Service
SERVICE
We welcome your technical questions and comments, and will publish those we think are of interest to our readers. Because of the volume of mail received, we cannot return any personal replies. Please limit your "Service" letters to technical subjects only, and keep them as brief as possible. Send them to: "Service,"CYCLE WORLD, 1499 Monrovia Ave., Newport Beach, Calif. 92663.
LOSING TEETH
I own a 1980 Honda CB750K and was wondering if 1 could lower my rpm by 500 to 1000 by changing the number of teeth on my front or rear sprocket or both. If I did, would I increase my mpg and extend engine life? I’m getting only 41.3 mpg now at speeds of no greater than 60 mph.
I also have a rough idle when the engine is warm or hot. I’ve had the carbs checked and adjusted and changed the plugs to no avail. Any ideas?
Wm. D. Wilson Pineville, La.
Dropping one tooth on the CB750’s rear sprocket w ill lower engine speed at 60 mph by about 100 rpm, and adding a tooth at the drive sprocket (if there is room) w ill lower it about 240 rpm. Small reductions 'in engine speed such as these will often provide a slight improvement in fuel mileage and more relaxed cruising, depending on the size and power characteristics of the engine, but higher gearing is not
always a guarantee of better mileage or longer engine life. The factory gearing on most bikes is a compromise that allows for reasonable economy without leaving top gear too high for adequate roll-on acceleration on the highway. If an engine is geared too high, larger throttle openings may be necessary to maintain speed, and detonation is more likely during sudden acceleration, neither of w hich is conducive to better or longer engine life.
Again, it depends upon the engine. Large engines and those with wide pow er bands normally get better mileage when geared up, while smaller engines or those with peaky powerbands ojien suffer a drop in mpg with a taller final drive. The Honda 750 is in between; a fairly large engine, but also an engine that develops peak pow er at high rpm. Depending on rear tire size, your 750 should be turning between 4500 and 4600 rpm at 60 mph. This is not an unreasonable cruising rpm for a high winding engine like the Honda’s, and dropping the> engine speed by as much as 500 to 1000 rpm will only put the engine farther out of its powerband, making a downshift necessary for upgrades, winds, or passing, as well as preventing the engine from cruising where it works most efficiently.
We would try a change of one or two teeth at the rear sprocket or one tooth at the front, then check mileage carefully to see if there is a worthwhile improvement, avoiding any large drop in rpm that moves the 750 off the bottom of its powerband. Honda itself is now beginning to overgear some of its 1982 road bikes for easier highway cruising, but the factory is also making small changes in exhaust tuning and other areas for better low-end power.
The dohc Honda 750s are not famous for idling smoothly when hot, and the impression of rough idle is added to by clutch rattle, which can become pronounced as the 750 accumulates miles. If the bike has a lot of miles on it and the idle becomes unusually rough, however, you may want to do a compression and leak-down test to make sure the rings, valves and guides are in good shape, and take a look at the cam lobes to check for wear.
Enduro Light
My letter concerns the article, "Long Range Test, Yamaha YZ465G" in the April, 1981 issue of CW. On page 54 you show a YZ465G with enduro prep, includ ing a Preston Petty headlight and an IT taillight. Are these lights operational or just for looks? If they work, how can I get my 1978 YZ125 to do the same?
Richard T. Weil
East Islip, N.Y.
The lights on the YZ465 were operational, but only bright enough to satisfy enduro rules requiring working lights, not adequately powerful for road use or competition night riding, as in Baja, for instance. The YZ465 uses an external flywheel with a single ignition coil beneath it, but there is room on the other side for a second coil, so Test Editor Ron Griewe installed an early DTI 25 ignition source coil and used it to power the Petty headlight and the IT taillight. One end of the DT coil was grounded to the backing plate and the other soldered to a single lead that split off in a T junction to power the headlight and taillight, both of which were grounded.
The DT coil has an output of 35 watts, enough to make the lights glow but not enough to provide good illumination. An IT or PE headlight, used with the same setup, would probably throw a more useable beam, as the 35w Petty headlight is used only to satisfy legal minimums. Petty now has a 60w version of the same light, and that should also throw useable illumination.
Unfortunately, your YZ125, as well as the YZ250, uses an internal flywheel surrounded by the ignition coil, so there is no way to mount a second source coil for light output.
HONDA CB135 LIVES!
Regarding the question about a Honda CB135 in the Jan., 1982 Cycle World, 4londa did indeed produce for the domes tic market a CB135 and CL 135 in 1970. Probably, rather than a quest for extra power, it was a means to get around traffic restrictions in Japan forbidding the use of freeways to machines under 126cc. The bike had a bore and stroke of 46 x 41mm,^ compression ratio of 9.5:1, a five-speed transmission, 50.4 in. wheelbase and a maximum speed listed as 81 mph. In a photo from a Japanese magazine the bike looks virtually identical to the American version of the CB125 of the same year.
M. Moore^
In Mr. Dodge’s letter he identified the bike as a 1974 model, but presumably his bike is in the same series, or was brought over and registered in this country at a later date. In any case, it ’s nice to hear the bike really existed. Thanks for the information. ^
BEAN OIL FOR THE H2
I recently purchased an old '12 Kawasaki H2 that I am restoring and have a question concerning engine lubrication. I was wondering what would happen if I tried running a castor based oil (Castrol to be more specific) in the injection system. I get different answers with everyone I ask, so I hope you gentlemen can give me a nice definite yes or no, or why or why not.
I already know it falls out of suspension pretty fast if used in pre-mix and is incompatable with regular petroleum oils, but don’t know what would happen in an injection system. This old H2 has engine oil pressure fed to the bottom end bearings and the intake ports. If you think bean oil would be okay, then what weight (30 or 40) is best, and is it smart to run it in the transmission as well (in case some should ever seep through to the crankcase)?
T.W. Wübbenhorst
Houston, Texas
Castrol R works well in oil injection systems, provided the viscosity rating is within the recommended range for the bike so that the oil flows properly in the injectors and pump. The 30 wt. R is compatible with most oil injectors. Castrol R. a mix-4* ture of bean oil and synthetic lubricants, is a racing oil that provides excellent lubricity under high temperatures and loads, and it is often used as an insurance against piston seizure in two-stroke racing engines that have to be run hard without proper break-in time. The R mixture is ashless, sop it also prevents the type of carbon buildup that can cause preignition in racing engines.
The only problem with Castrol R is that it burns to a gummy varnish, rather than a powdery carbon like regular mineral oils, and this means more frequent mainte nanee and cleaning for two-stroke pistons and rings. This is no problem on a racing engine that is torn down at regular inter-> vals anyway, but means extra work on a street bike. Castrol R also produces more smoke than most injector oils now in use. In other words, if you like the smell and racing associations of Castrol R, ami many people do, it will work in your bike, but will require a higher level of maintenance. Also, unless you are racing your bike or riding very hard there is no real advantage to using Castrol R rather than a mineral oil like Castrol Injector SAE 30, or any of the many other good mineral and[ synthetic injector oils now available.
Oil engineers at Castrol tell us there is no danger in using a mineral oil in your transmission while using Castrol R in the injector system, because even if there is minor gasket leakage between the separate compartments of the cases, the bean oi\ will mix with mineral oil in small quantities without coagulating, blocking oil passages, etc.
CAR COILS
I ride and repair a stock ’66 BSA Lighting. Recently I had to replace one of my coils and paid a premium price for it. I’ve heard of people using department store car coils with ballast resistors. Can I use this setup with my machine, and what is the function of the ballast resistor?
Henry Schoenenmann
Tolland, Conn.
As long as you can find a good place to mount them, there is no reason you can’t use a couple of 12-volt automotive coils on the BSA. A ballast resistor is normally located between the battery and the switcitf side primary terminal on the coil, and it has a couple of functions. One is to limit the rate of battery discharge if the ignition is left on when the engine is stopped, and the other is to regulate the flow of current through the points so surges in primary voltage don't damage the points. Most baE last resistors use a nickel alloy or iron wire element with a high resistance/temperature co efficient, so resistance to the flow> of current increases as the ballast resistor becomes hotter.
On some coils, the necessary resistance to primary voltage is built into the primary windings of the coil itself, so no external resistor is required and a full 12 volts can be wired straight to the coil. But coils with external resistors are normally intended to have only about 8 volts flowing through them, so removing or bypassing the ballast^ resistor will usually cause burned points or a very short points life. On automobiles, the ballast resistor is normally bypassed at the ignition switch or solenoid to give the coil a full 12 volts while cranking, but voltage goes through the resistor once the engine is running.
In other words, if the coil comes with a ballast resistor, or requires one on the installation instructions, use it. But if you buy a resistance-type coil, then no ballast resistor is required and you can wire the coil directly to the primary wire from the ignition switch.
FUEL SWITCH
In April of 1981 I bought a new 1980 Suzuki GS1000GL, which has to be the finest machine on two wheels. I have put 8600 mi. on it and have had no problems at all, except during the first two months. The salesman who sold it to me said I could go ahead and use leaded gas, bi^jj before you knew it I had to have the car-1 buretors overhauled twice. I finally read my owner’s manual which said to use unleaded fuel only. Since switching I've had no problems at all and the bike runs fine. The moral is, don’t always listen to the salesman and take the time to read the
owner’s manual. 1
Jerry Hinson Alta Loma, Texas
The salesman was partly right; you should be able to use regular leaded without suffering any troublesome lead fouling of the valves or combustion chambers—in theory, at least. The problem in practice is that most gas stations now move a lot more unleaded gasoline through their storage tanks than they do leaded regular, so the leaded regular—or premium, if they have any—tanks are more likely to have dirt, water, or other contamination in the fuel. An inline fuel filter will catch some of the contamination that might get through the screen in your petcock filter, but as longas your bike doesn’t need the lead, you are probably better off sticking with unleaded when it’s available.
CB450 CARB FIX
We’d just finished testing the new Honda CB450 Hawk when we mentioned to Honda that the bike had some rideability problems, mostly a feeling of lean and surging carburetion in the midrange. It turns out they’ve encountered the same problem on some of the new US models and have issued a service bulletin to dealers recommending a main needle jet change to eliminate the condition. The fix is very simple, merely a matter of removing the gas tank, pulling the vacuum cylinder, spring and piston off the top of each carb, removing the needle set screw from the needle recess at the top of the vacuum piston, pulling out the needle and dropping in a new one. Then, of course, you reassemble the carbs and reinstall the gas tank.
Honda is already changing this part on the assembly line, and will allow dealers to change the needle jets under warranty for those already sold. All the 450 series Hondas are affected by the change, but we have no frame serial number information as yet. That information should be available to dealers as soon as service bulletins are sent out. Si
