. Stiffening the front end tends to do that, but that may be complicated in practice. A steering damper will work as well, which may explain why older motorcycles with friction dampers were less likely to do this than the current generation of bikes.
Lurching Ducati
/ own a 1978 Ducati 900GTS with 4000 miles on it. When the bike is started after sitting for a day or more my initial shift down into first gear causes the bike to lurch forward and stall. But when making my initial shift up into second and then back down into first the problem is eliminated. What could be the cause of this? I have seen this occur to other bikes including my 1972 Honda 750 but not to this extent.
Scott Bury
Parma, Ohio A lurch like this is very common to wet clutch bikes and is caused by the clutch plates sticking together after several days of rest. It’s more noticeable in first gear than second because of the increased torque multiplication in the lower gear. It’s a condition to be more endured than fixed, though it may be possible to improve the situation by switching to clutch plates of a different friction material, using lighter viscosity oil, or even changing to a synthetic based oil. None of these are guaranteed to eliminate the lurch, but they might help.
Battle of the Twins Yamaha
/own a 1981 Yamaha XV920RH. I understand that these have been raced in Battle of the Twins races. If you have any information regarding performance mods or parts suppliers, specifically carbs and exhaust systems or anything else, I would appreciate it. I want to build a fast street going Yamaha V-Twin.
Murray Barber
Boulder, Colo.
The Battle of the Twins Yamaha that you’re thinking of was built by Bob Work and Don Vesco, and relied on the development work done on the 750 Virago engine for ferences in engine power or current draw. The only drawback to electronic ignitions is that they’re more difficult to troubleshoot if something goes rwrong. With points you have mechanical parts that you can look at and tell their condition; with an electronic ignition, you have transistors embedded in a block of epoxy. Still, with the current reliability of electronic ignitions, that’s a small drawback.
Single vS. dual shocks
I am the owner of a 1977 CR 125 and I was thinking of making some changes in the bike, but something is puzzling me. This bike has a dual rear shock system. Could I replace it with a single shock system like the new motorcycles have? Where could I get the parts?
William Avellan
New York, N.Y.
No, it’s not possible to change over to a single shock system without spending enough time or money to buy a new bike. And, there might not be as much reason to change as you would expect. The early single shocks were designed with linkage systems to give a steeply increasing spring rate: the further the wheel moved, the stiffer the suspension. In the latest motorcrossers, this steep progression has been abandoned in favor of spring rate progression curves that look very similar to those provided by dual shock systems. In practice, the best of the dual shock sustems (Husqvarna’s) works about as well as the better single shock systems.
Rather than trying to make the iconversion, you should simply switch to the best dual shocks available, either Fox, Works, or Ohlins. Or save your money until you can buy a later model bike.
Tires — mixing and matching
# read your article comparing sport tires. Your test results were that the Dunlop K391 Sport Elite and the Metzeler sport compound tires are the best sport tires you can buy. Also, you state that the Dunlop K391 had superior traction over the Metzeler on the rear tire; however, when it came to the front tire, the Dunlop K291 had more wiggle in the handlebars than the Metzeler. What I would like to know is if I could use the Metzeler for a front tire with the Dunlop K391 as a rear tire? Or would this cause a handling problem because I mixed the two brands.
Michael Lockwood
FPO San Francisco, Calif.
It would be wonderful to answer a question about tires with a straight yes or no, but it’s going to have to be a simpler question. Yes, you can use a Metzeler in front and a Dunlop in the back, but (here comes the hedge), yes, that might cause handling problems. Then again, it might not.
Production class racers routinely mix and match tire brands to make the most of traction at both ends of the bike, so there is definitely merit in considering different brands at different ends. Occasionally, this may lead to stability problems. We encountered one of those recently when mixing tire brands on a Honda Interceptor. With the tires we had mounted, the bike became violently unstable at speeds over 120 mph. With different tires, the Interceptor became stable again at high speeds.
Another point to consider is that a wiggle in the handlebar could be as much a function of the rear tire as the front. We’d recommend sticking to one brand of tires on a motorcycle unless you have very specific reasons to do otherwise, and are willing to change again if your combination doesn’t work.
Magna top speed?
#n your April issue, you say that a V65 Magna will do 145 mph in 5th gear and 173 mph in 6th gear. In your magazine’s Summary department you say that the Magna’s top speed is 137 mph. Which is right?
Jon Geear
Lyons, Colo.
Both sets of numbers are right, but they’re telling you different things. The first set gives the V65’s speed in different gears if (and that’s a big if) it would pull red line in that gear. These numbers are calculated from gearing information and rear tire diameter.
They do not tell you how fast the Magna will actually go. That we measured with a radar gun to be 137 mph. The speed in gears at redline is provided only to give an indication of the bike’s gearing.
GPz injection modifications
I am a proud owner of a 1983 GPzl 100, equipped with digital fuel injection. I have installed a Vance & Hines 4-into-l header. My pipe shows that I am running a lean fuel mixture now. I would like to install four individual velocity stacks. My question is: How or what can I do to riehen my fuel mixture?
Farrell Lawrence Bainbridge, Ga.
Do not, repeat, do not, modify the airbox on an injected Kawasaki. The fuel injection system depends on intake pressure measurements (among other things) to calculate the amount of fuel required. If you make changes in the intake system, you’ll hopelessly confuse the injection, and it’s very unlikely your motorcycle will ever run right until restored to original condition.
If you want to alter the mixture ratio from the injection system, Motorcycle Performance Specialties (MPS) offers a
modification to the injection system that allows some tunability. We haven’t tried it so we can’t say for sure how well it works, but this modification is the only way we know of tuning the Kawasaki injection. MPS’s address is Box 1395, Casselberry, Florida, 3207,
(305) 695-2154. '
If you make major alterations to your GPz’s engine and intake system, it’s unlikely the injection will be able to cope. If that’s the case, you can remove it and install carburetors in its place.
Big Horn intake manifold
I’m looking for a ram intake manifold for my Kawasaki F9 350. Do you know any distributor that would carry this part? If not, could you give me specifications such as length of manifold, degree of angle, jetting of carb, etc, so I could make one?
Dale Dixon
Durham, N.C.
Kawasaki Big Horn 350s have been out of production for more than eight years now, and it’s not likely that you’ll find anyone distributing hop-up parts for them. However, information should still be available on how to make them faster. We’d suggest you contact Woody Kyle at Honda of Winter Haven (2602 Ave. G, NW, Winter Haven, Florida 33880, (813) 293-1279). When Kawasaki 350 engines were competitive in short track racing, Kyle built the best ones. He would be a good source for recommendations on how to give your bike more power, including information on intake tuning.
Fork seal removal
I’ve found a quick and easy way to remove uncooperative fork seals. After removing the fork from the bike, fill it with oil (it’s best to take the spring out first) and replace the cap. Remove the retaining ring over the seal and protect the cap with a suitable socket. Now take Dad’s LTD (automatic transmissions aren’t totally worthless) and have someone hold the fork horizontally with one end against the front bumper. Edge up to a tree or fixed object and push until the seal pops out. Have someone else hold the fork leg because their feet will be wet with oil when the seal comes out. A small price to pay!
Jay Woodall
Lafayette, La.
We’d reserve this as a last ditch effort to remove a stubborn seal, as there are less messy ways that work well (and leave your Dad less irritated by the oil all over his car). If you remove the stanchion tube from the fork leg, and remove the snap ring above the seal, you can use a square shanked screwdriver to pry the seal out. It’s a good idea to put a thin piece of plywood between the screwdriver shank and the fork slider to keep from denting the top of the slider. Tap the seal out lightly, rotating your position between taps.
This technique has worked so well for us we haven’t had to resort to alternatives, but there are special slide hammer type seal pullers available from automotive tool suppliers.
Airbox modifications
# own a 1981 Suzuki GS450 and I thought it would run better if I took the airbox cover off. But to my surprise, it ran much worse.
Is this because the carbs are jetted so lean that they need to be strangled by the airbox cover in order to have the proper mixture? If this is true, would the bike run significantly better if I had rejetted the carbs to run with the open airbox? And how difficult would this rejetting be?
Keith Jensen
Upland, Calif.
Opening up the airbox causes problems not because of lean carburetion, but because the carburetion is set up to work with the restriction of the airbox. Remove that restriction, and even perfect carburetion will then become lean, leading to poor running. If you rejet the carburetors to work without the airbox restriction, a power increase is likely, thought whether the 5 percent or so increase is significant is up to you to decide. You’ll also have to decide whether the power increase is worth the added intake noise; the reason the air box was restrictive was to dampen intake roar.
Rejetting to compensate for airbox changes varies in difficulty: it can be simple, calling for only an increase in main jet size, or it can be very demanding, requiring changes in all carb circuits. That can be a problem if carb tuning parts (needles, pilot jets, and the like) aren’t readily available. It’s not a modification that we recommend unless you’re willing to become a carburetion expert.
Turbo pistons
in your March, 1983 article, Souping the Seca 650 you mention a 761cc piston kit. Can you run this particular kit in the Turbo Seca or would you need to have special pistons? Also, would the compression ratio given by this piston kit be acceptable for the Turbo as it has a low standard compression ratio? Would I have to do any jetting changes with this kit?
Pierre Vanwienen Las Vegas, Nev.
The piston kit we used wouldn’t be suitable for the 650 Turbo. The Turbo couldn’t live with the high compression ratio, and the pistons would probably need thicker crowns to cope with the higher pressures and temperatures in any case. We don’t know of anyone making oversize pistons designed to be used in the Yamaha Turbo, but that doesn’t mean a power increase isn’t possible. On our long term test Turbo, we simply disconnected and plugged the hose leading from the intake system to the carburetor waste gate. That gave a substantial increase in boost pressure and power, and our bike survived thousands of miles with this inexpensive modification.
Japanese parallel Twins
^rom the late 1960s, what Japanese motorcycles were or are built as fourstroke parallel Twins of 650 to 750cc? I know only of the Yamaha 650 and 750 Twins. Did the Kawasaki Z750 fit this category? Are there any built by Honda and Suzuki?
Scot Graham
Minneapolis, Minn.
Kawasaki has actually built two parallel Twins in the size range you mention: The KZ or Z750 in the late 1970s, and the W1 650 in the late 1960s. The Z750 was a unit construction dohc Twin that shared many design features with the Zl. The W1 650 harked back to an earlier time with its separate gear box and push rods; its engine looked very English. The other Japanese bike in this class was introduced last year: the Suzuki GR650.
Chokes and mixture enricheners
I sure enjoy reading your service column each month. Some times I learn something new, and sometimes I get a few laughs. The January column was one of the latter with the first letter, “Cold Start Solution.” The main reason people have trouble starting their Yamahas is that they try to choke them, and Yamahas don’t have chokes. In fact, most Japanese bikes do not have a choke. When you push that cold start lever, or pull it, depending on the model, you open a separate fuel system that gives a richer mixture for cold starting, and it only works properly with the throttle closed.
It is a very good system and flooding is no problem as with a choke. I work in a Yamaha shop and most of the people who bring their bikes in for cold starting problems just haven’t learned to keep the throttle closed when using the start lever.
Ray Martin
Spokane, Wash.
You’re right, and we probably should have made the distinction between starting systems that use chokes and those that use mixture enrichment circuits. Chokes partially block off the carb throat, requiring a higher intake port vacuum for a given amount of air flow. The higher vacuum pulls more gas up through the normal fuel metering jets than would usually be the case, and provides the rich mixture required for starting a cold engine. Enrichener circuits are, as you point out, a completely separate fuel system that bypasses the carb slide or butterfly. They’re basically miniature carbswithin-carbs that provide just the right mixture for cold starting.
The fix that wasn't
I find Mr. Benjamin King's fix given in your November 1983, Letters column for the Suzuki GS55OES's cold blooded tendencies very enlightening. In my years as a service technician, I have yet to find vacuum in a fuel line caused by an improperly fitting spring clamp. A loose fitting fuel line usually results in a
_ - w_ fuel leak, not an air leak. I suggest the fix for this particular problem will be found elsewhere. Michael Rush Magnolia, Del.
We agree; there isn't a good reason why a slightly loose fuel line should lean out carburetion. The most likely explanation for the GS55OES's cold bloodedness is its lean primary jetting, but that doesn't explain why Mr. King's change improved his bike. Perhaps the leak he cured was at the vacuum line connecting the automatic pet cock to an inlet tube. Or perhaps it will remain a mystery.
Seasoned pipes
;~~coiumn on exhaust pipe coatings did not mention oil coating. Dark brown-black thread cutting oil applied four or five times to a pipe that's still hot will leave a protec tive coating. The custom pipe on my XR1000 was oil coated. I have had no rust after 1200 miles, and touch up is easy (just add more oil). The only draw backs are that the color isn't as dark as black paint and isn't as uniform on close inspection. Paul Rogers Los Angeles, Calif.
