Service

SERVICE

Len Vucci

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.

GAS CRISIS I

After reading road tests of the 1977 Yamaha 250 Enduro I purchased one. I now find that after 600 miles I am averaging only 56 miles to the 2.4-gallon tank. I have twice taken it back to the dealer, who checked the carburetor, ignition, etc., and found all to be OK. The bike runs well but is still getting low mileage. All the road tests I've read state it should get 40 mpg.

What kind of mi-leage should I really expect? What might be causing the low gas mileage? What, if anything, can I do to fix it?

Jim Rowland Friendsville, Tenn.

Because you were not specific as to the type of riding (onor ojf-road) you are using your DTfor, we assume it’s a combination of both. In such a case, 25 mpg would not be considered abnormal, as off-road riding consumes much more gas than might be expected, especially for two-strokes. Fuel consumption for a 250 two-stroke under load in the lower gears can drop down into the teens, causing the overall gas mileage to plummet as well.

On the other hand, easy riding around town with some highway use will yield substantially higher figures, as our test reported. We think most dual-purpose bike owners are primarily concerned with onroad riding fuel consumption figures, so that’s what we publish.

So, based upon the info you’ve provided and past experiences with fuel consumption, we’d say your bike is performing normally. The answer to the following letter may provide you with several worthwhile pointers, however.

GAS CRISIS II

I’ve had my Suzuki GS750 for 2800 miles and am happy except for the mileage: just over 30 mpg. The dealer says this is normal but road tests say 40 mpg is what I should expect. Any suggestions (minor modifications or tuning) would be appreciated.

Stewart Akerman Pearl Elarbor, Hawaii

We can offer only general suggestions, but one or more might be applicable and boost your numbers.

Make sure your air filter is clean and oiled properly. Too much or the wrong kind of oil will severely restrict air flow, causing rich carburetion and a drop in mileage.

Check the condition of the breaker points and ensure they are adjusted properly and that the static ignition timing is correct. Make sure the centrifugal advance is working (use a timing light per the workshop manual’s instructions), as retarded timing causes a loss in both power and gas mileage.

Check the spark plugs for color; tan is correct. Dark sooty deposits mean the carb mixture is too rich. We make sure the carb jets are in accordance with manufacturer’s specs, set the float levels, and install fresh spark plugs of the correct heat range. The carburetor article in our March 1977 issue of CYCLE WORLD will be of value if any adjustments are necessary.

GAS CRISIS III

A friend and I each have a Yamaha XS750-2D, and share a common problem. Our fuel mileage is moderate to dismal: 40 mpg on the freeways, 32-36 mpg around town. He installed a set of K&N filters, but still has the problem. Performance has not been adversely affected, but plug coloration indicates an excessively rich condition.

Any suggestions you can offer would be doubly appreciated.

Robert L. Hovorka, Jr.

San Francisco. Calif.

Refer to the previous letter for generalities, with a couple of additional specifics: Your stock air cleaner should be clean, not oiled. Pay close attention to the float levels, as the Yamaha's CV carbs are less prone to mixture deviation than slide carburetors. See the curb troubleshooting article in our April 1977 issue for teardown/adjustment assistance.

XS750 INFO

I recently traded in a Honda 750 for a Yamaha XS750D. The Yamaha seems to be a good bike but I have a few questions about it which I hope you may be able to answer.

First, the rear wheel seems a bit hard to turn when the bike is on the center stand. I asked the dealer about this and he said all shaft drive bikes have a lot of resistance in the drivetrain. To give you some idea of the amount of drag, it is about equal to that of an average foreign car’s wheel while trying to spin it on the grease rack.

Second, and possibly related, is the problem of gas mileage. It doesn’t seem to matter whether I drive hard or easy, 40 mpg is the returning figure, tank after tank. Since I have only 1500 miles on the clock, can I expect to do much better later on? (My Honda got in the mid-fifties.)

Third, the bike is sometimes a bit hard to start. The kick starter will usually get it if the electric spinner fails to get the fire going, but this is a bit disappointing after owning the Honda (I never even used the kick starter on that bike except to see if it would work). I have always steered clear of the “black box” ignition systems; do you think one would help in this case?

Fourth, I have read where premium fuel was suggested for this bike, but the owner’s manual recommends regular or low-lead. Please comment on the selection of fuels.

Fifth, I have never owned a bike with a fuel petcock system similar to this one, which has “on,” “reserve,” and “prime” positions. Vacuum from the #1 and #3 cylinders seems to operate this Goldberg system and 1 am left wondering why Yamaha left the conventional spigots for these. What is the chance that this system will leave me stranded somewhere down the road? Can “prime” be used for normal operation?

So far this has been the best bike I have ever owned and I hope to put 100 grand on the clock before throwing it on the heap. Eldon H. Cook Davis Monthan AFB. Ariz.

(1 ) What you 're probably feeling is brake drag. With a blade screwdriver, carefully pry the caliper pads away from the rotor slightly. This should lessen wheel drag considerably. The normal on-the-road bouncing and jouncing will usually have the same loosening effect. If you're really concerned, however, have a shop check the rear hydraulic system, especially for operation of the pressure relief orifice of the master cylinder.

(2) Gas consumption figures may improve as the miles accumulate and the bike loosens, but probably not by much. The XS750D we testedfor our August 1976 issue returned 40.0 mpg, and our 750-2D (tested April '77) averaged 40.2 mpg. We therefore feel your mileage figures are acceptable; you may benefit from the "Gas Crisis" letters elsewhere in this column.

(3) We suspect a tune-up would solve the balky start problem. As to black box ignition systems, most of the manufacturers (including automobiles) are heading in that direction. We think they're better (with certain exceptions) too.

(4) Leaded gas (a) allows greater ignition advance (more power) without detonation and (b) provides additional lubrication to certain internal engine components.

Unleaded gas dramatically reduces deposit (lead, naturally) formations in the combustion chambers, especially on spark plugs.

We make our drag strip and performance runs on leaded gas. For all other riding we use unleaded, where possible.

(5) In our opinion, the vacuum-operated petcocks are the best thing since self-canceling turn signals! We seriously doubt they'd leave you dry. But, as you suspect, the "prime" position does circumvent the vacuum feature.

We're glad you ended your letter on a positive note—we also think it's a (100) grand machine.

STITCH IN TIME

In response to Sam McClure’s letter (“Service,” November 1977) I will bet my Lester wheels that he has one of the new inductive pickup timing lights. He is probably using it on spark plug wires #3 and #4. the logical pair to work with when tuning his Kawasaki 650 since these wires are on the same side of his machine as the breaker points. And he is more than likely following the instructions which came w ith his timing light just a bit too literally. If so, he can rest easy ... his problem is simple and the solution is even simpler.

The makers of these newfangled timing lights don’t bother to inform us that for many motorcycles the polarity of the ignition coil outputs is not the same for all cylinders. For example, on Sam’s machine, cylinders #3 and #4 share the opposite polarity (refer to the wiring diagram in the owner’s manual). The solution to Sam’s problem is simple: Just ignore the timing light instructions and connect the inductive pickup backward on spark plug wire #3. The timing light will now emit the same rock-steady stream of pulses w hich it did when it was connected to the #4 wire.

I learned this the hard way w'ith a new Sears/Penske timing light on my 1973 Zl. Luckily, I owned an old-fashioned timing light to check against the new' one, or I, too, might have chased around in circles trying to diagnose an erratic spark.

I hope this message reaches Sam before he spends any time or money looking for trouble where it maybe ain’t.

Mathew Hurwitz Auburndale, Mass.

Thanks a heap for the timely tip—I'm sure it'll help more than a few people with the same problem as Sam related. One could definitely chase around^ and around and never find the cause of erratic timing impulses. A gold star for you, Mathew.

TRAILER LIGHTS

I thoroughly enjoyed the trailer wiring article in your October 1977 issue, but where was it when I needed it? I bought an old dirt bike trailer that was wired with single-filament clearance lights, which is just asking for tickets. Two pinkies and some head-scratching later I came up with an old computer-logic circuit to get the job done.

The circuit allows the two single-ended lights to w'ork as brake and taillights, fourw'ay flashers and individual turn signals.

The circuit is composed of two 10-ohm, 10-watt resistors (A, Radio Shack 2761141 at 79'~ and two 50 PIV, 3 amp. high surge diodes (B, R.S. 271-132 at 69'1.

When the towing vehicle’s lights are on, current flows from the auto’s taillight to the trailer lights via the resistors and diodes, illuminating the lamps at less than full brilliance.

When the brake is operated, current bypasses the diodes, going directly to the trailer lights, illuminating them at full brilliance.

Because diodes only conduct in one direction, when the turn signal is flipped they keep the juice off the car’s brake lamps and other turn signal.

I closely followed the tap-in and insulating tips in your article. The only critical part of this circuit is the diodes’ polarity: They must be connected as shown in the diagram.

Bill Walsh Denver, Colo.

BLACK [BOX] MAGIC

I own a 1976 Suzuki RM370A. After 30 hours of trouble-free service I experienced a CDI unit failure. The dealer could not find any problems with the system, and simply replaced the blown unit. The new black box was installed, only to burn up the first time out of the shop. I returned the bike to the shop and still no problem was found. These $90 CDI units are breaking my bank.

At this time I am replacing the entire electrical system. Please advise me on precautions I should take in the future.

John Finnigan Woodbridge, Va.

First and foremost is a thorough inspection of all ignition leads and connections. A wire whose insulation has been chafed can short to the frame, causing a failure as you’ve described. Pay close attention to points where the wire is secured by cable ties, or where it might be squashed by the gas tank or other components.

Be sure the ground wire makes a good solid connection. Likewise, all the plug-in connectors should be tight—a loose connection, as well as a short, can wreak havoc on electronic components.

Not so easy to find is a wire which is broken internally, yet the insulation is intact. This occurs most frequently where the lead meets a connector or runs through a grommet. This type of break can be found using an ohmmeter to measure continuity while flexing the wire.

Pulling hard on the wire or connector will also reveal a break, but more than likely one you just created. In other words, don’t.

If a black box swap is indeed necessary, you can save some coins by using a replacement unit from Martek. The MotoTek CD100 is the item for the RM370, and retails for $59.95. The unit replaces not only the electronics module but the stock coil as well, and provides a slightly different advance curve.