The Service Department
THE SERVICE DEPARTMENT
GARY BRAY
IT'S NOT FAST ANYMORE
I have a 125cc Honda Benly Super Sport of 1961 vintage. According to the manual this machine is rated at 15 hp @ 10,500 rpm and will attain a top speed of about 85 m ph. My particular bike has a tach (no speedo) red-lined at 10,000 rpm. The problem is that the engine will reach 10,000 only in the first two gears. In third it will only turn 8,500 and in fourth it does a measly 7,000. If that is 85 mph, then those Yamaha 80s are awfully fast, because they whiz right by. I admit 1 am a heavy (200 lb.) 125cc rider, but don't see how that could reduce top rpm's by 3,000.
Oil consumption is fantastic, although the bike doesn't leave a smoky cloud behind. The rim on the rear wheel is always covered with a heavy coat of oil. As far as I can see the rest of the machine is okay. Nothing seems to be dragging or causing a lot of friction. The timing, point gap and plugs are as per the manual. And the bike starts easily. In short it "runs" well, it just doesn't "perform" too well. 1 realize that an overhaul is far overdue, and plan to give it the ol' "rings and valves" treatment myself later this fall. Will that cure the beast, or is there something else I should attend to while the bike is apart?
Bruce D. Davids
Santa Susana, California
A machine's performance will definitely suffer if the top end is in a weakened condition. The heavy oil consumption is probably due to extreme crankcase pressure caused by worn piston rings. The pistons themselves may also have outlived their usefulness. This will be indicated if wear is present in the piston ring grooves. A ring groove which is worn will allow blow-by and can cause piston ring breakage.
If this condition is found to exist, a cylinder rebore and the next oversize pistons would be the best cure. When work is undertaken on the top end, the camshaft followers and cam chain should also be carefully inspected for wear. Any or all of these components can hurt performance, if worn. In doing work on the top end, I would also replace the valve springs with new ones.
KNOCK KNOCK
Could you offer some advice on eliminating the front brake "knock" found on many low mileage BMW twins? Various local dealers and BMW enthusiasts have told me that this condition will go away with time and mileage, but after 5000 miles the noise is still loud and clear. The front brake shoes and linings appear to be normal, and various brake adjustment combinations have not helped.
Name Withheld
Boston, Mass.
A knock or clunk when the front brake is applied can mean the brake drum is uneven, or that a machining defect is present in the drum itself. Spiral-like grooves in the brake drum from poor machining will cock the shoes as contact between shoes and drum is made.
In many instances, mileage will indeed clear up the noise; however, it is annoying and the drum should be remachined if at all possible. Some brands of English motorcycles have also suffered this sort of brake trouble.
LOOSE SPOKES
The rear wheel of my 1965 Yamaha YA-6 125cc has developed a wobble of about V2-inch per side due to loose spokes. 1 have tightened the spokes but the wobble is still there. Is there any way I can remedy this situation? Also, are the frame, wheels and spokes on this machine able to withstand trials type riding as we have in the U.S.?
John P. McGuire Jr. Camp Douglas, Wise. Trueing of wheels requires, for the most part, patience. Merely tightening all the loose spokes will not bring the rim into alignment. The job of trueing the wheel will be made easier if a piece of stiff wire is fastened somewhere on the machine, or made stationary, so that it can be set close to the rim and used as a pointer.
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Through use of this wire as an indicator, the high and low points in the rim can be determined. To bring a low spot in the rim over, loosen the spokes on the far side of the low spot slightly and tighten the spokes on the opposite side. The reverse is true of a high spot in the rim. It is usually necessary to repeat this procedure several times until the rim is true.
The YA-6 should withstand the rigors of trials riding quite nicely.
AT A LOSS
I am at a loss. I just can't seem to find out the truth on detergent vs. non-detergent oil in bikes. I have a new BMW and want to start out right. The salesman says detergent ruins bike engines. The service man says it's fine. I was using Valvoline. The dealer says that's no good, use Torco; yet they sell Valvoline.
From your experience, can you give me the word on what you have learned? 1 have less than 1,000 miles on the bike.
Gary Macomb er
Northridge, Calif.
The truth is, it's easy to start an argument even among friends when you get on this subject. Most riders and mechanics have their own personal preference when it concerns the type of oil they use in their motorcycles. Since a personal liking can mean they like the brand name, or the color of the can, it becomes quite hard to get an objective viewpoint. Nonetheless, onward; possibly we can penetrate the fog surrounding the two types of oil that are in the mineral base category.
In theory, detergent oil carries, in suspension, whatever foreign matter the working parts of the engine may release. Detergent oils are a complex of many additives, and penetration is very good. However, many motorcycle engines use ball and roller bearings. Your BMW has this type of powerplant. It appears that a ball or roller bearing exposed to any quantity of metal in the oil would suffer abnormal wear, more so than the softer plain bearing. Working clearances in a motorcycle engine are greater than those used in automobile and truck motors. This means that extreme oil penetration is not of prime concern to the motorcycle engine.
Detergent oils do have one valuable asset. They combat acid, and other harmful deposits that form in an internal combustion engine, somewhat better than nondetergent oils. When a machine of any kind is used only intermittently, and never thoroughly warmed up, engine life can be shortened by the acid that is never burned off.
A big disadvantage of detergent oil, at least in a dry sump lubrication system, is its nasty habit of foaming. This condition can occur at high rpm, and complete engine failure has been known to occur. I must point out that this sort of trouble is usually encountered in racing equipment; however, a touring motorcycle traveling at high speed over a long period of time could suffer some sort of premature wear from oil frothing.
In deciding between the two, I think the non-detergent oil has a slight edge. It does a good job of lubricating and it is stable. Still, the best insurance of all is frequent oil changes, regardless of the type of oil used.
TEMPERAMENTAL
I recently threw a rod in my 1963 Triumph Tiger Cub. Upon taking it to the shop, I was told that trouble can be expected because the Tiger Cubs are a temperamental lot. Could you throw more light onto the subject than this? And could you tell me whether or not the new ones will behave similarly?
Jay Wilber Ann Arbor, Mich.
While the Triumph Tiger Cub is not the strongest motorcycle made, lower end failures are not particularly common. Of course, engine life will be shortened considerably if an attempt has been made to increase the power output or if the machine has suffered a great deal of abuse.
High mileage with little or no attention to the lower end and lubrication system can bring about a rod bearing's demise. Over a long period of operation, oil passageways in the flywheel assembly become filled with residue, thereby preventing proper bearing "lubrication. Excessive wear of the timing side main bushing will cause a drop in oil pressure. At high rpm such a pressure loss can have disastrous results.
Wear that occurs in an internal combustion engine throughout the period of its operation produces a certain degree of inefficiency in all the related internal engine components. To demand more of a machine than it was designed to give, or to add to its burden after several thousand miles have slipped by, is to invite trouble.
As for the Triumph Cub's temperament, it should be remembered that these motorcycles were engineered neither for racing nor long distance high speed touring, but rather as cheap transportation. There is no reason to assume that a similar problem will occur shortly after the machine is back in service, providing of course the repair work is carried out in a conscientious manner. The effect is known. An attempt to determine the cause should be made.
SQUIRRELLY
I have a Honda Scrambler with stock 19" rims but with 4.00 Pirelli Universal rear and 3.25 Pirelli Universal front. Stock tire are 3.50 and 3.00. The change has resulted in a marked handling loss at speed. Not only is the machine sluggish in corners (because of more rotational mass?) but straight line travel is sqiurrelly.
My front forks have no play, so it's not them. 1 thought that the tire changes might have reduced both the rake and the trail of the machine enough to cause this change. Is this it? Can I alter either of these factors easily and still retain the tires 1 have? Or can 1 change just the front tire to, maybe another 4.00 Pirelli.
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Ramon Dasch
Austin, Texas
For all practical purposes, rake and trail would remain constant since tire size, both front and rear, have been increased. It is true that the rotational mass is slightly greater; however, it is doubtful that the machine's handling characteristics would be noticeably changed due to the increased tire size, at least at speeds below 100 mph.
The difficulty in handling and straight line riding can probably be traced to a tire that has been improperly mounted and is crooked on the rim; or, you may find that one or both of the tires was manufactured with a crooked tread. This has been a fairly frequent occurrence with Pirelli tires lately. The evidence would seem to point in this direction, as no trouble was encountered evidently until a tire change was made. Naturally, if a tire defect is found, a new tire with a straight tread should cure the difficulty.
SOMETHING FOR NOTHING
I am new to the world of motorcycles after purchasing a Suzuki M-31 for my teenage daughter, followed shortly thereafter by the purchase of a Yamaha YKfor myself. Both of these bikes feature relatively low compression ratios, i.e., 6.3:1 and 6.8:1 respectively. I have been told that raising the compression ratio will increase power and performance without any adverse side effects. This sounds like "something for nothing" and 1 cannot help but wonder that the manufacturer would not overlook so simple (?) a solution for improvement.
Engine specifications always state the compression ratio, which reflects a wide variation offered on stock motorcycles, not to mention the even high ratios offered as options. Am right in assuming that there must be advantages and disadvantages in high v.s. low compression ratios?
L. D. Stevens Sebastopol, Calif.
The one thing motorcycle dealers and manufacturers don't want is uncalled-for warranty work. A higher compression ratio creates greater internal engine pressure and more heat. An occasional seized piston, while not serious, does cost the manufacturer if the machine is still under the guarantee.
Motorcycles having a low compression ratio tend to be somewhat less temperamental than the ones which are pumped up a bit. Off the road riding that calls for climbing ability and slow operating speed is best tackled by machines with a low compression ratio.
Both the Suzuki and the Yamaha will stand a boost in performance without ill effect. Yamaha even has a speed kit for the YG-1. Why should the manufacturer put all the goodies on to begin with? This way he gets you twice.
