Service

SERVICE

PAUL DEAN

Ask Carrie Underwood

Q Are there any effective ways to secure a bike’s oil cap? After I had a run-in with my girlfriend’s exboyfriend (also a rider), he dumped sand into my engine via the oil filler opening on my 2003 Honda CBR600RR. Swapping the engine for a slightly more powerful 2005 version taught me a lot about how my bike works, but I’d like to prevent such a thing from happening again. BMW has a few aftermarket locking oil caps for the R1100/1200, but none can be found for any of the Big Four’s bikes. It’s surprising that a locking gas cap is standard equipment but the oil cap goes unsecured and by far involves a more vulnerable part of the bike. Rob Mills

Chicago, Illinois

I could be wrong, but I believe the primary reason for the use of locking gas caps, whether on a bike or a car, is to prevent unauthorized people from taking gasoline out of the tank, not to keep them from putting contaminants in it-though it obviously works both

ways. There are millions of instances of people stealing gasoline from car and bike gas tanks, but there are far fewer cases of someone putting impurities into them. And I’ve never heard of anyone stealing used oil from a crankcase, which is probably why locking oil caps are not a hot item in the aftermarket.

Besides, if a thief attempting to steal gasoline encounters a locking cap, that person will most likely move on to another vehicle that is easier prey. But if someone angry enough to try dumping contaminants into a gas tank or a crankcase is thwarted by a locking cap, that person will quickly find another way to gain his or her revenge-slicing tires, bashing bodywork, smashing electrical components or even worse. For that kind of warped mentality, there is little in the way of prevention.

Rich and flameless

Q On my 1981 Suzuki GS750L, the No. 2 cylinder keeps loading up with fuel to the point where fuel dilutes the engine oil and makes that

sparkplug burn black. All four of the carb floats are set right and working properly. When the bike is at full throttle, the engine runs strong on all four cylinders, but when it is at idle at a stoplight, it starts to run rich on that cylinder. Henry Ruter

Pennock, Minnesota

A There are two possible conditions at work here, and one involves the float on the No. 2 cylinder’s carb.

You said all the floats were “set right and working properly,” but are you certain that the float needle is sealing tightly on its seat? If it is not, continual seepage of fuel into the float bowl will result, causing excessive richness that is most apparent at idle rpm when fuel consumption is at its lowest. If you see fuel dripping from that carb’s overflow hose when the engine is idling, the float needle is the probable culprit.

If there are no such leaks, then the problem is with that carb’s cold-start enrichment system. The enricheners involve a little plunger in each carb that normally remains closed but is popped

open when the “choke” is turned on. If the plunger on the No. 2 cylinder’s carb is leaking slightly-either due to wear or misadjustment-it will cause excessively rich running, particularly at low rpm. The orifice that allows the cold-start mixture to enter the engine is between the carb’s butterfly and the intake valve, so the intake vacuum that pulls the rich mixture up out of the float bowl is greatest when the butterfly is closed; that’s why the excessive richness is most noticeable at idle. But at large throttle openings, intake vacuum is significantly reduced and much more air flows through the venturi. This means far less enrichening fuel is drawn up into the intake tract, where it is then mixed with a greater volume of air. As a result, the air-fuel ratio is not exceedingly rich at large throttle openings, so the engine runs more or less normally.

Solution: Pull the entire cold-start mechanism from the affected carb and check it for dirt or damage that could cause it to leak. Upon reinstallation, adjust the cold-start linkage to ensure that all the plungers-not just the No. 2 carb’s-are fully seated when closed.

What goes with a belt?

QI own a 1992 Harley-Davidson FLHS that I purchased new, and I have a question about its drive belt and pulleys. When I recently replaced a tire, my mechanic said the original belt still looked all right. But considering the bike’s age and its 80,000 miles, I’m thinking about replacing the belt. Will a new belt run okay on the original pulleys or should the pulleys be changed? Don Newton

Lizella, Georgia

A Although H-D pulleys often are good for around 100,000 miles, you should at least replace the front one. Installing a new belt requires removal of the primary drive system, including the clutch and inner primary case, so it would be a false economy not to also replace the front pulley. A front pulley kit (pulley, seal, nut, etc.) sells for less than $100, and with the primary already removed, only about another half-hour of labor is required to install it. But if you don’t replace the pulley now and wait until it wears out (assuming you intend to keep the bike much longer), you’ll have to ante up for another complete primary-drive removal and replacement all over again.

If you replace the belt and the front pulley, you might as well go for a new

rear pulley. Quite a few riders replace the stock rear pulley when installing custom wheels on a brand-new bike; so you may even be able to buy an almost-new rear from a Harley dealer at a fraction of the cost of a brand-new one.

Cam sandwich

QA couple of weeks ago at a restaurant near a local off-road riding area, I overheard three dirtbike riders in the booth behind me talking about a special twistgip they said changes the engine’s power at low rpm. I didn’t know those guys and they seemed like a bunch of smart-asses, so I didn’t ask them any questions, but this doesn’t make sense to me. I’m a dirt rider myself (’07 Honda CRF250X), and I don’t see how a twistgrip can have any effect on the power an engine makes. Are these guys out to lunch or am I totally confused? Tom Boyle Rochester, New York

A Actually, if it was somewhere around mid-day at that restaurant, you all were probably out to lunch. Seriously, Tom, I think you misunderstood what those guys were saying, especially if they used incorrect termi-

nology. They probably were talking about the progressive Throttle Cam System made by G2 Ergonomics (www. g2ergo.com). The TCS consists of a billet-aluminum throttle tube on which the “cam” (the spool that the throttle cable winds around) is removable and can be replaced with one that has a different profile.

As the accompanying drawing illustrates, the spool on a stock throttle tube has a constant radius, so every degree of twistgrip rotation pulls the throttle cable an equal amount, which, in turn, results in a proportionate opening at the carburetor or EFI throttle body. But on the TCS, the spool is eccentric, with a shorter radius at smaller throttle openings and a longer one at larger openings. This progression means that as the twistgrip is turned from fully closed to about one-quarter open, the throttle cable is not pulled as far as it would be with the stock throttle tube; the actual opening of the butterflies/slides at the carb/EFI is not as great, either. So, for any given amount

of twistgrip rotation during the first part of the grip’s movement, the engine makes less power simply because the butterflies/ slides are not opened as far as they would be with a stock throttle tube turned an equal number of degrees. But as the twistgrip is turned farther, the radius of the spool increases so that the actual throttle valves are then opened just as quickly as stock-or even more so if the radius is greater than the Stocker’s.

As our Ryan Dudek wrote in a “Staff Stuff” segment in the May, 2008, Service, the purpose of the G2’s progressive system is to soften the initial hit of power as the throttle is first opened. This is ad-

vantageous on some dirtbikes that have an explosive off-idle power delivery, which can make maintaining traction difficult for some riders when accelerating out of slower corners on slippery surfaces. G2 is not the first company to devise a progressive throttle tube, but its system is the only one I’ve seen that comes with three different cams-one with a mild progression, one with a more-radical progression and one that is stock.

The science of sipe

QWhy are street and dual-sport tires not built with sipes in the tread design? Would this not serve as better wet-weather traction as with automobile tires? Is it a safety issue, and if so, why?

A.J. Jones Posted on America Online

A Sipes, for those who might not be familiar with the term, are the little slits cut into the tread blocks of car tires. They help a tire disperse water a little more effectively and provide additional sharp edges to help both forward and side grip in the wet. Even a few lower-performance motorcycle street tires are siped, though not nearly to the extent seen on most automobile tires.

Here’s why: squirm. The more individual tread blocks a tire has, and the smaller those blocks are, the more the tire tends to slide when subjected to cornering forces. Small tread blocks are unsupported, so they squirm easily; they are rubber, after all. And even though sipes are very narrow, almost like knife cuts, they nonetheless take additional support away from the tread blocks, allowing them to squirm even more. If you compare run-of-the-mill, OEreplacement auto tires with those designed

Got a mechanical or technical problem with your beloved ride? Can’t seem to find workable solutions in your area? Or are you eager to learn about a certain aspect of motorcycle design and technology? Maybe we can help. If you think we can, either: 1) Mall a written inquiry, along with your full name, address and phone number, to Cycle World Service, 1499 Monrovia Ave., Newport Beach, CA 92663; 2) fax it to Paul Dean at 949/631-0651; 3) e-mail it to CW1 Dean@aol.com; or 4) log onto www. cycleworld.com, click on the “Contact Us” button, select “CW Service” and enter your question. Don’t write a 10-page essay, but if you’re looking for help in solving a problem, do include enough information to permit a reasonable diagnosis. And please understand that due to the enormous volume of inquiries we receive, we cannot guarantee a reply to every question.

for high-performance cars, you’ll see that the latter styles very often are not siped at all for this very reason.

Because of the differences in cornering dynamics and contact-patch area between an automobile and a motorcycle, the average person driving the average car around the average corner at average speeds never notices any squirm-induced tire slippage, even though it does occur easily and often. But the average motorcyclist on the average bike cornering at average speed is much more likely to notice tire slippage caused by tread squirm; and the faster the comer, the more pronounced the squirm. This is why high-performance streetbike tires have few tread grooves and no sipes, and why slicks have neither. □