Cycle World Service

CYCLE WORLD SERVICE

Better front tire

I own a 1983 Suzuki GS450ED and commute very briskly over the Santa Cruz Mountains daily on a fast, sometimes tortuous route. Having worn out my front tire in 3000 miles, I read everything I could find on

selecting the best replacement. Since the rim is an 18 x 1.60-inch size (a WM-1 ), I seem to be limited to a 3.00, MH or 80, depending on the code used. The only tire currently available in this size is the original-equipment IRC, which rated poorly in your recent tire comparison. Is that all there is? Could I do better in Europe?

Jo Ann Docherty Boulder Creek, California

Yes, you could do much better in Europe (or Japan, for that matter) because of the far greater number of motorcycles sold there that require the 3.00 x 18 tire size. There are, however, several alternatives available in the U.S. that will offer improved performance. They are the Continental RB2, the Avon Speedmaster MK II, and the Michelin M38 PZ2. The Michelin is a racingcompound street tire intended for smalldisplacement roadracing, and would offer the best traction but the shortest life. If your dealer has any trouble locating any of these tires, have him contact Moto-Race, P.O. Box 861 ,Wilbraham, Massachusetts 01095, or call (413) 734-6211. Moto-Race is a distributor that goes out of its way to stock unusual sizes of tires.

CB1100FInterceptor wheel swap

I own a CBl 100F Honda, and have toyed with the idea of putting an Interceptor front end on it. They both

have 39mm forks, so the swap would be easy. Would this amount to anything more than just a waste of time and money? Would the two-inch difference in wheel size make the bike handle any better?

Peter Wildey Middlebury, Vermont

The CBl 100F was designed to work with an 18inch front wheel, and simply switching to a 16-inch wheel without other changes in steering geometry could lead to stability problems. We’d recommend against the switch.

Delayed rpm drop

IVhile working for Yamaha in the mid-1970s, I encountered symptoms of delayed rpm drop to idle on the TX500 twins, very similar to that described in the GS450L owner’s letter in your December, 1983, Service column. The cause on the TX500 was too little exhaustvalve clearance. The solution was to loosen the adjusters an additional .001-inch over the factory spec. I’m not sure why this worked, but once we did it, all our customers with similar problems on those 500s ceased complaining.

Paul Hye Denville, New Jersey

Slow rpm drop to idle speed is frequently a symptom of overly tight exhaust valves. It’s a product of CV carburetor characteristics and engine behavior. Tight exhaust valve clearances tend to drop idle speed; so to compensate the carburetors are adjusted to

open more at idle. But with CV carburetors, an excessive idle opening causes engine speed to hang up on trailing throttle. The cure is to set the exhaust valves to the recommended clearance, or, as you point out for the TX500, a slightly greater than suggested clearance, and then readjust the idle setting.

Bridgestone parts

/ have a 1967 Bridgestone 60cc motorcycle engine, and it needs a new piston and rings. I can’t find parts at any of the local shops. Do you know of a place where I can order Bridgestone parts?

Elmer Coffey Bean Station, Tennessee

Try Marsh Motorcycles,

36 North Road, Warehouse Point, CT 06088; (203) 6237795, or Fox Corporation, 1111 W. Racine, Janesville, WI 53545: (608) 752-9441.

Hot Honda

I am in the habit of switching off (with the kill switch) the engine of my 1980 CB750F at stoplights in town to help keep the engine temperature down. Will starting the engine at a green light and immediately driving away cause engine damage or premature engine wear by not allowing time for oil pressure to build up before placing a load on the engine?

Joe Leon Plymouth, Wisconsin

You are surely asking more of your engine with frequent stops and starts

than with the slightly higher oil temperatures that would come from letting it run at stoplights. We’d suggest staying away from the kill switch, changing engine oil at suggested intervals, and, if you’re still concerned about engine temperature, installing an oil cooler with a built-in temperaturecontrolled bypass valve.

Clean-air Kawasaki

# own a 1983 KZI 100, equipped with the Kawasaki clean air system that adds air to the exhaust ports. What does this system do to the engine? Are there any detrimental effects on performance or engine life? If so, is there any way to disable or modify this system without creating additional problems?

Paul Shook Colstrip, Montana

The charm of the Kawasaki air-injection system is that it has almost no detrimental effects on engine performance. The system relies on pressure fluctuations in the exhaust port to draw air through small reed-valves into the exhaust. There the fresh air can react with any unburned hydrocarbons to reduce emissions. By working in the exhaust, the Kawasaki system allows the use of a slightly richer air-fuel mixture. An enrichened mixture allows the engine to run better, but normally results in an unacceptable level of hydrocarbon emissions.

We would recommend leaving the system alone unless you were building an all-out racing engine; in a race engine, the air injection might interfere with exhaustpipe fine-tuning enough to decrease peak power. But for street use, disabling the system won’t give you any performance gain for your bother.

Two-stroke fan

7"he ideal cycle engine is a two-stroke, of course, so I enjoyed your How Motorcycles Work article on> the two-stroke engine. It does raise some questions, however. You say: “a singlecylinder two-stroke race engine of a given size will put out 10 to 20 percent more power than a four-stroke of the same size.” How does this relate to racing formulas that allow 500cc twostrokes to compete with 750cc fourstrokes? Doesn’t that imply that you should mean 50 percent?

Charles E. Maley Warsaw, New York

We meant what we said: The best two-stroke engines with a powerband suitable for racing might only produce 20 percent more power than the same sized four-stroke made with the best current technology. This assumes round cylinders for both; the latest oval-piston Honda NR engine may be more competitive. A two-stroke engine will, however, be considerably lighter than a four-stroke engine, and the bike it’s in can be lighter yet because its chassis won’t have to carry as much load. So the four-stroke engine will have to make substantially more power than a two-stroke to be fully competitive in straight-line acceleration. Besides, racing rules don’t have to make sense.

Drippy Carb

I am 12 years old and have a 1979 Suzuki DS80 with a problem. Gasoline is coming out of one of the breathers. I think there is dirt or something stuck in the carburetor valve. What do you think?

Manny Tavolilla

Tuckahoe, New York We think you’re right. The float valve in the carburetor isn’t shutting completely, so gasoline is filling up the float bowl and then running out the float bowl vent hose. The most likely cause is a piece of dirt stuck in the valve. The cure could be as simple as draining the float bowl, but it’s possible that the float valve is defective and requires replacement. The problem could also be a symptom of dirt in the gas tank, the cure for which would involve removing the gas tank and flushing it out. Finally, if you install a filter (available at motorcycle shops) in the gas line leading to the carb, you could keep this problem from reoccuring.