Performing valve-adjustment service on your engine requires just a few basic tools, which will probably pay for themselves the first time you set the valves.
In both concept and practice the valve mechanism of any four-stroke engine is pretty straightforward. Essentially, valves act as the doors to the combustion chamber, allowing fresh mix to flow in during the intake stroke, sealing the chamber during the compression and power strokes and providing an exit for the spent gas during the exhaust stroke. Despite the harsh environment in which they live, valves require relatively little routine maintenance. The average valve requires nothing more than a periodic lash inspection and the occasional adjustment to stay happy -- tasks well within the grasp of those even with little mechanical skill.
The first -- and perhaps most...
The first -- and perhaps most complicated -- part of the job will probably be removal of the bodywork that blocks access to the valve covers. However, bikes such as BMWs, Valkyries, and our Moto Guzzi example, allow you to get to the valve covers without stripping of the fuel tank or other bodywork. In that case, start by removing the engine covers that need to come off to perform the procedure. You can remove the covers in any order you like. In this instance, the alternator cover was removed first so we had access to the crankshaft rotor bolt, which simplified turning the engine over. Some bikes will lose a bit of oil when this cover comes off, so you might want to have a drip pan below it.
Anytime a valve is closed there needs to be a gas-tight seal between the valve seat located in the cylinder head and the valve face. To ensure that the valve is fully seated, there must be some slight valve clearance or "lash" between the valve stem and rocker arm or cam -- think of it as freeplay, similar to that which exists in a clutch or throttle cable. Lash may be adjusted manually via a screw or shim or automatically by a hydraulic device, but most often it's done by a hydraulic lifter, which is simply a self-adjusting valve lifter or tappet placed between the cam and pushrod.
As the engine warms to operating temperature the various chunks of steel and alloy inside the engine grow slightly due to thermal expansion. Obviously as the valve train components are brought up to running temperature they expand along with all of the other bits and pieces. If there is insufficient lash between the valve stem and its actuating mechanisms when the engine is cold, there will be even less clearance when the engine is hot.
If lash is reduced beyond a reasonable amount, several things occur, all of them unpleasant. First, the valve timing is changed slightly, and this may affect performance, and emission outputs. But there is an added complication: The exhaust valve stays alive because it's able to transfer a fair portion of the heat it absorbs from the combustion process directly into the cylinder head when it's resting on its seat. As clearance is reduced, the valves spend less time on their seats. This means the exhaust valve(s) has less time to give up its heat, leading to anything from detonation, pre-ignition and engine overheating, to a warped or burnt valve.
Finally, if the clearance is removed altogether, the valves may be held some slight distance from the valve seat, thus reducing, or even eliminating the seat-to-valve-face contact area, allowing combustion gases to leak past. Hopefully at that point the engine won't start and the valves will receive some long overdue attention. If the engine still has enough compression to start up and run, the likely scenario is an overheated exhaust valve, followed shortly by a torched exhaust valve and then a long tow home. Valves can lose clearance for any number of reasons. The most common is seat recession. Seat recession occurs as the valve pounds itself into the valve seat, reducing operating clearance. At one time lead was added to fuel to prevent detonation. As a side benefit, it was thought that lead acted as a "cushion" between the valve face and the seat to help prevent valve recession. Lately this theory has been somewhat discredited. Although there were a rash of seat failures when lead was removed from fuel in the late 1970s, (primarily effecting BMW and Harley-Davidson motorcycles) it was later found that both brands had simply been buying cheap valve seats. Nowadays valve seat recession is most commonly a factor only on new bikes or very high mileage bikes.
After blowing any dirt from...
After blowing any dirt from the spark plug wells to keep it from falling into the cylinders, disconnect the spark-plug leads. Remove one spark plug from each cylinder. This is done to permit the engine to turn over freely without the resistance of compression.
Loose valves create less of a problem, although if they become too loose, mechanical damage between the valve stem and its adjuster may occur. Loose valves tend to make a lot of noise, they also hammer themselves and the other valve train components, accelerating wear. In an extreme case, a loose valve in a pushrod engine may let the pushrod fall away from the rocker arm and destroy itself, as well as some of the other expensive bits. Generally loose valves occur due to normal physical wear between the components. If a valve develops a sudden affinity for large clearances, start looking for wear at the camshaft, rocker arm tip, or perhaps a collapsed or damaged pushrod.
Out-of-adjustment valves can make themselves known in a variety of ways, the most common being a distinct clatter from the valve area indicating loose valves, while tight valves may cause hard starting or a poor idle.
Before undertaking any work of this sort it's always better to have all your ducks in a row and a thorough understanding of what you're about to do. Take the time to read through the appropriate text. And gather up everything you might need for the task before you're up to your elbows in it.
Some bikes, BMW and this Moto...
Some bikes, BMW and this Moto Guzzi in particular, have datum/timing marks stamped into their flywheels. You'll need to remove the stubborn little plug to find them. The marks allow you to tell when the engine is at top dead center (TDC) on the compression stroke, which is when both valves are closed (they move up) and the followers or cam lobe are not pressing against them, so their should be some play between the valve and their actuators. You are checking and adjusting that play.
Here is a list of the tools you'll probably need:
- Shop manual
- Wrenches needed to remove the valve cover
- Wrench sized to fit the crankshaft rotor bolt (This is not a must have, but it will make the job much easier)
- Any special wrench required to fit the valve adjusters, again not a strict necessity
- A spark plug wrench to fit your plug
- A set of leaf-type feeler gauges
There are three basic methods of adjusting valve lash. The simplest and most common is the screw adjuster. It's been around for a long time and works just fine. Method number two involves shimming the valves. The shims are generally secured by a bucket-shaped tappet or lifter placed directly between the cam and the valve stem. The cams may or may not require removal to replace the shims, but either way adjustment is more complicated. Since few cruiser-style motorcycles use a shim adjuster, we'll address them separately. The third type of adjuster is the hydraulic lifter, which normally requires no routine inspection or adjustment.
Begin by letting the bike cool until the engine is cold to the touch. In most cases the fuel tank, and possibly some bodywork will have to be removed to gain access to the valve cover. Once the valve cover is exposed, wipe away any debris or dirt around the valve cover(s) and use compressed air to blow any offending matter away from the spark plug wells (in a pinch you can use a soda straw and your own lung power).
Rotate the engine in its normal...
Rotate the engine in its normal direction of travel while observing the valve gear of the cylinder you want to adjust. If you are unsure which way it turns, put it in gear and watch the rear wheel (or turn the engine by turning the rear wheel in the driving direction).
Remove the valve cover according to your shop manual's instructions. Valve covers differ greatly in their construction. Some are nothing more than a small cover that's unscrewed or unbolted. Others may be cumbersome affairs held on by 20 or more bolts. If the valve cover won't come loose after all the fasteners have been removed, it's presumable that the gasket has glued itself to the head and cover. If you are absolutely sure all the fasteners have been removed, place a block of wood, angled slightly upward, on one corner of the valve cover, and give the block a sharp blow with a hammer. That should dislodge the cover. Resist the temptation to pry the cover off. Chances are all you'll do is break something expensive. Don't try to hammer a screwdriver or chisel between the valve cover and the head. You will ruin the gasket surface in very short order.
Before the valves can be set you'll need to find top dead center (TDC). Manufacturers generally imprint either the flywheel, the alternator rotor, (if it's mounted on the end of the crankshaft), or the ignition rotor with a datum mark indicating top dead center. Depending on the engine design, there may be one mark, or a separate mark for each cylinder. Your manual will fill you in on the exact procedure.
To make turning the engine over easier, most bikes have some kind of bolt threaded into the crankshaft which may be used to rotate the engine. Sometimes the bolt is used to hold the alternator rotor. Occasionally it's installed specifically to use when turning the engine over. Every so often you'll run into an engine that has no convenient way of turning it over. The easy solution then is to position the bike so that the rear wheel is clear of the ground. Pop it into gear and -- with the spark plug(s) removed -- turn the rear wheel by hand to position the engine.
The feeler gauge cooresponding...
The feeler gauge cooresponding to the specified valve clearance (or lash)should slide between the valve and the adjuster with a slight bit of drag.
Valve clearances are checked and adjusted when the crankshaft/piston of the cylinder you are adjusting are at top dead center on the compression stroke. Start by rotating the engine in its normal direction of travel. Watch the intake valve as you rotate the engine. The valve will open on the intake stroke and close on the compression stroke. When the valve begins to close, start to look for the TDC indicator. When it lines up with its reference mark, check both rocker arms for freeplay. If the intake and exhaust rocker arms have some lash and the TDC marks line up, the engine is on the TDC of the compression stroke and you can check your valve clearance. If one of the rocker arms is depressed you're on the wrong stroke (even if the marks line up) and need to rotate the engine around again until both rockers are free. Your shop manual will list the appropriate valve clearances. If you don't have one, the clearances may also be found on the tune-up/emission information sticker located somewhere under the seat, on the frame, or in the owner's manual.
To measure the valve clearance select the appropriate feeler-gauge, and place the blade of the gauge through the gap between the adjuster and the valve stem. The blade should have some drag on it, but still be free to pull through. Using a feeler gauge takes some practice and is always somewhat subjective. Until you get used to doing it you can cheat a little. Say, if the manual calls for .004 (four thousandths of an inch) clearance, slide your .004 gauge through the gap. If it feels a little loose try a .005 blade. If it won't go, you know the adjustment is as good as it's going to get. If the .005 slides through the valve, it's still a little loose. Re-adjust it. You can also buy what's called a "go-no-go" gauge. These are gauges made with a step in them. The first step of the gauge is one size and the rest of the gauge is slightly larger. If the first portion of the gauge slides through but stops at the step, the valve is correctly adjusted. A typical go-no-go blade would be marked .004 - .006. Go-no-go gauges sacrifice a bit of accuracy but they make the job easier, and trust me, .001 difference won't mean a thing in the real world.
This valve was a little sloppy...
This valve was a little sloppy and needed adjusting. Once you have done it a few times, it just takes a few seconds. Before you write in to chastise me, yes, I know there is a special valve adjusting tool available instead of the wrench. No I don't have (or need) one. The small adjustable wrench works just fine, thank you. Repeat the clearance check and, if necessary, the adjustment process for each valve in each cylinder in turn.
Because the exhaust valve runs so much hotter than the intake valve, it's normally given more clearance. However, there are some oddball exceptions to the rule. So if something doesn't read right, cross-reference your info.
If the valve needs adjustment, loosen the lock nut and, using the appropriate tool, turn the adjuster to decrease or increase the lash. You'll find that leaving the adjuster nut a little snug will keep some tension on the adjusting screw, making it easier to accurately set the clearance. When the clearance feels good, hold the adjusting screw and tighten up the lock nut.
Recheck your lash with the gauge. If you find that the clearance has changed, don't panic. Tightening the lock nut often changes the clearance slightly. When the valves of the first cylinder are finished move on to the next and repeat the procedure until all of the valves have been adjusted. Novice mechanics may feel a bit intimidated by all of this. But with a little patience, the right tools and the right attitude, anyone clever enough to ride a motorcycle is certainly clever enough maintain it properly.
Not many modern cruisers use shim-adjusted valves, but they do pop up now and then. There are two basic styles of shim-adjusted valves. The more common places the shim atop a small inverted bucket. The bucket rests on top of the valve, over the valve stem and spring, and the cam acts directly against the shim. Generally when a shim-over-bucket design is employed, the shims can be removed and replaced without disturbing the cams. Less convenient is the shim-under-bucket style adjuster. This design places the shim underneath the bucket, between the valve stem and the underside of the bucket. This system complicates adjustment, as the cams must be removed to change shims. Since the shims are only available in set increments, valve lash tolerances are usually a little more generous.
The big disadvantage here is that not many owners are willing to go out and buy a box of valve adjustment shims. What you'll end up doing is measuring all of the clearances. If any are off you'll need to determine what size replacement shim you'll need and head down to your local dealer to buy one. That is, unless you are swapping one out that happens to be the size you need, or you have one from a previous adjustment. Keeping a record of which shims are in the bike can simplify this. If the correct size isn't in stock, you're screwed. You can reassemble and ride the bike the way it is, which means doing a large portion of the job twice or leave it apart until the shim arrives, which makes a great argument for screw-type adjusters.
Send your technical questions and comments to Motorcycle Cruiser's Technical Editor at email@example.com.
For more articles on how to maintain and modify your motorcycle, see the Tech section of MotorcycleCruiser.com.