Service

SERVICE

We welcome your technical questions and comments, and will publish those nr 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.

XL CARB SWAP

I own a 1980 Honda XL185S and I've been debating buying a bigger airbox and jumping the carburetion up from a 24mm to a 26mm. What kind of results can I expect in the torque and horsepower departments?

G.W. Ogden, Utah

While going to a larger carb is often an easy path to more power on a two-stroke engine, on a four-stroke like the XL185S it may cause more trouble than it's worth. If you take the time to get the needle, main jet, slide and pilot jet exactly right you may get a very small increase in power, but you are much more likely to experience a loss of horsepower and serious fiat spots in your throttle response. The carb on the XL is set by the factory to work precisely with the engine's porting, valve timing, air box and exhaust system. Adding a carb with too large a venturi diameter on an otherwise stock engine will only cause running problems. The XL will run best with the stock carb and airbox, unless you plan other engine modifications, such as a hotter cam, higher compression piston, diff erent exhaust system, etc. Unfortunately, it generally costs less to trade in on a larger, more powerful bike than it does to modify a smaller bike for equal horsepower, and the larger stock bike will provide much greater reliability.

CX SQUEAL

1 recently wrote you requesting help in determining what caused my 1978 CX500 Honda rear drum brake to squeal severely. After writing you, I pulled the rear wheel off again and finally solved the problem. I'll pass the information along so that it might be of help to someone else.

The problem was caused by the two pins which are integrally cast in the backing plate and located directly under the brake shoe return springs. The length of these pins was such that they put a slight bow in the return springs, causing the shoes to cock slightly away from the backing plate. By grinding the pins down so they did not push against the springs the shoes seat properly without cocking and this eliminates the squeal.

Ronald C. Hollett Cincinnati, Ohio

BONNEVILLE NOTES

Really appreciated the test on the Triumph 750 Bonneville (Nov. 1981). Must have read the article three or four times. 1 purchased a new Triumph in '79 and had all kinds of problems, most of which the dealer ignored, i.e. oil leaks, hard starting, etc. However, you must have experienced some of the other problems that are almost universal: (1) pipes discolor very rapidly, (2) tach and speedometer are not rock steady like the Japanese bikes, (3)> the gas cap hole usually spills gas onto an otherwise flawless tank, (4) very poor trade in value on most other bikes.

All of these are important to your readers, especially because of the high price. I finally traded the Triumph for a 1980 900 Custom. All of my problems were gone. I don't miss anything except the handling, the inimitable Twin burble from the pipes and 45 years of tradition. Almost feel like giving Meriden another shot.

G.W. Forrest Madison, Wis.

We did have some of the problems you mention with our test bike. Our Bonneville didn't actually leak or drip oil but, as mentioned in the test, it began to mist and seep from around the tach drive and banjo fittings on the cylinder head. It also leaked gas from the breather hole in the gas cap if the tank was filled right up to the neck (our Harley Heritage had the same problem, only much worse). As to the pipes discoloring, that is a matter of pro and con among motorcyclists; some think pipes are supposed to discolor and others think their pipes are ruined. In the past, the pipes on British bikes always discolored because they were made of light, single-wall tubing where exhaust heat could reach the chrome plating, so everyone expected it. Motorcycle pipes were supposed to be blue near the exhaust ports. The Japanese introduced double-wall pipes, weighing twice as much (with plating generally of lower quality), so now most buyers expect their pipes to hold their color. Depends on which you prefer.

British speedos and tachs have always been notorious for their short life and high failure rate, but fortunately Triumph is now using French-made Veglia instruments. Ours were both accurate and steady, though it remains to be seen if they will hold up with age and high mileage.

As to trade in value, British bikes do suffer, partly because there are few of them around, partly because the market is limited to a small number of interested buyers and partly due to their past record of poor reliability. Their collector status, however, generally keeps them from bottoming out as severely when they get old as many other bikes of the same age. Meriden is working hard to improve their product, however, so we can only hope their efforts will improve the trade-in picture.

CALCULATING SPEED

Please send me the formula which is referred to in the Letters section of your October, 1981 issue used to compute mph for a given engine rpm.

David Sarbey Hollywood, Fla.

Several calculations are used to get the figures in Cycle World data panels. Because speedometers and tachometers are not the most accurate instruments, we measure the rear tire circumference and use that measurement and the overall gear ratios to compute speed in gears and engine speed at 60 mph. The most basic part of the calculation is overall gear ratio, which is the product of the primary gearing times the transmission gearing times the final drive gearing. Using a Suzuki GS650F as an example, the primary drive has a ratio of l.977:1, the top gear is a direct 1:1 and the final drive is 3:1, so the overall top gear ratio is 5.93:1. Using this, the maximum speed in gears is the engine redline divided by the overall gear ratio, times the rear tire circumference in feet, divided by 5280ft. per mile, times 60 min. per hour. Factoring out a few numbers, the equation looks like:

redline tire circ. in ft. -x -gear ratio 88

Using this for our Suzuki example, we have

9500 x 6/70 = p, 5.93 88

To calculate engine speed at 60 mph, or one mile a minute, multiply the overall top gear ratio times 5280, divided by the tire circumference in feet:

top gear ratio X 5280 rear tire circumference (ft.)

ACCESSORY REPAIR

I own a 1979 Honda CB750K with the Hondaline backrest and luggage rack. 1 am very happy with the accessories but have not been able to find replacement hardware for the Hondaline products. All 1 need are replacements for some broken struts and spacers, but the Honda dealer 1 checked with said Honda does not distribute replacement Hondaline parts. Can you help me locate a possible source for these small but important pieces?

James Rice Laurel, Md.

Honda tells us the Hondaline catalog lists separate upper and lower luggage rack brackets and also separate part numbers for the saddlebags and their mounting brackets for the CB750K, so if you find a cooperative Honda dealer he should be able to order any of those components for you. Honda does not, however, break the parts listing down any farther than that, i.e. for individual spacers, nuts and bolts, etc. So if you need one of the smaller pieces, you may be better off making the brackets or spacers yourself, or having a machine shop make them for you. S3