The Service Department
THE SERVICE DEPARTMENT
JOHN DUNN
Due to the tremendous volume of Service Dept. mail, we are un able to reply to individual letters. We shall continue to answer as many as space permits in this column.
SCIENTIFIC SHIFTING
What is “torque peak” and how can it be determined for my machine? 1 can’t find it in my specs, nor can my dealer. The bike is a 1964 Triumph TR6/SS (English version of the TR6/SR — engine specs are the same as far as 1 can tell). 1 am interested in determining the proper shift points (by rpm) and have seen a formula for this using the ratios of the two gears involved and torque peak. Thank you.
Bruce Hatton APO San Francisco
“Torque peak” is the point in the rpm range at which maximum torque is achieved. This is controlled mainly by valve timing characteristics, and at this speed, maximum volumetric efficiency (breathing capacity) occurs. The torque curve will drop, mainly in direct relation to the volumetric efficiency being obtained. It is also affected by the mechanical losses due to friction and the degree of combustion efficiency being obtained. Maximum bhp does not occur at maximum torque, since bhp — torque x speed x a constant. In fact, horsepower increases with speed up to a point where the torque has dropped below a certain level due to the continued fall in volumetric efficiency.
We suggest that you write to Triumph Engineering Co. Ltd., Allesley, Coventry, England for information relative to the torque characteristics of your engine.
IS IT WORTH IT?
Could you please give me some advice on souping up my small Honda 50 Cub for maximum performance operation on the drag?
To start with, do they offer much potential for speed, compared with what you can do to some of the larger models? Cost does not seem to be /much of a problem.
If you see it advisable, 1 first plan to add the Columbia big bore and stroker kits, modify the head and add a Honda 90 carb. Is there any way possible to get fuel injection for the Honda 90 carb, ready built, kit form, blueprints and build from scratch, or are there any speed merchants who might build or modify an injector for the 90 carb?
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Would you recommend a shorter rod, oversize valves, having the pistons weighed and milled, and other such speed goodies that seem to help the big machines?
What sprocket size would you recommend? Would a hot ignition help any after all other electrical equipment is taken off? What do the professional racers do to get them going up to 80 mph and more?
Sorry to bog you down with so many questions, but I am pretty ignorant on these small motors. This new project of mine seems like great fun and a great challenge. (I’ll send you a picture and the results.)
Charles Kimball Tuscaloosa, Ala.
The Honda 50 Cub (Cl 00) is not equipped with the most satisfactory engine and transmission when considered for your intended use. The Cl 10 unit would be far more suitable and I would suggest that you replace your present engine with one if possible. Better still, if the rules will allow, try and obtain a 65 Sports engine. It may even be worth considering reworking the bore or stroke, or both, to remain under 50cc if this is necessary. This would be a far superior unit, having an overhead camshaft, larger intake arrangement and gear oil pump.
An increase in valve and carburetor sizes, together with an increase in compression ratio would be the correct approach when tuning any of these units.
It would not be an easy task to adapt fuel injection to an engine of this small capacity. To obtain the correct fuel-air ratio, even at full throttle and at maximum torque where the fuel requirement is at its maximum, only a minute amount of fuel would have to be injected. To my knowledge there is no fuel injection equipment available which is capable of accurately metering the very small amount necessary. If you intend to use alcohol, where the fuel-air ratio is not so critical for ultimate performance, a continuous spray system could possibly be developed, similar to the Hilbom injection system.
All the units mentioned are equipped with flywheel magneto ignition systems and will do the job.
If you must use the Cl00, the Columbia Eng. kit is the right approach.
DREAM ON
I am writing you requesting information on the interchangeability of Honda Dream parts. There are very few 1960 Dreams here, therefore the dealer cannot help me.
Ben Dekker
Edmonton, Alberta
The 1960 (model C-71) Honda Dream differs in nearly every respect from the later model (C-72). Only a few minor parts are interchangeable.
We suggest that you write directly to American Honda Motor Co., 100 West Alondra Blvd., Gardena, California, concerning the availability of spare parts and information for the C-71 model.
This model is easily identified by having a separate oil tank and a dry sump lubrication system. All later models have the oil contained in the crankcase.
IT'S NOT WHAT IT USED TO BE
I have a 1965 Bridgestone 90 which 1 use quite extensively. The engine runs well and gives a smooth ride, but while idling it has an irregular rhythm. A friend of mine says that this is typical of all twocycle engines, but 1 think it is due to retarded timing. Is this true, and if so, please tell me how 1 can adjust it?
Yogi Smith Campbellsville, Ky.
For many years, in fact right up to the time when the Japanese manufacturers began to make gigantic strides in twostroke development, it was accepted that one of the distasteful aspects of the twostroke engine was its inability to idle without four or eight-stroking.
This general concept has now been proven incorrect, as shown by the ability of the majority of Japanese two-strokes to idle smoothly at very low rpm. They have achieved this, in spite of increased maximum crank rotational speeds, by careful attention to intake and exhaust arrangements.
Your problem cannot be attributed to this old excuse. It is not unlikely that the trouble is being caused by too much ignition retard. In fact, too much advance or a sticking centrifugal advance could be the cause. Check your ignition timing as suggested by the manufacturer. The problem could also be caused by a malfunction or incorrectly set carburetor idle system, incorrect carburetor fuel level, or a faulty ignition component.
DAMAGED SCREWS
I own a Yamaha 80 YG1-K and am almost completely satisfied, except for the pot metal screws used to hold the engine together. The screws have a philips type head and several have become destroyed by a slip of the screwdriver. Have you any suggestions?
1 plan on boosting power with some of the commercial speed equipment and if so, would a smaller back sprocket be feasible to increase my top speed?
Stephen Hunt Lancaster, N. B. Canada
Removing engine set screws with a philips type head can often be a problem, particularly on Japanese machines. The torque (turning effort) required to break these usually small diameter screws loose, is far in excess of the recommended tightening torque, due to the thread distortion taking place in the alloy engine. A properly fitting driver with a welded on “T” handle is one solution. Before trying to turn the screw, enter the driver into the screw head and give it a sharp tap to make sure it is properly seated. Pushing hard on the “T” driver and turning it at the same time will usually remove the screw. Do not allow the driver to slip. If the screw is still reluctant to turn, repeat the procedure.
To remove screws where the head has been severly damaged, select a drill the same diameter as the screw head, and drill away the head completely. After all other screws have been taken out, the component can be removed leaving the headless screw in position, which can now be removed by using vice grips.
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Hammer operated impact drivers are recommended if engines are being worked on frequently. These take standard 3/8thsinch drive accessories, sockets, philips drivers, etc., and can be purchased through tool supply houses and auto parts stores.
Under ideal conditions, with a smaller (less number of teeth) rear wheel sprocket fitted, a gain in maximum speed will most likely be achieved. However, under adverse conditions such as a head wind, or even a slight up gradient, top speed may be impaired by pulling a gear ratio that is too high. The factory takes all these things into consideration when the standard gearing is selected, and the resulting ratio is generally a good compromise. In fact, most machines are slightly over geared by the manufacturer in the interest of fuel economy at normal existing speeds. Performance can, in most cases, be improved by pulling a slightly lower ratio than standard. This would require the fitting of a slightly larger rear wheel sprocket or a smaller final drive gear box sprocket.
A power increase from the engine would mean that a higher gear could be used; thus a higher speed, without a loss of acceleration.
SMALL BLOWERS
What do you think of the possibilities of super-charging a Honda CB-160? Aside from cost, what major problems would be encountered assuming a boost of half an atmosphere? Do you think the engine would be over stressed so as to greatly reduce its life? Would the increased temperature be detrimental to the engine — especially exhaust valves?
Of course, 1 realize this would be a complex undertaking, but it would be so fantastic that I am seriously thinking of attempting it. If supercharging appears feasible, I believe I can use my engineering background to design a system, and who knows, it may even run.
Your opinions will be greatly appreciated.
William L. Bryant Birmingham, Michigan
Your biggest problem would be obtaining a suitable compressor. To my knowledge, there are no superchargers generally available for a small capacity motorcycle engine. It is possible to supercharge some, of the larger capacity motorcycle engines by employing a unit originally manufactured for small displacement automobiles.
Even by applying 5 lbs. boost, a 40% increase in power output has been achieved in many cases with automobiles and, providing a suitable unit can be manufactured, there is no reason why you should not expect similar results from a small capacity motorcycle engine. With low pressure supercharging, it is a proven fact that the average automobile engine is not subjected to undue stress.
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The most efficient type of positive displacement supercharger is the “sliding vane compressor;” it will provide more effective boost at low rpm than most others. The “Roots” type supercharger, which is also of the positive displacement type, is more easily lubricated, but is not so efficient at low speed.
Contrary to the opinions of some “experts,” I beljeve that the high speed centrifugal compressor has many advantages, even when used on a motorcycle or automotive engine, by virtue of its light weight mechanical simplicity, combined with a high mass air flow and compact dimensions. The theoretical argument against this type of unit for automotive use, is its typical boostrpm curve, where the pressure is roughly proportional to the square of the impeller speed. However, even with a constant drive ratio, it is possible to obtain sufficient boost over a reasonable range that could be acceptable for sports type motorcycles.
FLYWHEEL TRIMMER
I received a suggestion from you on stellite facing the tappets for my A10 BSA Scrambler. It worked great. Thank you.
1 have a series of questions about my 1963 model AIO BSA.
1. One of my flywheel centre bolts ripped out threads and all. It jammed between the crankcase halfs and wore a groove about 3/64" deep in the centre of the flywheel before I could shut the engine down. Could 1 take a cut off the face of the flywheel, thereby lightening it a bit, or would it cut down engine performance?
2. I was planning on balancing the engine. Will a static balance be sufficient? Do you take the percentage of total piston and con rod assembly?
3. 1 am currently running 1 1/2" intake valves with a 1 3/16" monobloc carb. The ports sizes have been increased a bit. What size should the ports be for best performance?
4. Another problem 1 am having is piston-ping under 3,000 rpm. The engine has Wellworthy pistons with 1/8" cut off the crown and Webco track drag camshaft. The spark is retarded from original specs and 1 am burning 130 octane aviation fuel. But 1 still have the ping when 1 load her up under 3,000 rpm. Do you think 1 should go back to 9.5 to 1 and sacrifice a bit of power for possible piston failure?
5. Where do you figure the best place to change your gearing, at the engine sprocket or the rear wheel?
B. Sutherland Winnipeg, Canada
A reduction in flywheel weight may affect smooth engine operation and tractability at low engine speeds. If material is removed from one side of the flywheel, it will be necessary to remove a similar amount from the other side, to maintain the same degree of dynamic balance. Actual out-of-balance forces inherent in the 360 degree vertical twin, are no better than a single cylinder engine.
A fairly high degree of dynamic balance is achieved by the factory when the crankshaft is originally built and, providing the added or subtracted weight is evenly distributed when a static balance is carried out, a reasonable measure of dynamic balance will be maintained. However, to have the assembly dynamically balanced would be the correct approach.
It is generally accepted that the reciprocating mass includes the full weight on the piston, rings, wrist pin and pin locking clips, together with the suspended weight of the connecting rod taken at the small end. To measure the weight of the latter, support the small end in an accurate balance with the connecting rod, laying in a horizontal plane.
The ratio you are using between intake valve and carburetor size is good for top end performance and reasonable volumetric efficiency at maximum engine speed. The ping (detonation or pre-ignition) at full load, below 3,000 is most likely due to low gas velocity and poor mixture distribution.
The Wellworth pistons, with 1/8" machined from the top, most probably provide a reasonable combustion chamber shape from a strictly combustion point of view. A reduction in compression ratio may not be the answer. The pre-ignition may be caused by, as previously mentioned, poor mixture distribution, combined with poor volumetric efficiency at low speed. This latter might be due to cam timing. It would be well worth advancing the camshaft timing to obtain an earlier closing of the inlet valve.
When changing gear ratios, always use as many teeth as possible on the engine and gear box output sprockets.
