Service
SERVICE
We welcome your technical questions and comments, and wi//publish those we think are of interest to our readers. Because of the volume of mail received, we cannot return ani.' 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., Ne14 port Beach, Ca4f 92663.
TILTING FOR TRAVEL
My Suzuki TM100 (1975) could use a few more inches of rear wheel travel, and I'd like to remount the upper shock bracket, farther forward and lower. I notice that on most bikes the shock is mounted at a junction of two frame tubes, except for my buddy’s, a used CR125 with the shock mounted on a straight frame member.
Is this a weaker spot, and is it hazardous to the frame?
Frank Lagace E. Townton. Mass.
Before the factories went to long travel suspension, tipping rear shocks forward used to be the only way you could get more wheel travel. Nice to see the art is still being practiced.
But it isn't easy. First, make sure you have enough clearance between the tire and the rear fender, frame loop, etc. If your tire will rub before the shocks bottom, you're gonna be in trouble.
Now that you know how much more travel you can allow, take off the springs, bottom the shocks and push the bike down. (Note: Better put the thing on a milk crate bejore you dismantle the suspension. )
The upper shock mount should be where it will let the suspension bottom bejore the tire hits something. If that place is a frame tube junction, ftne. The joints are less likely to bend than a straight tube is. IJ the right place is a straight section, you can stiffen the mount by splicing in another piece of tube, running as nearly straight forward, from shock mount toward steering head, as you can manage.
SMOOTHING THE IDLE
Following Yoshimura’s instructions for better low-end and mid-range, I modified the carb slides of my ’73 Z-1 Kawasaki to ’75 specs.
It works, except for the idle. Once the bike warms up it runs too rich and the idle climbs 500 rpm. Pilot jets and main jets are unchanged. I’ve tried going to slides with leaner cut-outs, but the idle speed still changes and the engine went too lean off idle. Help!
Chris Barber Greensboro. N.C.
You've made internal changes in the engine, right? Fine. W hat you need to work on are the pilot jets. They're what control the idle. Go leaner, one step at a time, working with the adjustment screws between
changes. Then, when the idle is rig/Il it/z the engine warm, you might, repeat might. need to try different slides, but because the `75 slides worked beJbre, we'd guess they `re right.
SUZUKI INTO SUZUKI
The engine in my 1976 Suzuki GT550 is getting a bit tired. And I have a chance to acquire a completely rebuilt 1978 Suzuki GS550 engine. Will it bolt into my 1976 frame and if so, what troubles or changes should I look forward to?
Frank Guido New Hartford. N.Y.
What you have there is a basic engine swap. When Suzuki changed from the twostroke multis to the Jour-stroke multis, they changed just about everything. The GS frame is completely different from the GT frame, and the Jour-stroke 550 Tour is taller than the two-stroke 550 Triple. It could be done. As the racing shops say, all you need is the money. But we wouldn't recommend it. Reworking the frame, making the various pieces ft, installing new controls, etc., is lots of work, and more than the average home mechanic can do properly.
You'd be money and time ahead rebuilding your GT, or trading up for a GS.
100 MPH WOBBLE
Last summer I bought a new 1979 Honda CB750K. I’ve put about 4,000 miles on it, and there is only one thing that bothers me about the bike. The engine has amazing power, the bike is very fast and handles nicely, but just over 100 mph the things starts to wobble. I can’t understand why Honda would put an engine capable of good acceleration to 125 mph in a frame which is only stable to 100 mph. Is there anything I can do to get rid of the wobble? Could it be my riding style? If you have no suggestions this will be the last Honda I’ll ever buy.
Terry Matthews Kitchener, Ontario, Canada
The high-speed wobble you describe can be caused by many things, including accessories and luggage. Does your bike have a handlebar-mount fairing? How about a luggage rack with 10 or 20 lb. of luggage or gear strapped on? Many handlebar-mount fairings can cause wobbles, and the only cure is to remove or replace the fairing, or else come a little closer to obeying the speed limit. A luggage rack is probably the worst possible place to carry loads. In a demonstration given by the Continental Tire Co. at a motorcycle rally in Germany, a 20 lb. weight on a luggage rack changed the wheel loading and started a BMW R65 wobbling at about 40 mph. After the same weight was moved to a tank bag, the wobble disappeared. ,
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If your bike is stock and wobbles at speed, check wheel alignment and tire wear. Assuming that the rear tire is stock, note whether or not it still has the original profle. If the tire is worn flat in the middle from straight-line riding or dragstrip-sty/e burnouts and fast starts, it could make your bike wobble. Fitting a non - standard, than-stock tire could also cause the trouble, and a new stock tire should stop the wobble.
If that doesn 7 do it, make sure your bike 's rear shocks are not worn out. To test them, have a friend hold your bike's handlebars while you push down fast and hard on the seat, letting go as the shocks compress. Do the shocks rebound and stop, or keep bouncing? Another way to detect worn out shocks is to go through a bumpy corner. If the rear end of the bike bounces up and down (like a pogo stick), then the shocks need replacing, preferably with a set of aftermarket high performance shocks with a longer life than the stockers.
Next, check for swingarm bushing wear. To do that, put your bike on the centerstand, remove the rear wheel and unbolt both shocks. Then grab the swing arm and pushpull it from side to side. If you can detect side-to-side play, replace the bushings. Instead of using the oil-less nylon (plastic) bushings, get a set of bronze high-performance bushings from Ontario Moto Tech Corp. (6850 Vineland Ave. ^16, North Hollywood, Calif. 91605). The bushings must be accurately line-bored after installation, a job which can be done by Ontario or any competent machinist.
If the rear tire has a like-new prof le, the wheel alignment is perfect, shocks good and the swingarm bushings taut, make sure that both stock rear view mirrors are on the bike. In some cases, a bike will wobble with one or both mirrors off' and not wobble with the mirrors on. If all else fails, try tucking in when you exceed 100 mph. That probably won 7 stop the wobble completely, but it may not be as bad.
A caution about riding out wobbles. A speed wobble will be made worse by slamming the throttle shut. If your bike is wobbling, gently roll off the throttle and apply the rear brake, don 7 chop the throttle and grab a big handful of front brake. We’ve never been in a wobble so bad that we couldn 7 get stopped without crashing—and we've seen the scarev side of some big tankslappers—but presence of mind and a steady throttle hand are required.
TUBELESS RIMS
I have a 1978 Yamaha 750 Special. The bike comes with cast alloy wheels and tube-type tires. 1 am now considering installing tubeless tires on front and rear. Can you foresee any problems with this changeover?
Gary Lefebvre Flushing. N.Y.
Yes, there's a problem. Yamaha's cast w heels aren 't air tight and aren 't designed to use with tubeless tires. Cast wheels can be either air tight or non-air tight, depending on the casting density and coatings applied to the inner surface. The Yamaha wheels just plain won't hold air without a tube installed. It is possible to coat the inside of the wheel with a sealant available at many automotive supply shops, but it would be better to use a tube and not risk sealing problems of the tubeless tires on the wrong rim. Even if you have a tubeless-type tire you want to use on the Yamaha, you can install a tube within the tire and run the tires.
PINGING YAMAHA IT400
I purchased an IT400F Yamaha in December. My only problem with the bike is that after several flawless desert races this year I have a loud pinging in the barrel. My cousin also bought one of these motorcycles but his doesn't ping like mine. Could it be the oil 1 use. Pennzoil two-stroke?
Gary Muleonery Studio City. Calif.
The loud pinging in the barrel could be caused by improper carburetor jetting or a faulty CD I ignition that's advancing too Jar. Check the jetting first. If the pinging is noticeable in the mid-range, from one-quarter to three-quarter throttle, raise the needle in the carb slide by lowering the clip one notch. If the problem only occurs during full-throttle operation, put in a larger main 'jet. If the pinging continues with several sizes larger main jets installed, the problem is probably in the CD I unit which is advancing too quickly. This is assuming you are using the correct plug and it's gapped according to specs.
WATCHING THE BACK DOOR
I own a Honda CB400F and it’s a great running bike. Around town the mirrors on the bike are stable (mirrors are Sudco), but out on the highway at speed the mirrors vibrate really bad and it’s hard to make out cars behind me. When I want to see what’s behind me, 1 have to steady the left one to get a clear view of the car or truck back there. Is there anything that can be done to get rid of the vibration?
Paul Snyder
Dallas. Texas
The major motorcycle manufacturers have been trying to eliminate mirror vibration with various techniques and some of them have been successful. Honda in 1979 introduced mirrors on the 750s that had hollow shafts and weighted mirror heads that effectively eliminated vibration problems with the mirrors on those hikes. This year Honda has put similar mirrors on other models. We've tried mounting the Honda hollow shaft mirrors on other brands oj motorcycles and found them to effectively cancel the vibration problems. They just might do the trick for you. Suzuki is now using a similar hollow shaft mirror, hut before going to the hollow shafts Suzuki used a rubber insulator on the mirror stems. The rubber insulators didn 't work as well as the hollow shafts, hut they could help alleviate the problem.
FRONT END SHIMM
Got a problem with my 1977 Honda 550K3. The front end shimmies between 40 and 55 mph whenever I let go with one £iand or hold the bars loosely. It’s mueh worse during decel and if you relax your .grip completely it will shimmy quite heavily from 52 down to 30 mph. I have owned the machine since new and it has never been hit or dropped and now has 12.000 mi. on it.
Second question. Here in Germany, îionda sells a 902cc CB900FZ Bol d’Or. Do you know if these will ever be sold in the USA? From what you know, are they superior/inferior to the machines sold in the USA?
AÍ Poliakoff APO. N.Y.
To start with, check wheel alignment and don 7 rely on the adjustment marks stamped into the swing arm—they’re often inaccurate due to production tolerances. Next, check the swing arm bushings for play, and make sure that the rear tire isn 7 worn flat in the middle. Make sure that the steering head races are not dimpled. If a non-stock rear tire is fitted to the bike, make sure the profile *isn 7 flatter than that of the stock tire.
Aftermarket accessories could cause the problem, particularly a fairing or a luggage rack. To be sure, remove the accessories and try the bike without them—in one case, simply the weight of the rack itself upset a CB550K. Finally, don’t ride with your hands off the bars. The condition you describe, while annoying, is common to certain Honda models from about 1970 to 1977, and isn’t dangerous unless you ride with no hands on the bars.
As for the 900, you can expect to see that bike or a variation in the U S. in the near future. Vehicles imported into the U S. must be certified as meeting U S. emissions standards, a lengthy and expensive process which is best left to the vehicle manufacturers. But you could buy a certified, U.S.market CB750F and install a 900cc crank and big-bore pistons.
