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.
CLUNKY YAMAHA
I purchased a new 1978 XS11 Yamaha this June and with 5000 km on the speedometer I am experiencing a problem with the front fork.
When first starting off. especially over bumpy ground. I get a loose clunky feeling which can be felt through the handlebar. At very low speeds going into turns, if a crack or bump in the pavement is hit, I get a slight wobble in the fork. At moderate or high speed there seems to be no problem.
The oil was changed in the forks at 3000 km as per the service manual instructions using the correct amount and viscocity. There are no oil leaks from the seals.
I have checked the steering head bearings for tightness and adjusted as advised by Yamaha. 1 could find no torque setting for the bolt and had to play this by feel. With the front wheel off' the ground, grabbing the forks does not produce any movement that would indicate the steering head being loose. I have no accident damage in
the forks. There is no dealer in the area, your advice would be appreciated. D.G. Frederick Happy Valley, Labrador, Canada
Although the XSll's front fork provides an excellent ride, it isn 7 the greatest for torsional rigidity and is the weakest link in the Yamaha's chassis. So even at its best a hard rider will fnd fault with the fork when he's charging over bumpy bends.
However; it shouldn't display the clunky feeling you're getting and the only way to eliminate it is by checking out the whole front-end assembly, bearing in mind that the fork yoke pinch bolts have to be tightened in a specific order.
First, slacken the axle clamp bolts on the left slider along with the top triple clamp bolts, plus the steering stem top nut and clamp bolt. Tighten the main wheel axle nut (76 lb.-ft.) so that the wheel is pulled against the spacers and fork slider. The bottom triple clamp bolts should now be tightened to between 12 and 16 lb.-ft. (1.7 to 2.2 kg.m.) ensuring that the tops of the legs are adjacent to the upper surface of the top triple clamp. Now adjust the clearance of the steering bearings so that the fork will just swing from a center position without binding when the wheel is clear of the ground.
The fork legs can then be aligned by pumping the suspension with the front brake on and as a second check, viewing the parallelism of the legs by eye from one side. The steering stem top nut can now be tightened down and the clamp surrounding it bolted up to the same torque as the bottom triple clamp bolts (they're the same size, 8mm). This done, the top triple clamp bolts can be tightened (again to 12 to 16 lb.-ft.) along with the wheel axle clamp bolts (front, then rear—14.5 lb.-ft. or 2 kg.-m.)._>_
Asa final check, go over the brake caliper bolts for tightness as welt as the fender supports—the clunking could come from there. And on the stock Bridgestone front tire, the pressure is critical for the best handling. This should be betw een 28 and 36 psi.
ILLINOIS TIPS
I read your Service column with sincere interest each month but haven't seen anything to help me with my current problem-replacing mufflers on a '73 Honda CB350 and a '72 CB450.
Can you tell me how I might detach my current deteriorated mufflers from the otherwise good exhaust pipes and replace with a suitable muffler? I'd like to avoid the high-priced, complete exhaust pipe and muffler replacement Honda has available.
Any info or any place you can direct me for this would be appreciated. I’m looking for a more economical replacement and the size and length is secondary.
C.A. Rhines, O’Fallon. 111.
There are several possible muffler replacements you could use, in particular those made by Jardine, MCM or Dunstall, but we couldn't guarantee whether these would be available from a local dealer. And it is important to try the muffer for size before you buy. So that cuts out mail order.
We suggest you contact Russ Johnson at Nichols Motorcycle Supply (4135 W. 126th St., Chicago, III. 60658', (800)323-4488). They are wholesale suppliers of mu filers in Illinois and should be able to put you onto the nearest dealer with a wide range of stock.
BIG K BREATHING
I own a '77 Kawasaki Z1000 w ith an RC 4-into-l pipe and a Martek 440K ignition. In your recent articles on the Dunstall Suzuki and the 750cc Suzuki endurance racer you sparked a desire to go to Mikuni's 29mm carburetors with, possibly, stacks.
My questions are: Do you think the Kawasaki’s head would benefit from such a modification without a port and polish and would this adversely affect low speed and mid-range response and smoothness?
I read somewhere that the Z’s head seldom needs work except in radical modifications. Is this true or false?
K.A. Riley, Lincoln. 111.
In short, the answer is yes. You can expect a marked improvement in top-end power by the simple replacement of the stock 26mm Mikunis on the '77 KZ1000 with a quartet of 29mm Mikuni smooth bores, particularly if you remove the air intake silencer and filter and accordingly jet up the carburetors.
Better breathing on the smooth bores results from not just the larger throat diame-> ter. There’s a packing piece inside the throttle slides that minimizes the interruptions in air flow found on the conventional Mikunis. However there’s a catch if you want to use the bike on the road. The smooth bores have elementary idling circuitry since they are specifically designedfor wide open performance. But if you can handle the lumpy metering at town speeds, go ahead.
A more elegant alternative is open to you, though. You can use the 28mm Mikuni carburetors fitted to the pre-KZ900 models and the ’79 KZ1000. Much of the restriction in top-end power on these models is caused by the air intake so if you use the recalibrated 28mm carbs with stacks you ’ll get the best of both worlds—performance and manners.
The head on the KZ1000 is more than adequate for road use. It’s only when you want it to breath the amount of air that 31mm CR Keihins can flow for racing that modifying the porting shows gains.
TRADE, NOT TRICK
I wish to get more travel out of my Yamaha DT250’s shocks. I only want about 2 in. more travel and I don’t really want to get a new swing arm.
I would like to make the 250 into a 400, but don’t want to replace the engine. The only differences other than the bore and stroke are the alternator and ignition.
The 250 uses a magneto and the 400 uses a CDI system. Do I have to make these changes or are my systems sufficient? To save the expense of making a 400 how could I get optimum performance from a 250 engine?
Can I bore out a 250? What kind of carburetor and expansion chambers, etc. can 1 use?
M. Kwazinski, Chicago, 111.
We hate to say this but the amount of time, effort and money that you ’ll put into improving your DT250 Yamaha will be much greater than simply trading it in for a DT400 model.
Projects of this sort inevitably founder in a sea of hidden rocks. For example, increasing the rear wheel travel will provoke problems with the now inadequate front fork, so that will have to be uprated. And the similarities between the 250 and the 400 are more subtle than you might imagine. We know people who’ve tried it the hard way. Believe us, it doesn 7 pay off.
YAMAHA ECONOMY
I am writing you regarding the optimal engine rpm for mileage, driveability and prolonged engine life for my ’78 Yamaha XS750E. I have heard several theories on this subject, but would like your opinion.
One theory has it that high revs are best (over 5000) for normal driving. Proponents> of this theory claim the following:
1. Faster circulation of oil, thereby eliminating hot spots in the engine and reducing overall temperature of the engine.
2. Higher volumetric flow of air through the engine and therefore a “breathing cooling effect.”
3. Piston speeds more appropriately match flame propagation speeds therefore reducing and lugging detonation (knocking).
The other theory says that you should drive or run the engine at the lowest rpm possible without lugging the engine. In my case with a three-cylinder 750cc fourstroke, an rpm around 3500 to 4000.
Proponents of this theory state that it is best to avoid needless high engine revs. If the engine isn’t under load, keep the engine revs down. The theorists claim:
1. The higher the rpm, the higher the piston and bearing speeds resulting in wear. Also piston travel increases.
2. Higher engine revs cause greater vibration which is annoying and can be damaging.
I’m sure you've had this letter before but I can’t find it in the past couple of year’s issues. I don’t expect a magic answer.
C. Scott
Minneapolis, Minn.
There can be no hard and fast rules when trying to obtain the maximum mileage and life from engines since they can be so variable in construction and character. For example, you couldn't apply the same riding techniques to a 125cc two-stroke as you would a 1200cc four-stroke V-Twin.
The best way to run your Yamaha is to treat it as though you were breaking it in. This means running the engine at as light a load as possible never using, say, more than half throttle except when you need hard acceleration for overtaking.
Since your engine produces more torque at higher revs (the max is 6500 rpm)you will benefit from running it to around 5000 or 5500 when going up through the gears as opposed to lugging it. Nothing will be gained (except thrills) from taking to the red line because although the engine is making more power it's in fact less efficient on fuel.
It is important to use the gearbox to keep the engine spinning. If you don't get response from a properly tuned engine then change down a gear, it'll run much more economically.
However, the speed at which you cruise the freeway depends on whether you want to pay for the extra fuel that high speeds demand. Running at higher revs will only wear out you and the engine prematurely.
As a postscript, we once saw an engine stripped that looked as if it 'd never been run. You could still see the machining on the pistons after 6000 miles. The reason? The owner had never taken it over 4000 rpm and never run it hard. El
