Harley Davidson – Why Is My Engine Lugging?

You may have heard the term; ‘engine lugging’ before and that it is bad for your engine…but what does it actually mean, what causes it and why is it so bad?

Essentially when you ride around in a higher gear at low rpms and then roll on the throttle, either to increase speed for overtaking or going uphill, then more fuel air is forced into the cylinders trying to overcome the inertia of the engine, drivetrain and motorcycle itself causing extra stress and potential long term damage.

So what is happening? Well if the motorcycle is being ridden at low rpms and in high gear lets say on a flat surface or downhill, then the force required to overcome the reduced rolling resistance is less. There is less stress and less work to be done by the engine components and gearbox. However when going uphill or trying to overtaking quickly in still a high gear, the extra heat induced by extra fuel going into the cylinders is not efficient enough to drive the pistons fast enough to overcome that initial strain on the components, this is where lugging occurs.

Pre ignition and detonation can occur which essentially means that the increased fuel introduced into the cylinders will combust too quickly or early which creates hot spots and explosive properties. As the engine is in a low rpm, the oil and or water if applicable in a liquid cooled engine, being pumped around the engine is not at optimum, thus engine cooling is reduced and not enough to counteract the increased cylinder temperatures.

This increase in combustion in the cylinders can also raise the internal pressure too high thus causing increase blow by the rings and thus causing increased pressure in the crankcase, leading to inefficiency and early deterioration of oil too.

The higher temperatures in the cylinders can also cause damage to the piston rings and crowns and cylinder walls as well as propagate early carbon build up too…all of which is detrimental to the health of an engine.

So, the best way to overcome engine lugging is to ride at higher rpms in gears or stepping down a gear before rolling on the throttle when wishing to overtake or go up a steep hill for example.

However, aren’t Harley Davidson engines full of torque that will allow them to power through low rpms and higher gears? Yes to a certain extent but still they will be affected to varying degrees depending on the age of the engine and any previous engine upgrades performed.

If we take a stock engine, then overtime the engine ‘loosens up’ in that it may appear to run more freely and be less sluggish. However this is mostly due to engine and transmission wear and is not indicative of an engine gaining power. Actually the reverse is true. With age, the engine will produce less torque and less horsepower, especially one that has been poorly maintained.

Therefore, in an older bike or one with a few thousand miles on the clock, even though the engine may be at a higher rpm, due to the engine wear, it may not have the torque and horsepower to overcome the inertial resistance on the motorcycle. Therefore you may find you’ll need to ride at higher RPMS or ride in lower gears to get uphill or overtake, whereas in a new engine, there was sufficient torque and horsepower to overcome the same inertia.

The irony here is that when riding at low rpms and higher gears when the bike was new and with low mileage has actually helped to cause this early deterioration too.

OK, let’s consider a motorcycle that has had performance upgrades, upstaging, different cams etc. Can this cause lugging too? Well yes and no?

It really depends whether the performance upgrades will forgive lower rpms and higher gears or not. And this also relates to the kind of tuner that is on your motorcycle and if it has been calibrated correctly too. Sometimes the work done and tune installed is not supportive of cruising or higher gears/low speed riding. It may mean that you either need to have the tune adjusted, change the cam to suit your riding needs or simply adopt a different riding style.

If you find you are still riding at higher rpms and the engine is lugging, I.e. it is struggling with power delivery at lower speeds with an increase in throttle, or there are abnormal sounds and vibrations, then this could be indicative of early engine wear, engine or sensor faults, poor fuel management either from a carburettor or fuel injection system or simple it has a poorly defined engine tune which needs to be modified or changed completely.

Keep in mind that the older your motorcycle and the more engine upgrades may cause lugging to occur at lower speeds and higher gears. Alway try to modify your riding style to increase rpms or ride in lower gears when requiring to add significant throttle.

You may find that when the bike was new or before engine modifications you could ride in too gear at 40mph but over a period of time you may realise that you really need to be in 4th or 5th at most to ride at that speed. This is a natural consequence of older engines.

However in general practice, it is better step down a gear before rolling on the throttle to increase rpms even if your motorcycle is supposed to have a lot of low end torque, it may not remain that way for long if it is constantly being used.

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