Monitoring Motorcycle Engine Health

I wonder how your engine feels... By Art Friedman.

Some folks never seem to worry about their engines, and some people fuss over them all the time. The former group is getting larger as engines become increasingly durable and fiddle-free. However, almost anybody would like to know if something is going amiss down there in the mechanical guts of their expensive powerplant. This allows you to head off disaster and reduce the repair bill, or at least avoid getting stranded out on that little unnumbered country road that nobody but you seems to know about.

Engine failure certainly isn't the only--or even the most likely--problem that can strand you, but for many owners it is the hardest to predict. It's also the most expensive and usually doesn't lend itself to roadside repairs. The following techniques help you keep tabs on an engine's health and can pinpoint problems when they do arise. Any one of these methods used by itself won't guarantee that you'll spot a problem, but when you combine some of these monitoring techniques, you should be warned of trouble. The approaches described are not particularly expensive, and many cost nothing. Some require you to invest in or borrow specialized equipment, but items like compression testers and filter cutter are inexpensive. Most of these tools can be used on other internal-combustion devices, which is how I found the burnt valve in my generator...

Sound and Feel

Personally, I find that an engine can always be counted on to sound and feel its worst as the moments when I least want it to fail, such as when I'm far from home, late on Saturday night on a three-day weekend, 20 miles from the nearest phone and it's starting to rain.

However, if you are intimately familiar with every sound and movement of your motorcycle, that first little click or whine of a loose valve or bad bearing will probably catch your ear, assuming it develops relatively quickly so you don't have a chance to get used to it. Likewise, a change in frequency or magnitude of vibration will also get your attention. Of course, vibration can originate in wheels, chains or other spinning parts as well. You can usually tell which if you shift gears or cut the engine and coast. If it changes frequency with the engine, it's at least in the front part of the drivetrain. If the frequency changes with the bike's speed, the problem is behind the output shaft or in the front wheel.

Just don't get too sensitive. In college, there was a fellow who had the same model as one of my motorcycles. He frequently asked me to confirm if engine sounds were alarming. I never heard anything unusual. Eventually, after taking it to the dealer for tune-ups every 400 miles, he stopped just fretting about noises he heard from the engine and started fiddling with things to eliminate them. Finally, just before leaving on a trip, he announced triumphantly that he'd exorcised the sounds. He was three states away when the valves—which he'd tightened up to get rid of those normal tappet noises—burned and ended the ride with an expensive clatter.


A few cruisers come with engine gauges as standard equipment. Coolant temperature is the most common engine function monitored on cruiser instrument panels. However, there are a number of other gauges you could add to keep an eye of the engine room if you are customizing your dash. These include oil temperature and pressure and cylinder-head temperature. The temp gauges are easy to wire in on most bikes. Finding a source of oil pressure may be a bit more complicated, though this function is monitored by an idiot light on almost all motorcycles.

Whether your gauge is an add-on or standard equipment, you should get to know its habits. I like to mark the normal range of a gauge with thin pieces of plastic tape. An increase in temperature can be caused by rising ambient temperature or greater load, but if those conditions don't exist, it should be investigated. It can signal all sort of maladies: carburetion problems, ignition out of whack, a variety of engine ills, non-engine problems such as increased drag or a simply a bad gauge or sender. A drop in oil pressure can also be a gauge problem or debris in the sender, but it's always cause for investigation.

Oil Consumption

For engines without gauges, this can be the first symptom of an internal engine problem, and it's easy to monitor.

Once broken in, which takes less than 1000 miles if done right (that is, you didn't baby the engine, but ran it hard during those forst few hundred miles), you engine should use oil at a fairly consistent (and low) rate. If you get into the habit of carefully filling the crankcase to exactly the same point each time you add oil (which may just be at oil changes on liquid-cooled bikes), then check the level at regular intervals, say every time it turns over another 1000 miles, any variation in oil consumption should be apparent. An increase in the rate at which you must add oil indicates a possible top-end problem, normally rings or valve seals.

Changes in the oil you use or the way you ride may change the rate of oil consumption. Some bikes may use more oil if you change to a thinner oil or if you ride fast for long periods in hot weather. You may also see an increase in oil consumption if the oil starts to break down after extended use. With time, you will get to know these normal variations, but an unexplained rise in oil consumption should be checked out, usually with a compression check.

Fuel Consumption

A sustained increase in fuel consumption is another potential warning sign, but it doesn't necessarily mean that you have an engine problem. It could be a dragging brake, a tight chain, misaligned wheels, low tire pressure, a change in driving habits, a dirty air filter, the effect of new accessories, a different fuel formulation, or simply time for a tune up. We always calculate fuel mileage at every fill-up, but it takes several low-mpg tanks to get our attention.

Spark Plugs

Back when ignitions and carburetors were crude and leaded fuel was the norm, spark plugs were a useful way to check on all sorts of areas of combustion. As much as motorcyclists like to moan about emissions regulations, they have given us more precise and reliable ignition and carburetion and eliminated the deposits and subsequent problems that came from leaded fuels. They have also made it possible for spark plugs to work well for a much longer period, even as engine output as increased. With some of the exotic-metal-electrode spark plugs currently available, it's not unheard of for a motorcycle spark plug to last 100,000 miles.

It's much less common for a spark plug to reveal signs of engine problems in a stock engine. A spark plug can still serve to help you get mixture dialed in after you have made some engine modifications and rejetted, but it's rare that it reveals carburetor or ignition trouble. The one possible trouble sign is a blackened appearance on the inner insulator, indicating that oil is getting into the combustion chamber. A wet plug also should get your attention. This seems to be most common when you are trying to start a bike that has sat for a long time. If the fuel has had an opportunity to stratify, the heavy components reach the combustion chamber first, and usually don't burn well, if at all. The plugs will often foul under these circumstances. The fuel problem can usually be detected by the stale-gas smell of the plugs.

However, if the carbs are delivering clean, fresh fuel to the engine and your plugs are wet with fuel, you have cause to suspect bigger problems. At it's simplest, this can be something blocking the intake or exhaust (perhaps a dirty air filter element), but you might also find ignition or carburetor problems.

Oil fouling, which sometimes is mistaken for gas fouling because the unburned gas coats the plugs when they get oil fouled and stop sparking, indicates a mechanical problem within the engine. A leak-down compression test can pin-point it.

Compression Checks

There are two kinds of compression tests, a simple measurement of compression and a test of the cylinder's ability to hold compression, called a leak-down test. Both can offer substantive information about engine problems.

Most home mechanics are familiar with the basic compression tester. This inexpensive pressure gauge measures maximum compression pressure as the warm engine is turned over with the starter. You compare the value obtained to the specification in your shop manual. If it's too low, you have a problem, though you can't tell exactly where without further testing. If pumping a small amount of oil into the cylinder improves the reading, the problem is probably rings. (A too-high compression reading on an accurate gauge usually indicates a significant deposit build-up in the combustion chamber.)

A leak-down test also measures pressure in a hot engine, but instead of using the engine to compress the air, you supply your own with a compressor. The testing tool sets the incoming pressure, usually at about 80 psi, and then tells you how well the cylinder holds that compression. Most importantly, it allows you to easily determine where the compression leak is. You can move the piston up and down in the cylinder, and thereby find a defect in the cylinder wall if pressure drops at a particular point in the piston's stroke. More likely, the problem will be worn or stuck rings or a poor valve seal, and you can determine which one by listening and with the addition of oil. By listening, you can determine where the pressurized air is escaping. If you hear it through the exhaust, your exhaust valves are not seating properly. Escaping air in the carb indicates that it's the intake valve(s). If you hear the hissing of escaping air at the oil filler or crankcase breather, the problem is probably stuck or worn rings.

A couple of points should be considered before acting on these readings. Unless it's completely in the cellar, a single low compression reading isn't reason enough to rip your engine apart. Ride it for half an hour or so and check again before you get out the wrenches. The pressure differential between cylinders is almost as important as the reading itself. A significant difference between cylinders is more troubling than if all cylinders are equally low.

Compression tests, especially leak-down tests, are the best way to monitor the integrity of your combustion chamber. Even if you don't want to invest in the tools yourself (a compression tester can be had for $20 or less and a leak-down set up, less compressor, is under $100), if your bike is piling up mileage, it's worthwhile to ask your shop to test compression when the bike is serviced.

Oil Filter Inspection

It's messy, but examining what's been caught in the element of your oil filter can turn up signs of trouble. If you can identify the source of material caught in your filter, you might be able to get right to the source of trouble.

To examine the contents of your filter, you need to separate the element from its metal skeleton. For drop-in filters, this means cutting the paper element from the its metal supports. Spin-on filters require the metal can to be cut open. There are filter inspection kits sold just for this purpose (you can find them at truck supply and aircraft supply outlets) or an old-fashioned can opener might work. Just make sure that whatever you use doesn't leave any filings, or you may "find" signs of trouble that are merely bits of filter can.

Spread out the paper of the element and examine what's trapped in it. Sometimes washing it with solvent helps separate the debris. You are looking for bits of metal. Dragging a strong magnet through the debris ferrets out any ferrous metal. Most of what's in the filter is normal wear material, mostly bits of clutch. You might turn up a small amount of metal, but it should be a SMALL amount. A minor fraction of a teaspoon is the maximum. The pieces should also be small, the consistency of powder. Something as big as a grain of sand could mean trouble. Certainly any identifiable chunks are signs of distress and demand further inspection. Discovering the small end of a cnnecting rod in the oil filter often leads us to believe that there might be trouble.

Magnets can also pick up harbingers of engine trouble. Although magnetic drain plugs have become rare, you can accomplish much the same thing by place one or more strong magnets on the outside of your spin-on oil-filter. Griot's Garage sells a band of magnets intended for this purpose. Leave the magnets on when you cut the filter open. Metal shavings will cling to the inside of the filter can at the points where the magnets are attached. Again, a teaspoon full means trouble.

Oil Analysis

Spectrographic analysis of a sample of used oil drained from your engine reveals levels of wear metals and other materials contained in it. Higher-than-normal levels indicate forthcoming trouble. The problem may be determining what is normal for your engine, riding regime and other factors. For this reason, oil analysis is best used as a trend tool. In other words, you if you are going to use oil analysis, it should be every time or every other time you change oil.

You can get oil analysis kits at aircraft and heavy truck supply stores. Get several kits from the same lab, so that you get consistent testing and so that it can plot trends. Most labs supply a bellows-type container, which can be used to suck up a sample of the oil, either from the drain stream or through a supplied hose. Fill out the form packed with kit and return it with the oil sample.

You will get a report a week or so later showing the levels of all the materials detected. If some material's level is excessive, you may get a notice that you should sample again soon. The report may also include suggestions as to the cause of the elevated level of the material(s). If the lab finds a major jump in something, you may be advised, sometimes by phone, to pursue further inspections.

Oil analysis is probably the most expensive of the various techniques for detecting symptoms of mechanical illness in your engine, but it's only about $15 to $20 per kit (which includes all costs except mailing the kit to the lab). It is the best way to find bearing problems, gear wear, and other issues that won't show up in other inspections and tests.

Internal Examination

When red flags pop up, there is one final method of checking out the problem before you rip the engine apart. There are a couple of types of viewing devices that you can insert through spark plug holes, crankcase filler holes or other orifices to take a look at the suspected trouble area. Finding a motorcycle shop that has such a fiber-optic viewer or borescope may be difficult, though again those people dealing with expensive engines--truck and aircraft mechanics--may have one.

By spending a little extra time giving your engine periodic check-ups you can catch problems before they blow up and damage other components. And, like the truckers and aircraft owners who use these techniques as standard operating procedure, you also get piece of mind and confidence in your machinery.

For more articles on how to maintain and modify your motorcycle, see the Tech section of

What hidden damage lies undiscovered inside your engine? There are easy ways to find out.
A loud pipe can make it hard to hear your engine's aural warnings.
This oil-temperature gauge clamps on to your bike's handlebar. Installing the pickup may be a bit more demanding.
If they are broken in properly (i.e., not too gently), modern motorcycle engines should use very little oil. An increase in oil use bears investigation.
Spark plugs can tell you about the condition of your combustion chamber(s).
Properly used and analyzed, you'll get much more useful information from a leak-down tester than you will from a simple compression gauge.
This inexpensive oil-filter cutter kit lets you examine what has collected inside your spin-on filter.
A band of magnets strapped around a spin-on oil filter will collect the bits of ferrous metal for easier examination.
If one valve requires more frequent or larger adjustments than the others, it can signal problems with the valve seat or camshaft lobe.