Why Does My Dirt Bike Rev High When I Start It? (With or Without Throttle)


why does my dirt bike rev when i start

Revving is the increase in the speed of a machine or engine when the accelerator is pressed. Sometimes, riders intentionally rev their bikes. This, however, should not go on for an extended period as it could end up damaging the engine’s cylinder walls because of overheating.

Reasons Why Riders Rev Their Bikes

  • To forbear from having a fixed engine speed
  • To check on the functioning of a recently repaired engine.
  • To change gears.

What Are the Effects of Over-Revving?

  • Valve damage – during revving, the valves stay open, and if they remain so for a long time, a valve float occurs.
    A valve float is when the valves jam in between close and open and leads to loss of power by the engine.
  • Clutch ruin

Reasons Why Dirt Bikes Rev

Vacuum Leak

A vacuum leak occurs with the introduction of extra air into the fuel and air mixture from the carburettors.

This happens when low pressure draws air between the carburettor and the cylinder head. A vacuum leak can be caused by:

  • Cracked vacuum pipe
  • Distorted intake manifold
  • Malfunctioning throttle

How Do You Know If There Is a Vacuum Leak?

Rough Idle

This is when there is a problem with the ignition.

For the bike to start, it needs a spark to ignite the mixture of fuel and air, and however, if there is a vacuum leak to the mix, it becomes difficult to start as there is little or no ignition.

High or Low Idle RPM

Idle RPM refers to the speed at which an engine operates at, measure in revolutions per minute when the engine is warmed, and the vehicle stopped.

The ideal idle RPM for bikes ranges between 600 and 800. During a vacuum leak, the oxygen sensors detect the extra air and signal the EFI (Electronic Fuel Injection system to release more fuel to act on the air, and this results into a higher or lower idle than usual.

Rough Acceleration

A vacuum leak results to a wrong fuel-air ratio, thereby causing acceleration to be slow and irregular.

Engine Light

In the case of malfunctions linked to the engine such as vacuum, a flashing warning light appears in the dashboard prompting you to check on the engine. 

Loud Engine Sound

Sometimes, due to the continuous sucking of air by the engine, I may make a loud sound, a sign that there is indeed a vacuum leak.

Debris in the Carb

When there is dirt in the fuel, it may block the carburettor jets.

Any dirt like rust can lead to significant damage to the engine and is one of the causes of revs in bikes.

It is necessary to have a filter which should be cleaned regularly to prevent dirt from entering the engine.

Fuel Starving

Fuel starvation happens when the fuel supply is inadequate for the regular running of an engine.

This causes the driver to stop and is a result of contaminated fuel (mostly by water), clogging of carburettor jets, defective fuel pump or vapour lock.

Any factor that affects the regular supply of fuel to the engine may cause extensive damage and low performance of your bike.

It is, therefore, necessary that you fix such issues beforehand.

A defective fuel pump is easy to detect as you are likely to hear whining sounds from the engine when running.

You may also experience misfires and a decrease in power while riding.

Slow and rough acceleration and increased gasoline consumption are also frequent.

Contaminated fuel causes the engine to be sluggish and could completely shut down.

Needle and Seat Assemblies

The needle seat is a valve that regulates the amount of fuel that enters the carburettor.

It works in such a way that fuel flows when the needle is off the seat and stops when the needle is in the position.

When the needle seat is faulty, it can lead to the flooding of gasoline into the carburettor.

Very high speed can cause a valve float, and thus it is necessary to control the motion of the bike using a dirt bike Rev Limiter.

A rev-limiter is an appliance contoured in dirt bikes to control its speed, thereby protecting the engine.

There are two types of rev limiters;

  • Fuel control
  • Spark control

Fuel control rev limiters work by shutting off the engine’s computer system. In turn, this prevents fuel from being released into the carburettor.

Thereby slowing down the bike.

On the other hand, spark control rev limiters curb over speeding by ceasing operations of the spark plugs.

Spark plugs produce the spark used for igniting an engine, and once they are shut down, there is no ignition, and the bike slows down.

Running Out of Fuel

When your gas tank is almost empty, the bike is likely to rev.

This is as a result of insufficient flow of fuel to the engine carburettor.

High Idle Screw

An idle screw functions by changing the position of the throttle plate to increase or decrease an engine’s idle speed.

Idle speed is the speed at which an engine operates at when in a stationary position and is measured in RPM.

When the idle screw is high, it raises the RPM of a motor, causing a rough idle when the engine is restarted.

Conclusion

Revving is not a severe engine issue. However, revving could be a sign of engine error and needs to be checked to maintain your engine and to ensure your safety while riding.

Regular over-revving is not advisable as it could impair the normal functioning of the engine.

Several measures such as using dirt bike rev limiters should be put in place to handle the issue on revving.

You must remain observant for all the signs and symptoms of revving, inspect your bike thoroughly and take it to a mechanic for repair.

Regular servicing of the bike is also essential in its maintenance.

Daniel Mose

Daniel is a big fan of dirt biking. I own KTM 250 SX-F. I love learning how to maintain this dirt bike and ensure it lasts long while serving me without any issues. Join me in this journey as I document stuff that we, as dirt bike lovers, ought to know.

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