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.
READER RETORT
Reference the Yamaha 305 clutch problem of J. E. Jensen, Napa, California mentioned in your January 1966 issue.
While the Yamaha clutch design may not he the best or strongest (wouldn't it be nice if every bike could have a H-D or BMW clutch?), I think there is a simple solution to his problem.
If Mr. Jensen would change the transmission oil from the recommended SAE 30 to a good grade of 10-30, much of the clutch grab will disappear and the noise associated with the grab is lessened. My own YDS-3 had similar problems until I went to 10-30 oil in the transmission. Now, after 14,000 miles on the original dutch, no further problems have developed.
Also by going a tooth or two smaller on the countershaft sprocket, some clutch strain is eliminated on starting. The resulting lower overall gearing also provides a more flexible machine with no top end loss.
Mr. Jensen's comment on the dealer service should be heeded by the importer, i find many, if not most of the "New Era" dealers, provide poor parts service and have rather limited service departments. Isn't it about time some of these dealers woke up?
John R. GrinseI McChord AFB, Wash.
IS IT WORTH IT?
Can you tell me if a bore or stroke kit or both will help my Honda 90 C-200 by increasing horsepower? If so, how much horsepower? The kits 1 refer to are by Circle Industries; are they worth it?
Also, what the heck makes my bike burn valves? Two and a half complete sets in 8 months, running on my third set now.
Maybe I've run it too hard — 8000 miles in one year. But also I came to the conclusion that maybe it's because there was no diffuser in the exhaust pipe for back pressure on exhaust valve.
Tony Stalnaker Butler, Ga.
A bore or stroker kit will most definitely increase the maximum horsepower of your engine; however the most gain will be obtained at peak torque. The stroker kit referred to retails for $38.95 and increases cubic capacity to 111cc. Combined with a Weber 276 duration cam ($29.95), an increase of 40% maximum torque can be expected.
The exhaust valve burning being experienced could be caused by a lean mixture due to the removal of the muffler diffuser. Insufficient exhaust valve tappet clearance could also be the cause. Continued high speed running at maximum rpm can cause exhaust valve and seat burning, due to loss of valve control.
HAPPY HONDA OWNER
In answer to Mr. Steven Strade's disapproval of boastful Honda 160 owners, which appeared in your December 1965 issue, I cannot help but disagree with him more. In Milwaukee, 160 owners do not have to boast of their machines' performance. It has spoken for itself. I must admit that my 160 has not taken every 250 scrambler I have met off the line, top end, and all the way, but 1 do have a number to my credit.
I sincerely believe that the Honda 160 is an underrated machine. Under what 1 can only describe as perfect conditions, five miles of no wind, no hills, and no curves, I have consistently hit speeds in the 90s, twice burying the needle, and three times attaining 98 mph. I realize of course there is speedometer error to take into consideration, but how much? As for acceleration: that was a fast 0-60 time of 12.6 you reported in your road test of May 1965. I have yet to see a Super Hawk taillight as far as my first gear will carry me. Old Yamahas are no sweat and people have laughed as I whizzed by some ancient Harleys I know. My compliments to Honda for putting out such a fine, fast machine.
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Perhaps you could answer a few questions for me. What would be the best jet for my bike with straight pipes, megaphones, and standard mufflers? What is the possibility of valve damage with the incorrect jet?
You are certainly doing a fine job with your publication. We have always turned to it for the last word in disagreements. Thank you.
Garret Mcintosh Milwaukee, Wise.
The most practical way of determining correct mixture, irrespective of what exhaust system is being used, is by a plug check.
To obtain a good plug reading it is necessary to cut the engine as quickly as possible, when on full load (throttle fully open) at maximum rpm. To achieve this it is best to run the machine at maximum speed in top gear, and then simultaneously switch off the ignition and disengage the clutch, so that the engine stops instantly.
An alternative method is to set the carburetor for a dead throttle. (Throttle stop backed off, so engine stops when throttle is closed.) As in the previous method, the machine is run at maximum speed in top gear and this time the throttle is snapped shut and the clutch disengaged simultaneously, allowing the engine to cut clean. This method requires a little knack and some practice may be necessary before a good clean reading can be taken. If the engine is not cut off cleanly, or is allowed to fire on part-throttle opening, a true reading will be masked by the irregular explosions.
The appearance of the plug tip gives a good indication of mixture strength. A white appearance denotes a weak mixture, while a sooty black indicates that carbure -tion is too rich. For correct main jet, and mixture strength, the appearance should be a smooth, sometimes shiny ebony. This method is used by the majority, but unfortunately certain fuel additives can indicate to the inexperienced a lean mixture by leaving a whitish deposit.
The condition of the portion of the ceramic insulator exposed to combustion gives a more reliable indication. A black or very dark condition indicates rich mixture, while extreme whiteness combined with small bead-like particles clinging to the insulator indicates a very lean mixture. For correct mixture indication the insulator should appear mainly white with slight discoloration.
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A plug that is too soft (too hot) for the engine would tend to run hot and give a lean mixture indication, so be certain that the plug being used is hard enough (cold enough) to start with.
When any modifications have been made to the engine or exhaust system, always start jetting on the rich side and work back to the correct mixture.
WHAT DOES IT ALL MEAN?
Would you please tell me what is meant by the terms gear ratio and compression ratio? 1 have been trying for months to find someone who can explain these two terms to me. After everyone else has failed, I am depending on you.
Richard Stroh bach Washington, D.C.
The term gear ratio is used to indicate the difference in the rotational speed of two or more shafts geared together by means of meshed gear, belt or chain drive.
It is necessary on all vehicles powered by an internal combustion engine to have the rotational speed of the driving wheels or work head reduced in relation to engine speed to obtain sufficient torque, achieve this, it is necessary to employ a reduction ratio between the engine and the driving wheels. This can be achieved by using a direct drive between the engine crankshaft and the rear wheel; however, the most common method on a motorcycle is to use a chain drive having a small drive sprocket attached to the crankshaft, and a larger driven sprocket on the rear wheel. To find out the gear ratio it is necessary to know the number of teeth on each sprocket and then divide the number on the drive sprocket into those on the driven sprocket. Example: Engine drive sprocket has 25 teeth and rear wheel driven sprocket has 100 teeth. Therefore the gear ratio would be 100 = 4. This
25
would be referred to as a gear ratio of 4 to 1 (4:1).
In order that we can take full advantage of the engine power curve it is desirable to have a gearbox within the transmission system, so that the ratio between engine and rear wheel speed can be changed, by moving a lever. This allows good acceleration and pulling power at low speeds, and good cruising speed at moderate engine rpm in high gear. The ratio between engine speed and rear wheel speed is referred to as the overall ratio, irrespective of what gear is selected. The ratio between engine speed and gearbox input shaft speed is referred to as primary drive ratio. The ratio for each selected gear in the gearbox is referred to as the gearbox internal ratio. Lastly, the ratio between the gearbox output shaft and the rear wheel is known as the final drive ratio. To find out the overall ratio in any given gear, it is necessary to know the values of the primary drive ratio, the gearbox internal ratio and the final drive ratio. This would be written as follows: GIS v TR v RW — Overall Gear Ë X K X GOS = Ratio Where GIS = Number of teeth on gearbox input shaft (clutch spfocket)
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E = Number of teeth on engine
crankshaft sprocket IR = Internal ratio of gearbox RW = Number of teeth on rear
wheel sprocket GOS = Number of teeth on gearbox output sprocket (final drive sprocket)
Example: 50/25 X 2 to 1 X =30/15= 2X2X2 = 8
Therefore the overall ratio equals 8 to 1 (8:1).
Briefly, the compression ratio of an engine is the ratio between the swept volume of the cylinder (bore X stroke) and the cylinder head volume when the piston is at top dead center. On a 500cc single cylinder engine, with a head volume of 50cc, the compression ratio would be arrived at as follows:
(500+50)_ = 11 to 1 50
Simply, the head volume is added to the swept volume and the total then divided by the head volume.
ON THE LEFT BANK
Recently I bought an antique 1936 BMW R5 500cc. The machine was in fair condition, considering its age. The motor ran without missing, but it needed a new clutch. I removed the transmission and while I was waiting for a new one I cleaned the carburetors and points. Upon installing the new clutch and replacing the transmission, I discovered the compression in the right side was low (30 psi). I removed the head and installed a new intake valve and guide and ground the exhaust valve.
The motor only runs on the left bank, and there seems to be a considerable amount of blow-by coming out past the intake valve. You can feel the pressure and see the fuel being blown out the carburetor when the engine is run.
On a closer examination it appears that the intake valve is closing late, allowing the compression to escape out past the valve before it closes. The pushrods both appear to be the same length and it makes no difference which is used where. 1 have backed the tappets back to give plenty of clearance and still it does not fire. The plugs can be exchanged from one side to the other and still nothing. I have tried to obtain some manuals or other data about this machine but to no avail.
1 have built several basket cases but this has me stumped. I'm writing to you hoping that you may be able to help me. 1 would like to build this bike up to its original condition if possible. Your recent article in CW on BMW sure came in handy.
Lindy L. Shanoltz Riverside, California
From the contents of your letter it would appear that you did not alter the valve timing when the new clutch was installed. I can only assume that the new intake valve is not seating, possibly due to an incorrectly fitted valve guide, which is protruding too far into the port and holding the valve off its seat.
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A simple test is to remove the cylinder head in question, stand it on the bench so that one port is pointing straight up and pour gas in until the port is almost full. There should be no leakage past the valve into the combustion chamber. If there is not, then try the other port in the same manner. Should either valve leak, then it is quite simple to discover why the valve does not seat.
WHY BMW SIDECAR SUPREMACY?
/ would especially like to know why so many of the sidecar racers use BMW engines and what advantages they have over chain-driven motorcycles? Could you tell me how sidecar racing motorcycles are classed?
Ken Sandberg Courtenay, B.C., Canada
I presume your questions are mainly concerned with the Grand Prix World Championship sidecar class. This series of races, together with the world championship solo classes, are controlled by rules laid down by the FIM. Maximum engine capacity for sidecar racing engines is limited to 500cc.
There is a distinct lack of suitable engines which are capable of meeting the severe demands of a successful GP sidecar racing outfit. For some time 500 Manx Norton engines installed in conventional solo motorcycle frames with the sidecar bolted on, dominated this class. To achieve higher maximum speeds and improved cornering stability it was necessary to reduce the total height of the machine and reposition the engine to gain a lower center of gravity. With higher cornering speeds, the big single Norton engine was subjected to other than the normal mechanical stresses associated with a high output engine. Due to the large flywheel mass and the severe cornering effect of the machine, mechanical failures rose alarmingly.
Although today a few Norton-powered outfits run fairly successfully, after extensive engine modifications, they are no match for the overhead cam BMW-powered machines. The inherent design and layout of the BMW engines lends itself more readily to the requirements of the modern motorcycle racing sidecar outfit. Center of gravity can be kept very low, and due to the low height of the engine, the overall height of the machine can be kept to a minimum, allowing a good streamlined shape, combined with amazing cornering stability. The engine is also much less affected mechanically by the severe cornering forces.
In the early 1950s Gilera raced sidecars using the 500cc four-cylinder engines and were very successful. The main advantage of the BMW is not the shaft drive to the rear wheel. However, the shaft drive does not appear to cause any adverse mechanical or handling effects when applied to the racing sidecar outfit. On a high performance solo I would prefer a well designed chain-driven final drive every time.
