Service Department
SERVICE DEPARTMENT
JOHN DUNN
DESMO QUIZ
How does a desmodromic head work? I know that it is a system for opening and closing the valves that does away with pushrods, rocker arms, etc., but I can’t figure out just how it does it. I studied the drawing of the 125 Ducati in the December CYCLE WORLD, but I ended up more confused than ever.
Mark Rasmussen Harlem, Montana I won’t attempt to get deeply involved in this subject at the present time, but only give a brief description of what desmodromic valve operation implies. The valves are mechanically opened and closed and there are no valve springs. A conventional cam opens the valve, and the valve is returned to its seat by a special type of rocker arm that is actuated by a secondary cam. Actually, both cams are synchronized and control the position of the valve throughout its operating cycle. Desmodromic valve gear is very expensive to produce, and all components have to be manufactured to very close tolerances. Some designs have employed a light return spring to compensate for expansion variations within the system.
SLOW TIRE
I own a 1965 250cc Yamaha Catalina YDS-3. It has 6,395 miles on it and is stock, except for a 4.25 x 18 4-ply tire 1 put on the rear wheel.
I get 73 mph for top end!
I cleaned the motor of carbon and changed oil every 1,000, so what is my problem? Is it possible to put carburetors from a Yamaha 305cc on my 250cc? Also, can I put a smaller sprocket on the rear or in the gearbox to get better top end? What can I do to hop up my dead bike? 1 have never “rodded” very much, so what is wrong?
Mike Christenson Montevideo, Minn. I cannot think what prompted you to employ a 4.25 tire, but would advise that by doing so, you have increased the effective engine speed to road speed ratio. If you persist in using the big tire, I would advise that you lower the overall gear ratio by fitting a rear wheel sprocket with four more teeth than standard. The reduction in speed is most definitely due to the effect of the big tire, making the machine too highly geared. I would suggest that you resolve your present problems before contemplating the use of large carburetors.
LEAVE ’EM IN
Could you please inform me whether taking out my baffles really gives an actual horsepower increase? Does it change torque any, thus giving better acceleration? I realize that better gas mileage and speed can be obtained, but 1 was wondering if a rumor I had heard about “getting five hp extra” with the baffles out was true.
Does taking out the baffles alter the back-pressure to which the engine is set, and thus harm it at all?
My cycle is a Honda C95 (1966, 150cc). Would the foregoing be any different when applied to a two-stroke 250cc machine?
Derwyn Lea Killarney, Manitoba Canada
Canada
I am sure you are aware that we do not advocate or encourage the use of motorcycles on the street without silencers fitted. The exhaust system employed on the majority of modern motorcycles is very efficient. Years ago, removing the silencer and employing a turned open pipe did produce very good results from a performance point of view, because the manufacturers did not pay too much attention to developing an efficient system for street use. Their main intention was to cut down the noise level.
On today’s machines, the exhaust system, including silencer, has been very highly developed. The removal of the silencer and rejetting will not produce an improvement in performance. Actually where twocycles are concerned, a vast reduction is often experienced when the exhaust system is removed or tampered with. So, leave the perishing thing alone; there is enough prejudice against motorcycling by the uninformed public.
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REBALANCING
/ am hoping you can give me some information and advice about my Jawa 250cc Sport motorcycle. As always, when information is requested there is a goal, and in my case it is a higher cruising speed.
The top speed of my particular machine is near 70 mph and is sufficient for my needs. However, there is a vibration point reached at about 55 mph (4,200 rpm), that limits the comfortable cruising speed sharply. I have been unable to find out if this is the norm for this engine, but I presume that it is. Therefore, to achieve the increase in cruising speed, this vibration point must be moved either in relation to the road speed or in relation to the engine rpm. The first movement could be achieved by using a different gearing. The second movement could be accomplished by changing the engine balance.
1 feel that perhaps a combination of the two ideas might give me what 1 am looking for. For instance, a slight increase in overall gear ratio, say one tooth on the countershaft sprocket, and a small change in the balance point of the engine, plus slight engine modifications, such as matching the ports, and filling the flywheels, etc.
Larry J. Barnhart
Marion, N. C.
You have obviously given your problem a great deal of thought. Rebalancing an engine, using a different balance factor, should not be treated lightly; however, I agree with you, that it will probably be the only solution to the problem. There are no definite rules for determining the balance factor to be used. This value varies greatly, being influenced by engine and frame design characteristics and also the method used to mount the engine into the frame. Some manufacturers employ a cylinderhead steady - a stout bracket that is bolted between the cylinderhead and the frame. This improves engine and frame rigidity slightly, and in some cases, changes the amplitude of vibration to a level that is more acceptable. Rubber mounting the steady where it bolts to the frame can, in some cases, produce encouraging results. If your machine is not equipped with a head steady it is definitely worth experimenting with.
I do not know the balance factor of your particular engine. It could be anywhere between 50 and 75 percent of the reciprocating weight. This can be checked before you start to rebalance the crankshaft assembly. The reciprocating weight is generally accepted as being the total weight of the piston, including rings, piston pin, pin clips, and a percentage of the con-rod weight. To obtain this weight it is necessary to weigh the small end of the rod. Place the crank (flywheel assembly) on a flat surface with the crank pin positioned at 12 o’clock. Position the rod in a horizontal plane, with the small end resting on an accurate spring balance, which will then indicate the percentage of rod weight required. Add this weight to the total weight of the piston and related components, the sum of which is the total reciprocating weight. Place the crankshaft on a pair of parallel straight edges. If the assembly is rolled gently on the straight edges, it will always come to rest with the crankpin at 12 o’clock. Hang weights in small increments from the small end of the rod until the assembly is completely balanced (the crank pin coming to rest at no specific position). If the weight you have hung on the small end is half the weight of the previously checked reciprocating weight, the balance factor will be 50 percent. If the weight were two-thirds of the reciprocating weight, the factor would be 66 percent and so on.
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If you desire to change the balance factor it will be necessary to either lighten the heavy side of the crank or add more weight. This can be achieved by drilling the rim of the flywheels. This must be done equally on both wheels to maintain good dynamic balance. When experimenting, treat 50 percent as being a minimum value and 80 percent a maximum. I suggest you work in increments of 5 percent until the vibration period is moved out of the normal operating speed range of the engine. For experimental purposes, it is a useful practice to tap the holes drilled in the flywheel rim so weights of different values can be screwed in to change the percentage of balance. If holes are drilled in the crankcase wall so that they line up with the balance holes in the flywheel, the factor can be changed easily without dismantling the engine. On a two-cycle engine, access can also be basically obtained by just removing the cylinderhead and barrel.
If this work is successfully carried out, it will not be necessary to make a change of gear ratio to cure your vibration problems.
SOMETHING OLD, SOMETHING NEW
I’m planning on buying my first bike. Not knowing much about them, 1 had intended buying a used one until I began reading CYCLE WORLD. SO much importance seems to be put on breaking in a bike properly that I don’t know which way to go. Either I buy a used one and risk the chance it may be a lemon, or buy a new one. So, what I need from you are tips on how to keep from getting burned when buying a used bike — also what danger signals to look and listen for when breaking in a new one.
I’m thinking primarily of Triumph street machines: TR 6 or Bonneville (old); T100R (new).
Leroy Hollis
Wilmington, Calif.
Your lack of experience (not having owned a motorcycle) will put you at an obvious disadvantage, whether you purchase a new or used machine. It would be of great help to you if you had a reliable friend with a sound knowledge of the type of machine you intend to purchase.
Breaking in a new motorcycle is largely a matter of experience and feel. Also, during this period it is most important that the whole machine be kept in proper adjustment and tune. The break-in period can be very frustrating, even for an experienced and knowledgeable rider. During this period it is not uncommon for several minor malfunctions to occur, which, if overlooked and not attended to, can cause permanent damage. I have always considered that it is best for a newcomer to start with something older, particularly in the big machine category. However, my line of thought does not always hold good, for I have known a few who have started with new large displacement machines and carried out fine jobs of break-in, and in fact, taken to motorcycling like the proverbial duck to water.
When purchasing a used machine, a good indication as to how it has previously been cared for can be obtained by looking at the following points:
1. General appearance — does it appear to have been crashed (dents, grazed front fender or head lamp, bent foot pegs, torn seat, etc.)?
2. Check wheel alignment. This will give an indication of frame alignment, damaged forks, etc. Check wheel bearings for play.
3. Spin both wheels to see if they are true and check for loose or missing spokes.
4. Condition and adjustment of the rear chain often provides a good clue regarding the type and frequency of routine maintenance.
5. Look for evidence of oil leaks from the engine, gearbox, primary drive case, oil tank, fittings and oil lines. Check oil level and condition of oil, Also, look for any leakage from the front and rear shocks; leaking rear shocks will usually require replacing.
6. The general condition of the heads of nuts, bolts and screws will often indicate whether non professional repairs or maintenance have been carried out. Also, look for any broken bolts or studs.
7. Inspect the fuel tank for leakage, checking welded seams, mounting points and where the fuel tap bosses are welded.
8. Note the angle of the brake actuating arms (at hub);with the brakes hard on the angle of the lever should not go past center. If they do, this indicates worn brake linings.
9. Inspect the condition of all control cables. A frayed inner cable will fail in the near future, sometimes with dangerous results.
10. Check battery level and note whether there is any corrosion of the terminals, the battery carrier and other components in the immediate vicinity of the battery.
11. An excessive amount of gasket cement around an engine cover will usually indicate that some unprofessional work or investigations have been carried out.
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12. Check over the electrical system, making sure that the lights and horn are working. With the engine running, check that the generator is working.
13. When the engine is warm, listen for any unusual noises. (If you do not have a knowledgeable friend with you, base your judgment on previous experience obtained by listening to engines of the same type.)
14. If you have the opportunity to ride the machine, try accelerating from a low speed in top gear. Any loud heavy noises (knocking) from the engine will usually indicate an adverse engine condition. Accelerating hard will show up any clutch slip.
If you purchase a used machine, have it completely serviced and adjusted at the first opportunity.
PISTON ACCELERATION
I wrote to Mr. Jennings in January, 1966, about a “Technicalities” article he wrote that month on piston acceleration, but I received no reply. So, I am again writing, this time to you, for the same reason. What is the mathematical formula for piston acceleration?
John V. Lortie
Thief River Falls, Minn.
The following formula was developed by Hepworth and Grandage (piston and piston ring manufacturers) for computing the maximum piston acceleration of an engine. Piston accelerations in excess of 100,000 ft/sec2 produce inertia forces on the reciprocating parts which, in turn, limit the maximum crank shaft speed of a given engine design. They also have a limiting effect on piston ring control. This figure (100,000 ft/sec2 )cannot be applied generally, because it is mainly proportional to large displacement cylinders (500cc per cylinder).
The Honda 250cc four has a theoretical piston acceleration in the region of 180,000 ft/sec2 at maximum operating speed. With the smaller cylinder displacement (62cc in the case of the Honda four) a higher value can be employed.
The main factors influencing maximum piston accelerations are engine speed, stroke, and connecting rod length between centers. Altering any of these factors will alter the value of the MPA.
MAXIMUM PISTON ACCELERATION
N2 x S 2189 (1 +
Where
N = Engine rpm S = Stroke in inches n = Connecting rod centers/stroke ratio
A hypothetical engine operating at 7,000 rpm and having the following dimensions would have a maximum piston accelera-
tion as demonstrated in the following computation: N = 7,000 rpm S = 3.00 inches C — 6.00 inches n = 2 to 1.
_ 7,000 x 3.00 1 2,189 (1 + 2x2
— 83,900 ft/sec2
When the bore and stroke of an engine have been fixed, the piston acceleration can be adjusted by altering the center to center length of the connecting rod. A longer rod will decrease the value of Z and a shorter rod will increase the value. The same engine as demonstrated above is now shown with a 5.00-inch connecting rod instead of the 6.00-inch rod. It will be noted that the value of Z has increased.
„ _ 7,0002 x 3.00 , 1 Z~ 2189 (1+ 2 x 1.67 ) — 87,200 ft/sec2
OIL SEEPAGE
I presently own a 1966 Yamaha YM-1 350cc. My problem is an oil leak which first appeared when the machine had been ridden 200 miles. Present odometer reading is 2,000 miles, and after three checkups by my dealer, the old leak still persists. What actually is causing the problem is the right crankcase half which, when the engine gets hot, slowly lets oil seep out at one particular spot. The harder the bike is ridden the more oil I lose. This has seriously affected my oil mileage, which is about 150 miles a quart when the bike is ridden at a steady 65mph. 1 wonder if you could help me solve my problem. The only solution I can think of would be to replace the right crankcase half.
I wonder if there are any products on the market that would stop such a seepage when applied to the crankcase.
Kurt Michl Posen, III.
From the contents of your letter I would assume that the oil leak is being caused by a porous crankcase casting. If this is so, and you purchased the machine new, it would be best to have the crankcase half replaced under the terms of the Yamaha warranty.
Alternatively, if the porosity is restricted to a small area, it would be practical to drill a small hole through the crankcase and tap it to accept a small pipe plug. If the porosity is over a greater area, or if you don’t like the previous suggestion, take a medium half-round file and file the area of the porosity to a depth of approximately 1/32 inch, obtaining a rough surface. This area can now be filled with an epoxy glue or a plastic metal like “Devcon.” Devcon is widely used in industry, and when hard, can be filed, drilled or tapped. Another method would-be to dismantle the engine and have the porous area welded (heliarc).
