Service

SERVICE

Paul Dean

Simpler is simply better

With all the recent controversy concerning Yamaha's new YZ250F four-stroke motocross bike and its huge displacement advantage over the 125cc two-strokes it's allowed to race against, I have a question: Why can't the limits for engine displacement and engine weight be made equal for both two-stroke and four-strokes? The four-stroke engine manufacturers would be allowed the option of using either turbocharging or supercharging, and two-stroke engine makers would be permitted to utilize whatever technologies were available to them so long as the displacements remained equal. Why hasn't a solution like this one been implemented?

Don Cahill Tiffin, Ohio

Because for a host of excellent reasons, it simply would not work As a concept for equalizing two-stroke and four-stroke performance, your suggested model is much too complicated. As a mechanical application for competition bikes, which need to be as light and as compact as possible, it adds too much weight and bulk to the four-stroke. As a practical option for privateers, it would prove way too expensive. As an engineering reality, turbocharged or supercharged four stroke engines could not be made nearly as light as normally aspirated twostrokes of the same displacement, which scuttles your equal-weight hypothesis. And for use in the rough-and-tumble world of motocross, the power deliveiy of pressure-fed engines, especially the turbocharged variety, is too non-linear and unpredictable to permit an adequate level of bike control.

More than likely, racing organizations that must deal with this inequity will do so by implementing the simplest rule changes possible that allow the 125cc two-strokes to remain competitive with the YZ25OF four-stroke. That could in volve restrictor plates, a higher mini mum bike weight for the four-strokes or some other easy-to-police rule changes. Some sanctioning bodies might even ban 250cc four-strokes from the 125cc class altogether But this is one of those situations that calls for the application of some well-thought-out simplicity, not the infusion of more technology.

Cranky cranker

I'm having a starting problem with my 1987 Harley-Davidson Sportster. On occasion, when I hit the starter button, the starter acts like it's going to engage and then the circuit breaker kicks out. After I reset the circuit breaker, the same thing will happen again. If I then put the bike in gear and push it through the compression stroke, the starter seems to work just fine and kicks the engine over with no problem. I've tested the starter for a dead spot and it checked out okay. Can you help?

Jim Ewen Broadview, Illinois

If the battery and the rest of the electrical system are in good operating condition, your Sportster likely has a weak starter motor. Here's why the circuit breaker sometimes trips: Every once in a while, when you shut off the engine, the crank shaft just happens to stop with one of the pistons at the very beginning of its com pression stroke. When you then try to restart the engine, the starter motor doesn `t generate enough torque to push the piston past compression without first getting a run at it; so, the excessive current load placed on the starter circuit trips the breaker. By putting the trans mission in gear and turning the engine by hand, you push the piston past com pression, which then allows the starter to get the engine turning just enough to build up a little flywheel inertia before it encounters that first compression stroke.

The Harley factory shop manual for your bike outlines a whole series of tests you can perform on the electric starter but that procedure is way too involved to describe here. Those tests can accurately diagnose most of the problems that occur with a starter but they're not foolproof and there is no effective way to subject the starter to a meaningful load test. So, f the manual~ diagnostic procedure fails to identify any weaknesses in the starter you may have to cross your fingers, bite the bullet and buy a new one.

The Duke of Eire

I recently moved to Ireland and had my Ducati Monster 900 shipped over by sea in a container, a trip that took about 6 weeks. When the bike arrived, it started with no problems but will not idle prop erly Every time I come to a stop, I have to keep the revs up or else the engine dies. With the choke fully engaged when first starting, the engine fires, runs a while and then dies. Any suggestions?

!~rryi Varner County Cork, Ireland

Unless you are being harassed by a leprechaun who has contempt for Italian motorcycles, I suspect that your Ducati `s exposure to salt air during its journey across the Atlantic has caused blockage of the pilot fuel jets in the carburetors. Ordinarily, this shouldn `t happen; but neither of us knows the conditions under which the bike was transported, and a 6week Transatlantic existence in a wet, poorly ventilated cargo hold could easily result in those jets' tiny orWces getting plugged. You not only need to unclog or replace the pilot jets, you should disas semble and thoroughly clean both carbs.

Houston, we have a problem

My 1990 Harley-Davidson FLHS has a persistent oil leak that I can't seem to cure. The leak is at the left-rear corner of the rear cylinder, between the base of the cylinder and the engine case. The leak showed up for the first time at 8000 miles, although I didn't replace the base gasket until 17,000 miles. Just 2000 miles later, the leak returned, so I fixed it a second time. But in only another few thousand miles, the leak started again in exactly the same location. Both times that I repaired the leak, I used a genuine Harley gasket and strictly followed the procedure spelled out in the factory shop manual for my bike.

I've seen other H-D Evolution engines with leaks in the same spot as mine, but when I questioned the Harley people about this problem, they had no com ments other than the usual: "Did you clean the gasket surfaces? Did you use the proper gasket? Did you follow the proper torque procedures?" So, my ques tion to you is, do you have any ideas how I can cure this persistent leak?

Jason P Curtiss, ID Houston, Texas

Base-gasket leakage is a very common problem with H-D Evo engines. It ` an unfortunate by-product of the engine s' top-end design, in which the aluminum cylinders and heads are fastened to the crankcase with long steel through-studs. Since aluminum has a greater expansion rate than steel, the cylinders and heads expand more than the studs as the engine warms. Consequently, as the engine goes from stone-cold to its normal operating temperature, the clamping force that the studs exert between the cylinders and the? crankcase, as well as between the cyl~ inders and the cylinder heads, gradually~ increases. If that force becomes too great, the cylinders could distort, which can result in uneven piston and bore wear, and possibly even piston seizure. To prevent such an occurrence, the.1 recommended torque for the studs has been calculated to provide the maximum~ allowable clamping force when the en-I. gine is at full operating temperature; but this also means that when the engine is cold, the force will be considerably less than is needed to ensure that the gasket~ won `t start leaking.

Here s the multi-part solution. First, al ways-I repeat, always-allow any Evo en gine to get fairly warm before revving it up into the top half of the rpm range, be fore accelerating at or near full-throttle, or before placing it under any substan tial load. Otherwise, the reduced clamp ing force at the cylinder base can allow crankcase pressure to begin pushing its way past the base gasket.

SecOi~ do flZ~ the originaieqi4p-!~ ment Harley-Davidson gaskets; instead, round up a set of the rubber-coated steel base gaskets made by James Gaskets (800/523-7843). They're available at most H-D aftermarket shops and have proven the most effective cure for Evo base-gasket leaks. A side benefit of these gaskets is that even if they do begin to seep a little (which rarely happens), they won `t blow out completely, whereas most H-D gaskets that seep eventually will.

Third, while you have the cylinder off, lay a straightedge along the gasket surfaces of the cylinder and the crankcase to determine if they are fiat. If either one shows any warpage (the cylinder is almost always the culprit), it will need to be resurfaced by a knowledgeable machinist, preferably one who has performed this job before. If you follow these steps, the basegasket leak should cease and desist.

An idle inquiry

My 1991 Kawasaki ZX-7R has a very fickle idle. I bought it last year with 6000 miles showing, and except for a Vance & Hines exhaust, it has all of the original components, including the stock Keihin flat-slide carbs. At first, it would not idle after being run for any length of time, and playing with the idle-speed screw just produced erratic fluctuations that would ultimately end in a stall. I cleaned the carbs and installed a new air filter, but that didn't help. I richened the mixture, which made it idle better, but it still would stall and then be hard to restart. Eventually, it stopped idling altogether. Is this something that is economically viable to fix or should I start searching for a new bike?

James J. Sab~ Pittsburgh, Pennsylvania

Your ZX7R escalating idle problem is being caused by the carburetors getting farther and farther out of synchronization. On multi-cylinder engines with individual carbs, a smooth, consistent idle is possible only when each cylinder is handling its share of the work That requires each carb to be adjusted so its air-to-fuel ratio and degree of throttle opening are as close as possible to those of the other carbs. Other wise, some cylinders end up working extra hard while others are pretty much along for the ride. Allowing that unbalanced condition to persist-or attempting to cor rect it without using the proper equipment and procedures-leads to erratic idling, stalling and low-rpm hesitation.

Obviously, you need to get the carbs properly balanced-or "synched," short for synchronized. This requires a multicylinder manometei~ the most common of which consists of four long glass tubes linked to a small reservoir of mercury A hose on each tube attaches to a fitting on each of your bike four intake manifolds. Once the manometer is connected, each carb is adjusted and readjusted until in take-manifold vacuum causes the mercu py to rise to the very same level in alifour tubes when the engine is idling. Then the carb linkage is adjusted so that all four throttles begin opening at precisely the same time. Unless you `re an experienced motorcycle mechanic, I suggest you not try this job yourself Better to have it done by someone who has the proper equipment and knows how to use it.

Biased for bias

In reference to Phil May's Honda ST1 100 tire question ("Baby needs new shoes," February, 2001, Service): When I wore the bias-ply tires off my `96 ST, I opted for some stickier Michelin radials. Frankly, I didn't care for the feel of the bike with those radials. With bias tires, the ST1 100 would respond instantly to countersteering inputs and track with outstanding stability when going straight. With the radials, no matter what I tried, the bike would always hesitate for a heartbeat before responding to steering inputs, and it was not as directionally stable. This meant I had to start my turns earlier and make little micro-corrections when going straight. I wore out the radials but went back to bias-ply tires as soon as I could justify the cost of them, and the problem disappeared.

Claude Baggett Monroe, Louisiana

As I stated in my reply to Mr May, no one here at Cycle World has any personal experience with an STJ 100 that has been fitted with tires other than the original equlpment rubber My recommendations were based on Honda s' spec~flcations for the ABS version of the SL and on the model-specific fitment charts produced by some of the larger tire manufacturers. Since then, however, I've received several letters and e-mails from STJJOO owners informing me of a strong preference for bias-ply tires on their machines, al/for reasons that are very similar to yours. So, I thank you and all those other ST owners for the valuable feedback.