The Service Department
THE SERVICE DEPARTMENT
DUAL-TWIN FIXES
In your "Service Department" (March '67), you gave several tips concerning the Bridgestone 175 Dual Twin.
I experienced much the same performance from my Bridgestone as Mr. McIntosh, so I replaced the 16-tooth countershaft sprocket with a 15-tooth one, which would be equivalent to adding 2 to 3 teeth to the rear sprocket (as you suggested). I did this several weeks prior to reading your article, and have found that the performance was improved tremendously. Acceleration times have decreased markedly and top speed is at least as good as before. I have not liad the opportunity to check the fuel mileage, but I feel that it is also better. I did consider using a 14-tooth up front, but it is my belief that this would adversely affect top speed and fuel mileage. Perhaps the biggest advantage is realized in the 40 to 60 mph cruising range which previously required considerable shifting between 5th and 4th as hills were encountered; but now it is rarely necessary to shift, except for the steepest hills.
Mr. McIntosh also mentions that excessive smoking occurs during acceleration, and I have also experienced this along with considerable loading-tip with oil during idling. I have not checked the oil pump yet, but if it is set properly I will probably eliminate the injection system entirely and run on conventional twostroke mixture. This can be done, since the 175 does not employ any positive-force bearing lubrication, such as Suzuki or the new Bridgestone.
In fact, the loading-tip is so bad at times of low-speed in-town cruising that it will sometimes foul a plug, no matter what heat range I use. I've tried NGK, B7H, B7HZ, B8H, KLG FlOO, and AC 42 F. The Bridgestone manual recommends the B8H, but when I got the bike, the ACs were in it. I ran the 250 mile break-in period on the ACs, then switched to the recommended NGKs, which were in the tool box. I haven't had any plug trouble while cruising at highway speeds.
When I removed the cylinder heads recently (2,600 miles) there was not an excessive amount of carbon accumulation, contrary to my expectations. The hard-chromed bores, however, appeared to be pitted. The Bridgestone manual mentions "porous treatment," which supposedly creates numerous "oil retainers" in the cylinder walls to enhance piston lubrication. The pits do not appear to have any logical pattern to them, but could they be the oil retainers?
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When 1 first saw the pitted bore, I thought a hone job was in order, but I was advised not to hone a chrome bore. Why?
Another perplexing problem is the battery charging system. The battery goes dead every week or two regardless of how fast, how much, or under what conditions the cycle is operated. It is especially bad when the headlight is operated. This has persisted since the day I bought the bike, so 1 doubt that the battery is at fault. It's almost as if the bike were operating on a "total loss" ignition system.
I have not checked the generator, but the rectifier shows no evidence of burning, and I cannot detect any shorts in the system. The manual makes no mention of any sort of adjustment for regulating the generator output. All this would not pose much of a problem, except the bike simply will not start with a dead battery. Do you think I have a faulty generator, or what?
Thomas S. Buy an State College, Pa.
Thanks for relating your experiences, relative to a change of gear ratio on your 175 Dual Twin. The information may be of value to others owning this model.
Only dispense with the oil pump after you are fully satisfied that it is impossible for you to overcome the oiling condition at low speed. Remember that the reason the manufacturer employs a metered lubrication system is to provide correct oiling at all points throughout the normal speed range. A fuel/oil mixture of, say, 20 to 1 is a compromise between the lubrication requirements at idle and maximum speeds. If the ratio were correct for continuous maximum speed operation, there would be far too much oil at idle, causing the machine to load up and foul spark plugs.
The pitted bore ("porous treatment") is in order. The etching is obtained by an electrical or chemical process. A chromium bore can be wet honed, providing the correct grade of stones are used.
The electrical problem referred to may be due to incorrectly prepared battery or one that has had electrolyte spilled from it. Have the battery checked by an automotive electrical specialist. When fully charged at ambient temperatures below 80° F, the electrolyte should have a specific gravity of between 1.270 and 1.290.
The output of an AC motorcycle generator is not regulated in the normal sense. Field current is provided by a permanent magnet. There are usually three generating coils, and the output of each is governed by the strength of the magnetic flux and the speed of the rotating magnet. During normal running, only one or two of the generating coils are in circuit, and the third is brought in to meet the additional load when the lights are switched on. The lucas system differs from this principle by employing a zener diode for regulation. All the generating coils are in circuit all the time, and the zener diode acts as a kind of a dump switch when the battery becomes fully charged.
If your battery proves to be okay, have the generating system thoroughly checked.
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WHY THE GP?
What is the difference between a GP carburetor and an ordinary carburetor? The only differences I can see between the two are a remote float chamber, a velocity horn and about $80.
Can an ordinary carburetor with a detachable float bowl be made to perform as well as a GP?
John Curry Pullman, Wash.
I presume that you are referring to the Amal GP racing carburetor in particular and not racing carburetors in general, when you refer to "GP carburetors."
The Amal GP carburetor is a precision instrument, designed primarily for racing where maximum engine output is of great importance. They are manufactured with great precision to very close tolerances. The relatively high cost of these instruments is due to their not being mass produced — only limited quantities are made to meet the requirements of race engine manufacturers and individuals when optimum engine performance is the main consideration.
At full throttle, the GP carburetor offers no resistance to incoming air. The needle is offset and therefore does not restrict the air passing through the choke area, as a centrally located needle does on a standard type carburetor. The effective choke diameter of the mixing chamber is on the engine side of the throttle slide and not in the center of the choke adaptor, as on a standard carburetor.
A standard carburetor can be tuned to operate very well on a competition engine, providing the correct size is selected. The standard unit is easier to tune and will generally provide better low and mid-range response. However, where ultimate performance is required, the GP carburetor has no equal. In fact, certain experts in the field of carburetion regard the Amal GP as a standard when optimum air flow and operating efficiency at high gas velocities are the main considerations. The main advantage to be gained in employing a remote rubber-mounted float chamber is a reduction in float vibration, which, if excessive, will adversely affect the proper function of the float needle valve.
I believe that a GP carburetor is "wasted" on a street engine, because its optimum performance characteristics are very rarely used. The standard unit will generally perform just as well at anything other than the conditions where ultimate performance can be attained.
HONDA KITS
I am the proud owner of a 1966 Honda Super Hawk, and am happy to say that in more than 7,000 miles of riding I have had no mechanical problems. I use the bike a great deal for long-distance highway cruising and I am quite satisfied with the top end performance. However, for low-speed and around-town driving, the low-end pulling power leaves just a bit to be desired. I wish to increase the low-end performance without significantly altering the high-speed performance. I have considered two methods: Installing a HarmanCollins roller cam, or a 350 kit. Please compare these ideas in regard to cost, as well as effectiveness.
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Fred Strutton
San Luis Obispo, Calif.
The most effective way of meeting your needs would most definitely be achieved by employing a 350 conversion kit. Lowspeed torque and tractability will be greatly improved. Midrange torque will also be increased, providing more power at cruising speeds, this additional power being most useful when long upgrades are encountered. A small, but noticeable increase will also be obtained in maximum output.
There are several 350 conversion kits presently available on the market. They all seem to be reasonably well engineered. However, before you decide on which kit to buy, I would suggest that you inquire whether it is possible to obtain oversized pistons and rings, for these will be required when cylinder bore wear takes place and a rebore is called for. Alternatively, it will be necessary to have the cylinders resleeved.
After the conversion has been carried out, it will be necessary to rejet the carburetors to suit the increased displacement. Depending on the piston crown configuration, it is most likely that the compression ratio will be raised, and it will therefore be necessary to employ a one-grade-colder spark plug. With the higher compression ratio it may also be necessary to retard the ignition timing approximately two degrees.
RE-RINGING
I have read several motorcycle service instructions on installing new rings. None of them suggests ridge reaming — removing the ridge in a worn cylinder just above the upper limit of the ring travel. Granted, this is not necessary to remove the piston, as in most automobile engines. But it seems that the new ring might hit the ridge, particularly if the cylinder is installed with a new gasket that is a bit thinner than the old gasket. Do you advise removing the ridge as a precaution?
Winfield Sample Pasadena, Calif.
A ridge in the cylinder bore, caused by wear, would probably cause the top piston ring to break after new rings had been installed. If the ridge is small and the cylinder bore is not badly worn, it is normal practice to hone the cylinder before installing the new rings. If there is a pronounced ridge at the top of the bore, it is most likely that the cylinder will require a rebore and an oversized piston. If there is any doubt, take the cylinder to a dealer that is equipped to do reboring and ask his advice. Preferably, go to a dealer that specializes in the make of the machine in question.
When installing new piston rings, make sure that they have the correct end gap when installed in the cylinder. Also, be sure that they are fitted the correct way up. Most rings have "top" lightly marked on one face. The face marked "top" should face the top of the piston. If in doubt, always ask, because incorrectly installed rings will result in blow-by and the engine will burn oil.
