Service

SERVICE

Paul Dean

Vulcan illogic

I own an ’85 Kawasaki 700 Vulcan I bought from a friend who never had any problems with it. But now, if the engine is shut off when warm enough to put the needle beyond the second line on the temperature gauge, it won’t restart. I’m no mechanic, and after many trips to different bike shops, the problem remains. Someone said it might be an electrical problem, so I checked the sparkplugs and they seem to get plenty of spark after the temperature is up. Could the problem be a vapor lock? I’ve called a lot of Kawasaki shops, but no one seemed to know anything about Vulcans having vapor-lock problems.

When riding the bike, I have to plan ahead so that when I get to my destination, I have at least 15 minutes of free time to wait for the engine to cool down before starting it. I hope you have some suggestions, because I’m on a budget and can’t afford a new bike.

Eric W. Baugher Transfer, Pennsylvania

First, a couple of important points. Just because a sparkplug gets a good spark outside of the engine, where at mospheric pressure is just under 15 pounds per square inch, that doesn `t mean it will do likewise inside the en-

gine, where compression of the fuel/air charge raises the pressure to 150 psi or more. In that super-dense environ ment, the compressed mixture presents a much greater resistance to the flow of electrons across the plug gap, so considerably more current is required to make the plug fire.

Second, vapor lock usually occurs in fuel systems that have a fuel pump located downstream of the fuel tank. The pump therefore must “pull” the fuel from the tank and then “push ” it to the carbs. If engine heat causes some of the fuel in the line between the tank and the pump to vaporize, the pump often can’t suck any more fuel from the tank until the vapor condenses and again becomes liquid. Pumps designed to draw liquids don ’t do a very good job of drawing vapors.

To combat vapor lock, most modern cars (and some bikes) have electric fuel pumps located either very close to or inside of their fuel tanks, which allows them to push fuel all the way to their carbs or injectors. Fuel pumps can push trapped vapors in fuel lines much more effectively than they can pull them, so this pump location virtually eliminates the likelihood of vapor locks. Gravity-feed fuel systems-which is what your Vulcan has-behave much like those with pusher fuel pumps, so vapor lock is not the cause of your hotstarting problem.

You haven’t given me much diagnostic information to work with, but you can do some troubleshooting yourself. Next time the bike gets hot and fails to start, crank it over continuously for about 10 seconds, then quickly pull one of the sparkplugs and inspect it. If it is even slightly wet with fuel, the problem is either a weak spark or no spark at all. If it is bone-dry, the problem is a lack of fuel reaching the combustion chambers.

It ’s my guess that the problem is electrical, either in the ignitor unit or the ignition coils. To check the basic soundness of these components, take your Vulcan to a shop that has a coil tester and an ohmmeter, and the knowledge to use both. When your coils are put on the tester, they should arc a spark across a 7mm gap. Testing the ignitor is a bit more complicated, however, requiring measurement of the resistance between all 10 of its plug-in connectors.

Still, these tests might not give conclusive results. The problem only occurs when the engine is warm, and it is difficult to test the coils and the ignitor under those conditions. As absurd as it seems, sometimes the most costand time-effective way to find the cause of a heat-related electrical problem is to replace the suspected component and see if the symptoms go away.

Clunkety clunk

I recently bought a used 1984 Honda VF700S Sabre from my local dealer. The bike has about 16,500 miles on the odometer and runs flawlessly, with one exception: When the transmission is put into gear at a stop, it emits a very distinct “clunk.” It clunks regardless if I shift down to first gear or up to second when the bike is not moving. I want to know if this problem is causing damage to the clutch and/or transmission, what can be done to fix it if it is, and a ballpark estimate of what the repairs will probably cost. Carl A. Schumacher St. Louis, Missouri

More than likely, the noise you ’re hearing is perfectly normal. On some bikes, in fact, the clunk is even accompanied by a shock wave in the driveline that causes the bike to lurch forward slightly when the gear is engaged.

Such behavior is characteristic of bikes that use wet, multi-plate clutches. “Wet” means the clutch runs in the same oil that lubricates the primary drive, and “multi-plate” refers to the large number (sometimes upwards of 12 or 14) of clutch plates. Every other plate is a drive plate connected directly to the crankshaft, while the rest are driven plates connected to the transmission mainshaft. All Japanese and American-made motorcycles, as well as many of the streetbikes and all of the dirtbikes made in Europe, use this kind of wet-clutch design. BMWs, Ducatis and many roadrace bikes use dry clutches that usually do not experience this clunking problem.

When a wet clutch is fully engaged, the pressure exerted by the clutch springs forces excess oil out from between the plates. But there remains on the plate surfaces a thin film of oil that causes the plates to stick together slightly when the clutch is disengaged. So, even though the pressure between drive plates and driven plates is relieved when the clutch lever is pulled, all of the plates continue to rotate at engine speed when the gearbox is in neutral because there is nothing to stop the mainshaft from spinning. Shifting into gear, however, prevents both transmission shafts from turning, bringing the mainshaft abruptly to a halt the instant a gear is engaged. This sudden stop is what causes the audible clunk and forward lurch.

For the most part, no clutch or gearbox damage results from these clunk-and-lurch incidents. Some dam age can occur over time, though, if you rev the engine in neutral just be fore clicking the transmission into gear-a technique that, for some un known reason, is favored by many riders. It is far easier on the entire driveline to shift from neutral into first gear only when the engine is at or near idle. This also applies to cold starts: Don `t shift into gear when the engine is screaming away on full choke, especially since cold oil tends to make the clutch plates stick togeth er even more firmly. So does dirty, worn-out oil. Sometimes, simply changing the oil can greatly reduce the intensity of the clunking.

Get rich or choke

I own an old ’78 Honda 750 that has chokes for cold starting, and I also have a ’92 Suzuki Katana 600 that has enricheners. I know these are two different kinds of cold-start mechanisms, but they also require opposite techniques. The Honda likes to have the throttle opened slightly, and the Suzuki starts best if its throttle is completely closed. This is not just a peculiarity of these two motorcycles. I’ve noticed this same choke/enrichener difference when cold-starting other bikes, too. My question is, why do they require different techniques? Ralph Austin Panama City, Florida

A choke valve always is located at the intake mouth of the carb, upstream of the throttle butterfly or slide. If the throttle is left fully closed during cold starts, intake-manifold vacuum cannot work against the choke valve to draw the needed extra fuel up through the needle jet and into the intake stream. But if the throttle is slightly open, intake vacuum can then work against the choke valve, creating the additional low-pressure condition needed to draw more fuel up from the float bowl.

Enricheners are completely independent fuel circuits that allow large amounts of fuel to enter the intake stream through an opening on the engine side of the throttle butterfly or slide. When the enrichener is on and the throttle closed, intake vacuum draws fuel up through the enrichener opening. But if the throttle is open, the vacuum has little to draw against and thus cannot suck enriched fuel up through this orifice.