Service
SERVICE
Shakin’ to the Max
My friend and I both own V-Maxes, his an ’86 and mine an ’88. We share a common problem: front-end shake between 35 and 45 mph. If we take one hand off the bars to close a shield or zip up a coat, the handlebar shake really maxes out, if you’ll excuse the pun. We’ve tried different tires, from stockers to Metzeler Marathons and currently Dunlop Sports. After experimenting with tire pressures and trying a variety of front-fork air-pressure and rear-shock preload combinations, the problem has not improved. My friend has gone so far as to remove his entire front end, inspect everything carefully and reassemble it with new fork oil and proper torque specs, but to no avail. Should we be looking into fork braces or an aftermarket steering damper? Please clue us in, because the only local input we’ve gotten is: “Well, don’t take your hands off the bars!"
StevenW. McCarter San Leandro, California
This type of front-end waggle has occurred on many bikes we `ye ridden over the years, V-Maxes included, but only when both hands have been removed from the bars, not just one. The symptoms have been more exaggerated on some V-Maxes than on others, and often did not show up until the odometer had racked up a few thousand miles. But although this condition can be bothersome, we’ve never felt that it made the bike dangerous to ride. >
We ’ve never made an all-out attempt to rid a V-Max of its handlebar waggle, but we have discovered a few things that seem to contribute to the problem: 1) A V-Max doesn’t like any side-toside freeplay in its swingarm; 2) it works best with its steering-head bearings adjusted just a bit on the snug side; and 3) the waggle is least pronounced when the rear wheel is perfectly aligned with the front wheel.
To check for swingarm freeplay, put the bike on its centerstand, tighten the swingarm pivot bolt and nut to 72 footpounds of torque, then try to move the swingarm from side to side. If you can detect any freeplay whatsoever, replace the swingarm-pivot bearings.
There are several ways to determine if the rear wheel is aimed precisely at the front; even the quick-and-dirty eyeball method can tell you if the alignment is at least in the ballpark. But in the end, it really doesn’t matter. The V-Max uses shaft final drive, which means that its rear wheel has no provisions for adjustment. So, if the alignment is off the mark on your V-Maxes, there ’s not much you can do-aside from some major re-engineering-to correct it. You can, however, reduce the waggle 's intensity by installing an adjustable steering damper (a fork brace won 't help significantly) and keeping the tires properly inflated.
Plugging along
The manual for my 1991 Honda VFR750F says to change the sparkplugs every 8000 miles. But upon inspection, the plugs in my bike are not worn. The electrodes are flat and their surfaces are not rounded. Is it really necessary for me to change plugs at this particular mileage?
Also, why does rejetting motorcycle carburetors improve performance? In automobile carburetors, I’m told, rejetting is not important because the gasoline actually flows through circuits in the carburetor walls. Please explain.
Stan Smith Harvey, Louisiana
Generally speaking, the service-life recommendations listed in most motorcycle owner's manuals tend to be rather conservative. One reason for this cautious approach is the manufacturers ’ desire to minimize the service problems their bikes might suffer, particularly while warranties are still in effect. The companies are also bound by law to establish service recommendations that will help their engines meet applicable emissions standards for a certain mileage period. As a consequence, the engine must be kept in an excellent state of tune, a requirement that relies heavily upon-among other things-the condition of the sparkplugs.
On the other hand, the manufacturers cannot control the way in which their bikes are ridden and maintained, so they play it safe: They recommend service and replacement intervals to cover just about any reasonable set of operating conditions. The key, therefore, is to remember that what you read in the manual is only a strong recommendation, not an absolute ultimatum.
In reply to your second question, all production-bike engines must meet federal (or state) exhaust-emissions regulations, a requirement that usually necessitates stock jetting which is too lean for optimum power output. Rejetting simply adjusts the fuel-air ratio to achieve maximum power production.
During the latter days of carbureted automobiles (most cars are now fuelinjected), some carbs had fixed (nonreplaceable) jets-which is what your information source may have meant by “circuits in the carburetor walls.” This practice prevented any fuel-mixture tampering that could have altered the engine ’s legal emissions output. But otherwise, automobile carburetors traditionally have had replaceable jets. Some current motorcycle carburetors have fixed idle jets and non-adjustable jet needles, but replaceable main jets are still the norm.
Milling around
I own a 1973 Honda CB750. It’s a great ride, but it’s getting tired. I’m going to do a top-end job by installing new rings and grinding the valves. The previous owner bolted on a 4into-1 pipe and a set of smoothbore Mikunis, and I’m thinking about having the head milled, since stock compression is a conservative 8.6:1. How far can I go before having problems with detonation (I always use 93-octane super unleaded) and valve-to-piston interference?
• • •~-, Bill Anderson Onalaska, Texas
If you want to raise your CB750’s compression, do it by installing highercompression pistons rather than by milling the head. On an overhead-cam engine, milling the head reduces the camshaft-to-crankshaft dimension a distance equal to the amount milled off the head surface. When the cam is chain-driven by the crankshaft—as is the case with a 1973 CB750—lowering the cam causes the cam timing to be slightly retarded. To compensate, the cam sprocket must then have its bolt holes carefully slotted on a milling machine so that the camshaft itself can be adjusted for the proper valve timing independent of the sprocket’s required positioning in the cam chain.
This is why installing high-compression pistons is a simpler and, in the end, cheaper alternative to milling. What’s more, aftermarket pistons are likely to provide a much better highcompression co mb ustión-chamber shape than you would get from milling the head. You probably won ’t have detonation problems if you don’t exceed a 10.0:1 compression ratio, and you should allow a piston-to-valve clearance of at least. 060-inch.
Seeing the city lights
When reading about the Yamaha YZF750 in your March issue, I couldn’t help noticing the little light above the YZF’s headlights. My 1992 FZR1000 has a cutout for that same light, but I haven’t been able to locate one to buy. Can you tell me where I might get one?
` Steven DeMartino New Haven, Connecticut
That light, called a “city light,’’ is required by law in some countries, but the U.S. is not one of them. Because of that, and the fact that Yamaha is selling the YZF750 in this country for racing only, the U.S. distributor does not stock the light. Your best bet would be to try to buy one through a dealer in Europe, most logically in England where any language differences would not be a factor.
V-Four hiccups
My 1985 Yamaha V-Max has a problem I can’t solve. It runs fine for a while, then starts breaking up. It will be idling perfectly one minute, then sound like it just misses a beat before picking back up and continuing to run normally. Other times it will start missing at idle and die. Sometimes when I’m riding at road speed, it suddenly acts like it’s only running on two cylinders. It might keep doing that for a while, or it might clear up right away. Sometimes it runs just fine and doesn't act up in any way.
I've cleaned the carburetors com pletely. I've checked the fuel pump and filter to see that it's getting plenty of gas. I've interchanged igniter units, checked ignition-coil resistance and pickup-coil resistance. It has new plugs, correctly gapped. I've checked the kill and start switches, the battery ground and the wire connections. The bike only has 10,900 miles on it and has no mechanical problems. When it decides to run right it is extremely fast, but it can't be trusted. I'm at my wit's end.
Ronnie M. Harris De Queen, Arkansas
Based on your description, it ’s likely that the problem is with one of the VMax’s four ignition coils. At idle, the failure of just one cylinder to fire for a few revolutions could cause the engine to stall. And as you are motoring along on the road, the sudden loss of just one cylinder’s power could very easily be mistaken for the loss of two.
So far, you 've taken the right steps to track down the cause by verifying that all four coils have the prescribed amount of primary and secondary internal resistance. But an ignition coil can fail in ways that a resistance test cannot detect. Coils sometimes develop a tendency to perform perfectly when they ’re cool but misfire when warm. That condition can be difficult to diagnose but can often be detected bv performing a long-period spark-gap test on each coil.
The bad news is that you need a coil tester with an adjustable spark gap to perform this test; the good news is that quite a few motorcycle and automotive repair shops have testers of this sort, so you should be able to borrow one or have a shop in your area perform the test for you.
Yamaha specifies that the coil should be able to produce a spark that consistently jumps at least a 6mm gap without any sign of misfiring. If the coil passes that criterion, open up the gap as far as the spark can jump without misfiring, and simply let the spark-gap test continue uninterrupted for a long period-at least 10 minutes, preferably longer. If at any time the spark starts breaking up, even when the gap is dialed back to the 6mm minimum, you will have located the source of your misfiring problem. □
