The Service Dept

THE SERVICE DEPT

JODY NICHOLAS

HONDA HINTS

I have a 1970 Honda CB450. Will either of the following result in damage?

1. Using low-lead gasoline (compression ratio, 9.0:1). If this is not good, would some minimal change, such as retarding the ignition, make it acceptable for commuting and around-town use, i.e., freeway speeds for less than one mile on any given trip?

2. Setting the tappets so that a 0.0015-in. blade fits, but a 0.002-in. one does not, thus allowing more than 50 percent greater clearance than the manual specifies (0.0012). This does not produce any undue noise, but compression readings are only 170 Ib./sq. in. (185 specified as normal). Could the cams or cam followers be damaged?

Andy Hromiko Oakland, Calif.

Certain brands of low-lead gasoline will work satisfactorily in your engine without any changes to ignition timing. You’ll just have to try a couple of brands to find one suited for your machine.

Setting the tappet clearances using a 0.0015 feeler gauge is quite all right if you make sure the feeler gauge is a rather snug fit. It is important that you don’t set the tappets any closer than the recommended setting, but a little more clearance won’t hurt anything.

If you’re really concerned about it, you can buy a 0.03-mm gauge at almost any foreign car parts store.

EXHAUST CLEAN-UP

I have a 1967 Yamaha 250 twin-cylinder motorcycle with automatic oil injection. The bike now has over 8000 miles on it, and after replacing the piston rings and scraping the carbon build-up off the tops of the pistons and out of the exhaust ports, it runs much better.

What I’m wondering about is how can I clean the carbon out of the exhaust pipes and mufflers? Will it help the performance any?

William Barker Chicago, III.

Cleaning the carbon and burned-on oil out of your exhaust system will make your bike run better, and give

better gas mileage as well.

Remove both exhaust header pipes and mufflers. The header pipes can best be cleaned by drawing a piece of motorcycle chain back and forth through them, using a twisting motion. If you’re patient, almost all the carbon can be removed, and you should then blow out the pipes with an air hose.

The baffle tubes should be removed from the mufflers and heated with a torch until they are cherry red. Then tap the baffles against a piece of wood to shake the carbon particles loose, and use a wire brush to scrape off stubborn particles. When the tubes have cooled, a blast of air will blow the rest of the carbon away.

Mix up a strong solution of caustic soda and heat it almost to the boiling point. Immerse the mufflers into the solution and agitate them with a pair of pliers. Rinse the solution off with clear water and gently tap and shake the muffler.

It should be mentioned that caustic soda must be used with care to prevent burns, and care must be taken not to breathe the fumes.

WHICH GEAR RATIO?

In recent weeks among my many cycling friends gear ratio and rpm calculation have beet] discussed. We do not seem to be able to agree on any formula for calculating rpm at given road speeds.

I am confident that a formula published in the Service Department of your fine magazine would not only settle my problem, but would also help those who wish to know how rpms increase according to countershaft sprocket reduction and increased rear sprocket size. Certainly with this kind of information a person could more intelligently set the bike up to meet his needs.

I am an avid reader of CYCLE WORLD. Tor my money it is the best cycle publication on the market. Keep up the good work.

Alfred J. Johnson Atlanta, Georgia

Calculating engine rpm at any given road speed is a simple matter if a couple of constants are known. These constants are the final reduction ratio of the gear you want to compute the speed for, and the circumference (sometimes called rolling diameter) of the rear tire, in feet.

The primary reduction ratio is obtained by dividing the number of teeth on the engine sprocket into the number of teeth on the clutch sprocket. Because many of today’s motorcycles use gears, or a multi-row chain, between these sprockets, this reduction ratio remains constant as different size sprockets (or gears) are not available

So, We’re now concerned with the secondary reduction ratio, or the ratio

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between the size of the countershaft and rear wheel sprockets. We’ll use a BSA Thunderbolt for an example. The Thunderbolt has a triple-row primary chain, so the standard sprockets have set sizes. The engine sprocket has 28 teeth, which, divided into the clutch sprocket’s 58 teeth, gives a primary reduction ratio of 2.07.

The secondary reduction ratio is figured by dividing the number of teeth on the countershaft sprocket into the number of teeth on the rear wheel sprocket. In this case, the numbers are 47 divided by 20, which gives a secondary reduction ratio of 2.35. And to find the final reduction ratio, we multiply the primary reduction ratio by the secondary reduction ratio and obtain a figure of 4.87:1. This tells us that the engine will turn over 4.87 times for each revolution of the rear wheel. If the transmission’s internal top gear ratio is 1:1 like the BSA, the calculations are easy. If, however, the top gear ratio is not exactly 1:1 (say, 0.850:1), the result we first got of 4.87:1 would have to be multiplied by 0.850, which would in turn give us a final reduction ratio of 4.1 3: 1.

Now, to calculate the engine’s rpm at any given road speed, we will need another figure: the number of tire revolutions per mile. This figure is obtained by dividing the rear wheel’s circumference in feet into 5 280, the number of feet in a mile.

As an example, let’s use a rear tire circumference of 6.66 ft. (the circumference of a 4.00-18 Dunlop K70 tire in good condition), and divide it into 5280. We come out with a figure of 793, the number of times the rear wheel will revolve in a mile.

We still need one more figure before we can calculate our rpm at any given speed, however, and that is the number of mph/1000 rpm. We find this figure by multiplying our tire revolutions per mile (793) by the final overall gear ratio (4.87) and dividing by 60. Plugging into this equation we find that 1000 rpm will give us a speed of 15.6 mph. Therefore, at 6000 rpm, we should be traveling down the road at 93.6 mph; at 5000 rpm we should be going 78 mph, and so on.

Unfortunately, it doesn’t work out exactly because of several variables. One is the error which is inherent in practically every speedometer and tachometer on road machines today, and another is the tires’ propensity to “grow larger” as the speed increases, due to centrifugal force and heat which is generated, causing the air pressure to increase.

So, the only really accurate method of determining speed is with an elec-

tronic speed trap such as the one CYCLE WORLD uses in its road tests. But a good indication can be gotten by using the above formulas and having your speedometer and tachometer calibrated.

MACH III TEST REVISITED

Please explain why April '69 CYCLE WORLD gives the Mach HI mph/1000 rpm top gear as 12.8 mph, while this issue (Jan. '71) tests the same hike and, with the same overall gear ratio (5.84:1 in 5th), gives the mph/1000 rpm top gear as 13.0 mph. This 0.2 mph increase also gives an even 100 ft. /min. increase in piston speed at 8500 rpm.

1 note with interest that the new Mach ¡11 is 5.6 mph faster “on top” than the first Mach HI. I assume this is due to the extra 300 rpm that the '71 model will turn? When you give a top speed and list an actual rpm, is that rpm the max that you can get out of the machine?

Scotty Haralson Miami, Okla.

The mph/1000 rpm figure is calculated by measuring the circumference of the rear tire, in feet, and dividing that figure into 5 280. the number of feet in a mile. When that result is obtained, we then multiply it by the final overall gear ratio (5.84:1 on the Mach III) and divide by 60.

Because the 1970 and 1971 Mach Ills had slightly different rear tires, the circumference was also slightly different, which explains why we got two different mph/1000 rpm speeds.

Piston speed is calculated by multiplying the length of the stroke in inches by the red-line rpm, and dividing the result by 60. My guess is somebody’s slide rule slipped, or was misread, on the first road test data panel. But final gear ratio has nothing to do with piston speed at red-line rpm.

The 1971 Mach III just happened to go a little faster through the top speed traps than the 1970 model, but it was just slightly slower in the standing quarter-mile runs.

VIBRATION MEASUREMENT

I suggest that you measure the vibration of the bike that you road test.

This is especially important on road touring bikes. Perhaps you could give us a graph of vibration in vibrations petsecond against rpm of the engine. Thus readers could see where the main vibration comes in.

I feel that vibration causes a lot of trouble with bikes, for instance, light bulbs failing and parts falling off.

Jim Toler Chicago, 111.

There are no standards for measuring the amount of vibration produced by a motorcycle, and converting this mea-

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sûrement into a meaningful “quantity.” For example, sound level is measured in decibels, the unit used for measuring the relative loudness of sounds. One decibel (db) is equal to the smallest amount of difference in loudness the normal individual can detect, and the normal range is from 1 to about 1 30.

Vibration must be measured for its frequency and its amplitude, and then put into some meaningful figure which would be understandable. As you probably know, vibration levels, like sound levels, affect people differently. One person might not feel any discomfort sitting in front of a speaker at a rock festival, but 1, for one, would probably experience pain in my ears. Therefore, 1 feel that just about any vibration measurement that we might be able to make would be meaningless to most readers if it were expressed in some quantity such as cycles per second at an amplitude of so many mm.

In reporting our road tests, if the consensus of opinion of the testers is that the test machine vibrates excessively, we report it by using more common phraseology. We’ve also found that two machines of the same make, year, model and displacement will vibrate at different speeds, and often with decidedly different characteristics. Hence, we feel the measurement of vibration is best left up to the road testers, who report as they feel it.

POLARITY CHECK

/1 while hack in the Service De ’partaient, there was a letter from a fellow in Colorado who complained of being a long way from good service, and you mentioned the possibility of a check to determine whether he’d accidentally reversed the charge polarity on his battery. Here the nearest dealer is 300 miles away, and the nearest reliable service nearly 1000 miles distant.

To check polarity without a volt meter I proceed as follows: prepare two small pieces of lead strip, scraping them bright, and attach two lead wires, one to each pole of the battery.

Trepare a glass of water with a few drops of sulfuric acid. Now attach the wires to the terminals and immerse the lead strips in the acid water on opposite sides of the glass so they cannot touch. With the flow of current, the positive lead will com menee to discolor and, in a little while, will turn brown. Bubbles will arise from both leads, but larger and more frequent ones will arise from the negative lead.

Clinton Thomas Amazon Valiev, Brazil