These are 4 Reasons Why Your Motorbike Engine Is Not Knocking

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Because motorcycles are so simple, it is easy to fix any mechanical problems. Many of these repairs can be done by the owner.

What about unusual sounds coming from your engine? Motorcyclists are often concerned about the sound of a motorcycle’s engine knocking. Engine knocking should not be ignored and should always be addressed immediately.

What is the reason my motorcycle engine keeps chugging? Premature detonation is the most common cause of a motorcycle engine knock. This makes the piston unable to reach the top of the cylinder and complete it’s cycle which causes the piston to move sideways and knock the side of the cylinder wall. You can also have a knocking engine due to a warped wall, valve tapping, and lateral moving bearings.

Having done over a dozen motorcycle restorations, I’ve dealt with a good amount of engines that knock. I’ve taken engines apart and rebuilt them myself. This is what I’ve learned and and I can explain the reasons your motorcycle engine is making that annoying knocking sound.

Premature Detonation

Premature detonation, which is the most common reason why a motorcycle’s engine knocks, is the main cause. This could cause serious damage to your engine. If you begin to hear a knocking sound coming from your engine, it’s really only a matter of time before it seizes if you don’t address it promptly. This is not a sound that will go away on it’s own.

To understand the term premature detonation, you’ll first need to understand how a four-stroke motorcycle engine works (most motorcycles have a four-stroke engine). Two valves are found in each chamber or cylinder. One for exhaust, and one for intake. Each cycle will see the piston come down, and the intake valve will be open to let air in. That’s what creates a vacuum that sucks in air and gas. After they are inhaled, the valve closes and seals the chamber.

When that piston is pushing the gas and air back up, it’s creates pressure and compression and the spark lights off when the piston reaches the top. That detonation forces the piston back down and that’s what rotates the crankshaft which in turn gives power for the motorcycle to go.

The compression stroke refers to the stroke that comes up between the valves. Premature explosion occurs when the gas and the air ignite before it can reach the top. The friction and force created by ignition pushes against the piston’s attempts to work. As a result, the piston will turn slightly sideways for a split of a second. When this happens, the side of the piston is going to hit the cylinder wall and you’ll begin to hear that knocking noise.

The left image illustrates normal compression. The image to the right shows premature detonation. This is because the piston turns slightly, causing the knocking sound.

Even if your motorcycle is equipped with a 2-stroke engine, the mechanism will still work. The engine could cause a premature explosion for many reasons.

Premature detonation is the most common reason a motorcycle engine explodes. A lot of older motorcycles don’t have automatically advanced timing so you have to manually adjust the points so the timing is correct; otherwise your spark plugs will be sparking before the piston gets to the top of the cylinder and will tap the side. This is also called spark delay.

An incorrect gap size can cause spark delay. This can be caused by the spark plug’s gap being too close together.

Motorcycles that are too lean can also lead to premature engine detonation. When an engine runs lean, that means the air to fuel ration is off and there’s either not enough fuel or too much air. This can cause extreme heat within the engine and lead to detonation of the cylinder before it reaches its top.

If If you have dirty pistons, or carbon deposits at the bottom of valves, premature detonation can also occur. These deposits usually have a brownish-black color.

Carbon deposits formed on a piston in the engine I was rebuilding for my 1969 Triumph TR25W Trophy.

These deposits can quickly become very hot. The deposits can cause the piston to fail to reach the top of the chamber if the fuel is squirted in. You can think of the carbon deposits as burning embers; they’re unintentionally doing the job for the spark plugs but not doing it at the right time.

A motorcycle engine that is experiencing premature detonation can also be caused by low-octane fuel. Premium gas should only be used if you have a high-compression ratio engine, which most motorcycles do. Low octane fuel pressure can cause fuel to ignite without heat by combining air and fuel. Only pressure can ignite the fuel. The fuel you use for your engine must be able to withstand the pressure.

No matter what type of motorcycle you own, premium, higher-octane fuel should be used. Motorcycles usually get better gas mileage and it doesn’t take much to fill the tank, so this should only cost you a few extra dollars. You can find more information on what type of gasoline motorcycles should use by clicking here.

Warped Cylinder Wall

Warped cylinder walls can also be a reason why a motorcycle engine might not start. The cylinder wall can become slightly misshaped or warped. The affects of this will be similar to premature detonation but timing isn’t the issue, rather the piston has an obstacle it can’t overcome.

The cylinder wall must be perfectly straight and smooth in order to allow the piston to move upwards and downwards to give power to the motorcycle. Any type of warp will cause the piston to struggle getting all the way to the top of the cylinder and complete it’s stroke. The forces of the piston pushing up against another force can cause the piston to move slightly sideways for a split second, knocking on the cylinder wall.

This is often caused by engine overheating. The metal on the overheated engine will begin to warp, expand, and misshape because of the extreme heat it’s experiencing. This happens either when the engine is working too hard, there is a lack of coolant (on water cooled bikes), or there isn’t enough oil to lubricate the parts of the engine.

It’s also possible for the engine to have been rebuilt incorrectly if you had that done recently. As reliable as most mechanics are, it may have been machined poorly while they were working on it and the cylinder wall isn’t smooth. This is known as an “out-of-round” cylinder wall.

Valve Tapping

Interference engines can be found in nearly all modern motorcycle engines. The valve must be open for the piston to strike it. It is important to adjust timing correctly or replace the timing chain. If the valve is open just a little bit too much and the piston comes up, it’s going to slam into that valve and bend it. The sound of the piston touching the top is caused by the valve falling.

It’s also possible for the valves to be slightly open when detonation occurs which causes that valve to slam shut prematurely because the ignition that’s happening underneath the valve is forcing it to do so (which can also be caused by premature detonation). A motorcycle engine will make a tapping or clicking sound when the valve closes.

Valve knocking can be caused by a timing error, even if it is just one tooth. Although motorcycles have the same timing as cars, most have chains and not belts. It is located under the left-hand side cover of the engine. This likely wraps around the stator.

This type of engine knocking isn’t something that spontaneously happens. This is only caused if you’ve opened up the side cover where the timing chain is located, removed the chain and replaced it, or recently had it worked on.

Bearings Bottom End Knock

On the lower half a motorcycle engine, there is a crankshaft as well as a crankshaft bearing. The bearing is a circular shape and within it is a smaller circle.

The engine will have a bearing along the crankshaft and that’s what allows the arms to spin. A bad bearing can allow for a bit of lateral movement, as the piston goes up or down. This can result in the knocking sound.

Age is often the primary reason why a motorcycle engine’s bearings start to wear. However, this type of engine knocking is much less common than those mentioned previously. This could be the reason for a knocking sound coming out of the lower engine.

Overheating can also cause this. Again, the extreme heat the engine may be experiencing can warp some of the metal parts inside which could ultimately cause the bearing to malfunction if it isn’t perfectly straight and aligned.

You may also experience a bottom-end knock on your motorcycle if you don’t have enough lubrication. The friction that can build up between the crankshaft and the crankshaft bearing can cause some major damage if there’s not enough lubrication happening throughout the mechanism. A bottom engine restoration can be more costly than a top engine restoration.

A bottom engine knock can also be caused by an excessive amount of oil. Excessive oil can cause excessive oil pressure, which places stress on the crankshaft.

An engine that was too oily in it once caused me to crash my bike. The dipstick was incorrectly inserted by the previous owner. It was too short. Oil was added in order to reach the “correct” Level according to the dipstick, which means too much oil was added. For a short time, I ignored a bottom-end knock that started to happen. Before I knew what was happening, the engine blew up and started throwing rods. The engine looked like a chunk of Swiss cheese.

How to handle an engine knock on your motorcycle

It is not a good sign to hear your engine hum from your motorcycle. Some of the reasons it has happened could have been prevented, but once it has started happening there’s potential damage that has already been done to your engine. There’s likely some sort of fix you’ll have to make in order to ensure it’s full functionality.

Always address an engine knock promptly If you hear the dreaded tickling or knocking sound, turn off your motorcycle immediately. Do not start it again until you are satisfied that the problem is resolved. You should never take it to the shop in such a condition.

The level of your experience with motorcycle engines will play a major role in how you fix your knocking motor. The easiest problems are the ones that are most suitable for beginners. These fixes include checking the sparkplug gap on each sparkplug to ensure that it is the right distance. A lot of people don’t understand that when you buy new spark plugs, you need to set the gap at a certain distance; they don’t come stock at the distance your motorcycle needs.

This can be done quickly with simple tools. A spark gap gauge, which is about the same size and looks like a half-dollar coin, is what I use. These gauges are available at most auto parts stores and cost about $2. Check your owner’s manual to find the correct distance your spark plugs need to be at.

You can check the level of your motorcycle’s oil and make adjustments if necessary. Make sure there is the correct amount of oil added, meaning there isn’t too much or too little.

Lean motorcycles can cause premature detonation. Lean motorcycles can be characterized by a slow, hotter motorcycle and spark plugs that have little or no wear. You can read my article to find out how to tell if your bike is lean.

If you have a lean running engine, you’ll need to adjust your carburetor. This is done by turning the screw on its side, which adjusts what mixture it gives the engine.

If you are able to do so, you can also check the timing of your motorcycle. You will need to access the crankshaft as well as the points, and use a variety of tools to check the timing. If any of that sounds unfamiliar to you, it’s probably best you leave this test to a mechanic.

In all other cases, it’s best to take your motorcycle in to a professional mechanic and have them fix it for you. I am all about doing DIY fixes on a motorcycle, but engine fixes are something I don’t want to toy with if I know it’s over my head. A small error could endanger the engine’s life and cost you a lot of money.

How to Prevent Engine Knock

You can prevent engine knocks from happening to your bike by following these simple steps. The answers are all in how well you maintain your bike (unless the problem was caused by a careless mechanic who didn’t do their job right).

Always use high-octane, ethanol free, premium gas in your motorcycle. Your motorcycle is saving you a lot of money on gas mileage, so it’s only reasonable to put in the best gas that it needs.

Routine oil changes should be a priority. Just because you don’t ride your motorcycle very often doesn’t mean you don’t need an oil change very often either. Your oil should be changed every 4,000 – 5,000 miles or every six months, whichever comes first. Check out my article for more information about how often to have your motorcycle serviced.

A properly functioning carburetor and spark plug gaps are always a must. A good engine temperature will help prevent engine knock. Maintain a good engine temperature if your engine is water-cooled. If you have an air cooled engine, stay conscious of how hot your engine may be getting if you’re going high speeds or idling in traffic on a hot day.

It is a good idea to have your car checked every two years. This will also include a timing adjustment. It is important to keep your timing in check in order to prevent a motorcycle from knocking. Timing problems are often the cause of many of these problems.

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