Service
SERVICE
We welcome your technical questions and comments, and will publish those we think are of interest to our readers. Because of the volume of mail received, we cannot return any personal replies. Please limit your “Service” letters to technical subjects only, and keep them as brief as possible. Send them to: "Service, ” CYCLE WORLD, 1499 Monrovia Ave., Newport Beach, Calif. 92663.
FINDING PSI
How can I tell what is the correct tire pressure on my, or any other, cafe bike? I ride a stripped down '78 Yamaha SR500 with Dunlop K81 tires, 3.60-19 front and 4.10-18 rear. My bike weighs less than 300 lb. wet and I ride hard, solo and twoup. I weigh 135 lb. and usually carry little or no luggage. I’ve tried all different pressures and notice a loss of handling at severly reduced pressures, but plenty of traction at all pressures. Is there a way to tell by feel or handling? Should I maintain the stock pressures, or reduce by a couple of psi to maintain a similar to stock contact patch?
Mark E. Stull Ashland, Ore.
Motorcycle tires will work reasonably well over a fairly wide range of pressures, *A>ut there are limits. Tires are designed to work in pairs with a given tread profile on each tire, so a severe reduction in tire pressure on a street bike is never a good idea, as it allows the tread to distort out of its intended shape and also causes heat buildup in the tire carcass, leading to possible tire failure. Overinflation can cause a harsh ride and poor adhesion in bumpy corners. Tires work best within their recommended range, with the pressure varied according to the weight the tires carry. Rear tires normally carry more weight than front tires and need higher pressure; add a passenger and the ideal rear pressure goes still higher.
Another variable is tire construction. Tires noted for having relatively stiff sidewalls, like the Michelin M45, tend to provide better feel and absolute adhesion at the lower end of recommended pressures, while tires with slightly more flexible sidewalls, like the Continentals, work better at the higher end.
Yamaha recommends 26 psi front and 28 psi rear for loads of up to 198 lb. on the SR500, tires cold. For loads of 198 to 421 lb., or for solo sustained high speed riding, it’s 28 psi front and 32 psi rear. With your lightened SR, 26 and 28 psi should work fine for most street riding, but there’s no advantage in dropping pressures lower than that. Leaving pressures at 28 and 32 psi, however, is probably the best all around solution. That way you can carry a passenger or ride at sustained high speeds without changing pressure, and it’s unlikely there will be any noticeable change in handling when the bike is ridden solo.
In short, you can fine tune the tire pressure for load, feel and type of tire within recommended pressures, but intentional overor underinflation usually results in poor handling, reduced tire life, or both.
PARTS FOR SUPER RAT
I have a 1977 Hodaka Super Rat MXIOO. I need a left side cover with the gear shifter included, but unfortunately I can’t find any parts for the bike locally. Could you tell me where I might be able to locate some of these parts?
Timmy Creech Selma, N.C.
Try the Speed Spot, West Center Street, Centralia, Penn. 17927, phone (717) 875-3583. They stock a large supply of Jawa/CZ, Maico, Hodaka, Cooper-Am-* mex and Chaparral parts and accessories. If the part is not available from their stock they are willing to refer you to a likely source.
NEW HOPE FOR THE MZ
Spares for MZs? No problem. Ask Mr Nooteboom (March, 1982 issue) to write to my local MZ spares distributor, Barrie Rogers Motorcycles, 163 London Rd., Derby, England, who stocks all parts except frames for the 125, 150 and 250cc MZs. The 175cc models are unfamiliar to me, but 1 assume that parts like points will be common to the other MZ models which are very popular here, partly because spares are so easily available. Service manuals, including English translations of official factory workshop manuals for each model, are also available.
P.J. Gibson Loughborough, England
CHAIN BREAKER
I own a 1977 Honda CB750K. I have a problem, which I understand is not uncommon to this motorcycle. I have had two chains break, both times damaging
my cases. Both chains were of premium quality, I am told. They were always well lubricated and always adjusted properly. After the first break the sprockets, countershaft and swing arm bushings were replaced. The dealer who did the work said the alignment was fine. So far the only suggestion is to install a case saver between the countershaft sprocket and the cases. This would require a smaller sprocket, as there is not enough clearance* with the original. Are there any alternatives?
Skip Cuomo Hasbrouck Heights, N.J.
The first Honda CB750 K-O models were notorious chain breakers, and Honda had to upgrade the chain to prevent further problems. Honda claims there should be no breakage problems on the 77 model as long as a factory recommended replacement from your Honda dealer is used. They also recommend using a staked master link, where the ends of the link are peened over, rather than using a master link with a spring clip.
Another possible source of trouble on the 750s is improper routing of the battery vent tube. If an overfull battery leaks acid onto the chain, as when the bike is leaned over, for instance, the battery acid will quickly corrode and crack the sideplates and pins in the chain, and breakage willsoon follow. Make sure the vent tube is long enough to clear the bottom of the bike between the engine cases and the swing arm.
Case savers will sometimes prevent ease damage when the chain breaks, but are more often merely added to the usual list of damaged parts when the chain bunches up in the sprocket cavity. As you mention* a 17 tooth sprocket is required to make the case saver fit, rather than the 18 tooth drive sprocket fitted on later CB750s. The smaller sprocket may accelerate chain wear, making further problems more likely.
Beyond that, do a visual check of your wheel alignment whenever the chain is adjusted or a new one is installed. The axle alignment marks on the swing arm are not always accurate, and misalignment can cause a binding side load on the chain.
BRAKE STICK
I own a 1976 Triumph T140 with Triumph/Lockheed disc brakes. The rear brake drags excessively, causing the disc to overheat. (A friend’s identical bike has the same problem and overheated the disc to the point that it cracked!) I tried replacing pads, adjusting pedal freeplay, and checked the pistons for sticking in the caliper, as well as bleeding the brakes, all to no avail. A local dealer suggested the caliper mounting plate was bent (a common problem he said) so I replaced it with a beefier one—no results. It would appear that this is a design problem common in this model because Triumph switched to a top mounted caliper in ’77 or ’78 which they are using presently with no problem. What can I do?
Bruce E. Hunt W. Olive, Mich.
While it is possible the problem is in the caliper, if the seals are gummed up or have lost their resiliency, the trouble is most likely in the rear master cylinder. On the 76 TI40 the master is the lowest part of the rear braking system, so all moisture and condensation settle into the cylinder itself, causing rust and corrosion. Even regular bleeding often fails to flush the lowest level water out of the cylinder. The corrosion on the master cylinder wall prevents the piston and seal from returning properly, holding hydraulic pressure on the caliper pistons and causing the pads to drag. The same kind of bottom-of-the-system corrosion often occurs in the front calipers of bikes with discs, especially in climates that favor condensation—damp, or with widely ranging temperatures.
The only solution on the Triumph is to rebuild or replace the master cylinder. Rebuild kits are available, but it is critical on the Triumph to mark the thread position where the steel cylinder screws into the aluminum housing before disassembly. If the insert is not repositioned in the same spot it can cause loss of pedal travel and another case of dragging brakes.
The older T140s did have a problem with bent rear caliper brackets, but this usually causes the pads to be pushed too far apart, making it necessary to pump the brake pedal once or twice to get full braking and a firm pedal.
GS TRANSPLANT
The engine in my GS 1000 has passed away, but the rolling chassis is still in great shape. I can get a GS1100 engine for less than half the cost of any available GS 1000 engines (we’re talking $400 vs. $850). Can this engine be mounted in my frame, and exactly what changes would this entail? The bottom ends of these engines seem remarkably the same. I also like the idea of the rocker-arm adjustable valves and the extra power enough to have considered such a swap when I thought I’d have to pay a premium for it. I’ve just read in your magazine of mounting a GS1100 engine in a Moriwaki frame made for a Kawasaki engine, and hope you might be Jpable to give me some information with my own project.
Gregory W. Lincavage Houston, Texas
Vance & Hines performed a similar swap recently and say it was fairly Straightforward. The GS1100 engine will \fit right in the GS1000 frame, except that the lower left motor mount has to be updated to a mount from a GS 1100 or some material has to be milled off the engine case. At the front motor mounts the GS1000 uses a different mount for the left and right sides. To install the GS1100 jnotor you need either a pair of right or a pair of left mounts—V&H isn’t sure which were used, but you’ll need two of the same type. The only other problem is the exhaust system. Vance & Hines used a custom made exhaust, but with a stock system you may have to modify the fluffier support brackets to fit the GSl 100 mufflers. The headpipes themselves will fit around the frame with no changes. 13
