Service
SERVICE
Paul Dean
Horse sense
I have a 1993 Kawasaki ZX-6 D-4 and was wondering if you could tell me what the maximum horsepower output is for this machine. I have asked several dealers in my area and all of them have given me different answers. I'm confused.
R. Scott Cloninger
Dallas, North Carolina
Believe it or not, every one of those dealers might have been correct, depending upon their sources for horsepower ratings. In our July, 1992, comparison test of 60()cc sportbikes, for example, the ZX-6 (your '93 D-4 model is, in effect, a carryover '92 with different paint) pumped out 75.3 horsepower at I1,250 rpm on the dyno. But we’ve seen horsepower figures for that bike published elsewhere that ranged from well over 100 down to the low 70s.
The confusion stems from the fact that there are numerous different types of dynamometers, each of which produces different horsepower numbers
depending upon, among other things, where it connects to the motorcycle. Dynos that take readings directly from the crankshaft produce the highest numbers because the horsepower at that point is unaffected by the fric-
tional losses incurred throughout the driveline. Other dynos such as the Dynojet unit we use-take readings from a roller spun by the rear tire, and they tend to produce the lowest numbers because they 're affected by
everything in the driveline. And dynos that take readings from points somewhere between the crank and the rear tire-usually at either the countershaft or the rear sprocket-yield numbers that are somewhere between the highest and lowest, since they are affected by some of the driveline components but not all of them.
There are still other variables that can make readings from different dynos incompatible. Some dynos use water as a means of applying resistance for the measurement of torque, while others use oil: some very expensive dynos do the job with electrical current. The differences in efficiency and repeatability' between those types of dynos can be significant enough to invalidate any comparisons between the numbers they produce. On top of that, even similar types of dynos from different manufacturers are likely to deliver different readings on the same engine.
Still con fused? Take heart: So is just about anyone who tries to compare horsepower numbers that were acquired on different dynamometers. Some of those people just don 't know they 're confused.
Chasing the bandits
In Margaret Lambrinides’ “Battery Bandits” letter in your August, 1993, Service column, she said her dealer had installed a rebuilt alternator stator in an attempt to correct the low-battery problem with her XS650 Yamaha. This rang a bell with me, so I looked up the problem in my file of Yamaha 650 Society newsletters. It seems the condition she describes is not uncommon on ’78 and later XS650s.
Apparently, on some 650s the problem lies in the alternator rotor, not the stator. Most diagnostic tests will fail to show a lack of charge at idle, but centrifugal force can cause an internal ground in the rotor at higher rpm. The problem shows up as low or no charging output at cruising speed (above 3500 rpm).
I can’t say for sure that this is the problem with Mrs. Lambrinides’ 650, but it is a good place to look. Hey, anything to keep another 650 on the road.
Doug Somppi Ojai, California
Thanks for the excellent tip. Mrs. Lambrinides probably is already in hot pursuit of a used alternator rotor—if that is, her battery hasn 't gone dead.
Hibernation hints
I live in Connecticut, so my bike is in storage from around the first of November until sometime mid-April. 1 have a 12-volt, 1.25-amp battery charger that I usually hook up for about an hour or two every month during the winter. It’s neither a trickle charger nor one of those units that shut off when the battery is fully charged. I don’t have a voltmeter, so I never know when the battery is fully charged. What happens if a battery becomes overcharged? Will it damage the components in the bike’s electrical system?
Also, last winter I put Sta-Bil in my bike’s gas tank. It claimed to keep the gas fresh and eliminate the need to drain the float bowls before putting a bike in storage. My engine started fine in the spring, but when I blipped the throttle it acted like one or more of the floats in the carb were sticking. When I took it for a ride, though, the problem cleared up immediately and the bike ran fine. Do you recommend a different stabilizer and/or draining the float bowls?
Kevin J. Wright
New Milford, Connecticut
When a battery is overcharged, it doesn’t acquire a surplus of voltage that could run rampant and damage electrical components. Each individual cell in a battery is capable of producing a maximum of 2.2 volts under any conditions. This equates to 13.2 volts for a 12-volt battery (2.2 volts times 6 cells); 12-volt electrical components are designed to cope with at least that much voltage.
Bad things do happen to a battery that is overcharged, however. Primarily it overheats, which can cook the electrolyte out of the cells. Excessive heat also can warp the plates enough to render the battery useless.
You didn ’/ indicate what kind of bike you have or reveal your battery s ampere-hour rating, so I can’t give you a definite assessment of your charging regimen. But if everything is exactly as you describe, it’s highly unlikely that your battery will be damaged by a once-a-month replenishment with a 1.25-amp charger for an hour or two.
As far as the fuel stabilizer is concerned, what’s the problem? The engine might have stumbled a little when first revived after a long winter's nap, but the symptoms quickly cleared up and the engine ran normally thereafter. The Sta-Bil did its job, which is all you can ask.
Taming of the screw
I bought my '82 Yamaha 920 Virago used with only 3000 miles on it and in mint condition except for its badly gummed-up carburetors. In order to clean the blockage, I had to remove the idle-mixture adjustment screws, flush out the passages, then reinstall the screws. Now the engine idles poorly and occasionally backfires just off idle. I know the mixture screws are not adjusted properly, but the only thing my Yamaha and Clymer repair manuals tell me is that the idle screws are preset at the factory. My question is, how many turns open from the closed position should these screws be set? I haven't been able to find this information anywhere.
Michael Iczkowski Steger, Illinois
With virtually any carburetor, a good starting point for idle-mixture adjustment is to turn the screws all the way in, then back them out about P/4 to l1/2 turns. That should put you in the ballpark. Keep in mind, though, that this is just a general rule of thumb, not an absolute law ofcarb tuning.
Properly setting the idle invariably calls for additional adjustment of the mixture screws, first in one direction and then the other; very slowly and one screw at a time, until the most stable idle is achieved. As you fine-tune the mixture, you also may need to adjust the idle-speed screws fairly often to keep the rpm from going too high or too low for you to ascertain the correctness of the idle mixture.
Some experienced mechanics can adjust and balance multiple carburetors fairly accurately without any special tools. But most people need to use a manometer (a set of card balancing gauges) to get the carburetors adjusted evenly so that all cylinders enjoy the most equal combustion possible.
There's a good reason why the idlemixture screws are pre-set at the factory and the workshop manuals do not discuss their adjustment: emissions. The law requires the manufacturers to do everything reasonably possible to ensure that exhaust emissions will remain within prescribed limits for at least 25,000 miles. To comply, the manufacturers pre-set the mixture screws to the requisite levels, then attempt to prevent any further adjustment by either sealing off access to the screws or using devices that severely limit the range of screw adjustment. □
