Service
SERVICE
PAUL DEAN
Reverse engineering
Q I've decided to ask you a question that's been bothering me for quite some time. Why do current inline-Fours and other across-the-frame engines have their intake systems in the back and the exhausts in the front? In your reply in the April Service column regarding cylinderhead design ("Out with the new"), you explained that air does not like to turn corners; so if the intakes were at the front of the engine, the path to the cylinder would be as straight as possible. Another advantage would be the weight savings from not having to first direct the air over the top of the engine and route the exhaust down and around the engine. Please explain why this layout is not being used.
Jason Miller Oak Harbor, Washington
A There are numerous reasons, most of them related to the greater degree of difficulty that a front-intake! rear-exhaust motorcycle would pose in
terms of packaging. For example, unless you were willing to allow unfiltered air to enter the engine directly-which might be acceptable on a drag bike or a Bonneville racer but not on a street machine-where would you put the air filter? Just look at the profile of the motorcycles in question and you'll see the problem: There is no room ahead of the engine for a filter and an airbox containing enough volume to provide the desired intake-system tuning. Those components instead would have to be located under the gas tank, which is where they currently reside, with ram-air ducts feeding them intake air just as they do on contemporary sportbikes. Ultimate ly, the air would have virtually the same path it now does on a rear-intake engine. You could, of course, tilt the cylinders rearward instead of to the front, thereby allowing room for an airbox in front of the engine. But that would shift the bike's overall weight bias rearward, likely having a negative effect on the handling. Then there's the exhaust. Moving it to the rear of the engine would create a
nightmare for anyone trying to engineer an exhaust system long enough to have the necessary volume for both optimum engine tuning and sound-emissions com pliance. This challenge would be tough enough with forward-canted cylinders; angling them rearward would allow even less room for the exhaust. And once the header pipes exit the exhaust ports, where do they go from there? If the en tire system were under the tank and seat, the rider might as well sit atop a barbe cue pit, as all the exhaust heat would be concentrated just a few inches below. But if the exhaust were to have traditional low-mount mufflers alongside the swingarm, how would the rest of the system get there? It couldn't wrap around the sides of the engine, and routing it behind the motor would force compromises in other important design factors, including swingarm length and the proximity of the countershaft sprocket to the swing arm pivot.
All of this could be done, of course, but since there doesn't seem to be any
real benefit, why would anyone actually do it? Even if you were to implement such a design fairly successfully, you’d likely end up with-by current standards, at least-a pretty goofylooking machine. The short-lived Cannondale dirtbike had a front-intake/rear-exhaust engine, but it was only a Single and did not have to meet streetbike equipment standards.
Think of it this way: If motorcycles had always positioned their intakes in front and exhausts at the rear, some manufacturer would come along with a new concept that reversed those locations. And because such a design would overcome all the disadvantages I just described, everyone would herald it as one of the great engineering advances of our time.
Rules of the old road
Q With more than 50 years of rid ing under my belt, the past con tinues to ride with me. By that I mean that in the `60s, there were two rules we always followed: 1) After starting a bike, we let it idle until the cylinder fins were warm to the touch before riding off; and 2) at long stoplights or slow coasting down long hills, we would al ways shift into neutral and let the clutch out instead of "riding" the clutch. Both of these were said to be easier on the bike. Is either still valid? Were they ever valid? David Weller Laredo, Texas
A With one possible exception,
“slow coasting down long hills” was never a valid riding technique practiced by me or anyone I ever rode with, clutch in or clutch out. We always kept the clutch engaged when descending long, slow hills and downshifted to the gear that would provide a reasonable amount of compression braking without revving the engine too high. That lone exception was the rare occasion when we’d conduct an impromptu downhill coasting race; we then would not only shift into neutral to eliminate any clutch drag that might slow us down, we’d even turn off the engine to make cheating more difficult.
At long stoplights, however, it always has been advisable to shift into neutral. Back then, sitting for long periods with the clutch disengaged tended to overheat the pressure-plate release mechanism in short order,
causing the clutch to start drag ging and the bike to creep forward. Clutches these days are far better at enduring long periods of disengage ment, but it still is best to shift into neutral when stopped for more than 10 or 15 seconds.
Of course, if there were any cars ahead of us when coming to a stoplight or stop sign, we wouldn't pull too close to the last one, and we would only shift into neutral after the first car behind us came to a complete stop. That gave us the best chance of dodging out of harm's way if a car approaching from behind threatened to rear-end us. Again, that is still is a strongly advised tactic today.
As far as your engine warm-up procedure is concerned, that tech nique was practiced by many riders back in "the day," and the circum stances of the times probably justi fied it. But a lot has changed since then. Engines are built with better materials and more-efficient oiling systems, and oil itself has improved greatly. The advent of multi-grade oils alone has helped eliminate the need for long warm-ups, and so has the development of additive packages that keep a protective layer of lubricant on critical parts between the time an engine is turned off and then restarted.
Feedback Loop
Q Your helpful response to Paul Wellendorf ("Slammed or damned?") in the May issue answered his question about lowering his Suzuki SV650 but didn't deal with his other complaint about the bike's abrupt "off/on" throttle response. I also have an SV and found that this particular problem is fairly easy to deal with: Take the bike in for its 600-mile checkup. After that, the throttle responds much more smoothly. I noticed the same thing with my SV, but I only had to endure it for about a week before taking the bike back in for an oil change, valve adjustment and whatever else they do as part of the post-break-in service.
Given that Mr. Wellendorf bought his bike in the spring and that the throttle response is still an issue for him, I'm willing to bet that his SV is far overdue for this scheduled service. The abruptness won't go away completely afterward, but it should be reduced enough to eliminate one of his two reasons not to like the bike. Andy Kerr Winona Lake, Indiana
A Thank you for the useful input, Andy. Let's hope that Mr. Wellendorf has already had the 600-mile service performed and is much happier with his SV650's throttle response.
More than likely, what your dealer did to smooth your SV's response was either synchronize the throttle bodies and/or make a small adjustment to the throttle-position sensor. Under the best of conditions, V-Twins tend to have more-abrupt response than inline-Fours, and they're also more sensitive to throttle-body synchronization. Otherwise, the fuel-injection system is entirely non-ad justable, so there is nothing else a dealer or anyone can do to alter the fuel delivery other than install an aftermarket device such as a Power Commander or an Fi2000.
Incidentally, the reason I didn't reply to Mr. Wellendorl in regard to his SV's throttle response was that he never asked a question about it. He commented that he disliked the throttle response but his only inquiry ("My question is...") was about the effect of lower ing the bike.
Back then, if you'll recall, motorcycle engines were fortunate to last more than 10,000 miles before requiring some form of rebuild, yet it's not uncommon for today's bike motors to rack up 40,000 or 50,000 miles without a lot of TLC.
Some even are still running strong at
the 100,000-mile mark. So, while there certainly is nothing wrong with your long-standing propensity for extended warm-up periods, such practices really are not necessary today.
Ahead of the curve
Q What's the reason the front fend ers on many Harley-Davidson models don't follow the curve of the front tire when all other manufacturers' fenders do? W. Wilson Severn, Maryland
A This curvature mismatch takes place only on Sportster and Dyna models. Sportsters come in nine varia tions, seven of which use 19-inch front wheels while the other two (XL 1200C Custom and XL 883C Custom) have 21-inch fronts. Of the five Dyna models, four use 19-inch front wheels and just one (Dyna Wide Glide) has a 21. Even though the 21-inch tire has a shorter sidewall than the 19-incher, it is slightly taller. Apparently, to allow parts inter changeability between as many similar models as is reasonably possible, Harley uses the same front fender on all Sportsters and another common fender on all Dynas. Those fenders therefore have to be radiused to conform to the larger tire. The obvious result is that the fender does not accurately follow the contour of the 19-inch tires.
Four is more than five
Q Why have engines with five valves per cylinder fallen out of favor? What made the five-valver good and what caused its demise? George Picabea Sugar Hill, Georgia
A Two decades ago, when Yamaha introduced its five-valve engines,
that cylinder-head design not only provided excellent airflow capabilities for the state of the market at that time, it also gave the tuning-fork company a unique feature that could be employed as a powerful marketing device. Mean while, the rest of the performance oriented motorcycle companies-not to mention most automobile manufactur ers-forged ahead with four-valve-per cylinder development.
Today, with so much time, money and brainpower having been invested in four-valve heads over the ensuing years, that design has reached a level of sophistication that the five-valve layout cannot equal. Flow-bench testing has shown that in a five-valve
head, opening only the outer two intake valves results in outstanding flow rates and characteristics, but that when the center valve is also opened, the flow loses some of its smoothness and efficiencies. Five-valvers also tend to have relatively poor squish areas compared to their modern four-valve counterparts, as well as less-efficient combustion-chamber shapes.
With so much at stake in the hightech world of MotoGP, Yamaha had little choice but to switch to four-valve heads if it wished to remain competitive. Even in Formula One racing, where cost is no object and technology is paramount, the engineers spent many millions trying to develop five-valve heads but ultimately went back to four-valve designs. And of course, after a run of more than 20 years with its five-valve Genesis engines, Yamaha has also reverted to a four-valve head with the 2007 Rl.
The cable guy
Q I am the second owner of a 1994 BMW Ri 100RS. I've had it for 3 years, and it has a little more than 43,000 miles on it. My problem is that on two separate occasions, the bike has acceler ated very hard away from a dead stop. The first time it happened, I was pulling away from a stop sign and making a right turn into traffic. The bike suddenly surged forward, nearly causing me to lose control.
Candid Cameron
Q Just read the good articles on radial engines in the April issue. The one from Mr. Cameron ("Engines that fly") was very complete and under standable but there are still a couple of questions I'd love to have an answer for: Some years ago at Le Bourget Air Show, I saw a German guy offering some radial diesels (I even think they were twostrokes) inspired by the famous Junker 52 of WWII, but these radials had 2, 4, 6 or 8 cylinders, killing the theory of the mate rod on which the others attach to. 1-us website is www.zoche.de/specs.html. These would be really neat engines for a bike, but how does this work with an even number of cylinders? Philippe Murat San Francisco, California
A The major radiais were all fourstrokes, which are most practical to build in odd numbers of cylinders per row. This keeps firing orders even to help reduce vibration. On a nine-cylinder, single-row radial, for example, the cylinders are numbered sequentially, with No. 1 at the top and the others following in order (1,2,3,4, etc.) around the circle. Being a four-stroke, the engine requires two crankshaft revolutions to complete one entire combustion cycle, so the firing order is 1-3-5-7-9-2-4-6-8.
When the engines are two-strokes, however, such as the Zoche radiais you saw in France, the number of cylinders can be either odd or even, depending upon the designer’s whim. Since only one crank revolution is required to fire all cylinders, the firing order of an eightcylinder two-stroke radial would be the same as the cylinder-numbering order: 1-2-3-4-5-6-7-8.
Having an even number of cylinders does not negate the use of the masterand-link connecting-rod system. So long as the master rod’s big-end is solid, it can readily be flanged and pierced for any desired number of link rods-even or odd. Mr. Zoche’s twoand four-cylinder engines place all cylinders in-line on the same crankshaft throw, but his eightcylinder radial has two rows of four each. It is, in essence, two of his four-cylinder engines siamesed into a common crankcase. -Kevin Cameron
I took the bike to the BMW dealership for the 40,000-mile service and explained to the tech what had happened. I asked if there were any “service alerts” about RI 100RS acceleration surges, and he said there were none. He >
couldn’t find anything wrong other than the needed service, so he sent me on my way. Everything was fine for a while, but then it happened again. The second occurrence was almost identical to the first. Both times, I was making a right turn into traffic, and only by the grace of God was I able to avoid an accident. This dangerous, erratic behavior has caused me to leave the BMW parked far more than ride it out of fear of losing my life. Can you offer any advice? J.R. Willis Ruston, Louisiana
A My guess, given that I can't actu ally examine the bike, is that your
Product Update
2007 Honda CRF150R/RB
American Honda is conducting a Product Update Campaign to replace the camshaft on certain 2007 CRF150R and CRF150RB models. On affected units, a defect may exist in the camshaft that could cause it to break during extreme operating conditions. Honda will replace the cam and necessary related parts for original or subsequent owners at no cost.
Owners of 2007 CRF150R/RB motorcycles should contact an authorized Honda motorcycle dealer and make an appointment for the repair. The campaign will end March 23, 2010.
2007 Honda CRF250R
American Honda is also conducting a Product Update Campaign to replace the clutch outer on certain 2007 CRF250R motorcycles. On affected units, the rubber clutch dampers may deteriorate after extended use and time. American Honda has not received any customer reports of failure but has determined from its own ongoing durability testing that there isa possibility that current or subsequent owners could eventually experience the problem. Honda will therefore replace the original clutch outer and necessary related parts for current or subsequent owners at no cost.
Owners of 2007 CRF250Rs should contact an authorized Honda motorcycle dealer and make an appointment for the repair. The campaign will end March 23,2010.
For more information on either of these campaigns, visit the Honda website at www.powersports.honda. com and click on “Locate a Dealer” or call Honda Customer Relations at 866/784-1870.
BMW has a problem with the routing of its cables. This could involve either the throttle cables or the “choke” cable, which is actually a fast-idle cable for the fuel-injection system. In both instances, the bike accelerated while you were attempting a right turn from a dead stop. This gives us two pieces of evidence that points to the cables as the likely culprit. First, only when a motorcycle is traveling very slowly-such as when maneuvering around a garage or a tight parking lot, or when making a sharp turn from a dead stop-are the handlebars turned very far; once you’re moving faster than a walking pace, they never move right or left more than a few degrees. Second, the device that controls the R1 lOORS’s fuel-injection system-the ECU-doesn’t know if the front end is turned right, left or not at all, so it would not cause the engine to accelerate only when the handlebar was turned sharply to the right.
But one of the aforementioned cables would. To find out which one, start the engine, let it warm up until it idles steadily with the choke fully off, take the bike off its stand and sit on it, then turn the handlebar all the way to the right. Don’t just do this once; do it repeatedly. If the engine even slightly revs during one of these full-right-lock moves, a cable is catching on something, causing it to hang up. The throttle cable is routed under the gas tank, where it enters a junction block that connects to the separate cables for each throttle body.
The fast-idle cable also hooks into the junction so it can pull on the two throttlebody cables when the handlebar choke lever is operated. With careful inspection of the cable routing and condition, you should be able to identify and remedy the problem. □
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If you think we can, either: 1) Mail 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 949/6310651; 3) e-mail it to CW1Dean@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 10page 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.
