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Whether it’s a small ornamental lawn or a lush multi-purpose lawn – the lawnmower helps to cut back the cherished and tended area regularly for a dense and even growth. But the lawn mower won’t stay running all the time. Even the best device occasionally causes problems. Large walk behind mower with bagger, the first sign of serious problems is often a fluctuating speed. Find out why and what you can do here.

Ultimately, there are only a few factors that can cause an engine to run smoothly. The causes for a fluctuating speed, however, are far more diverse.

The clogged air filter

For even combustion of the gasoline, a predefined amount of oxygen and thus also of the air supplied from the outside is required. If the supplied oxygen fluctuates, the speed drops with the amount of supplied air and the engine runs irregularly.

  • Causes: dirty, clogged air filter
    • e.g. by sucking in dust and grass residues
  • Remedy: Open and clean the filter,
    • Disposable filters (paper filters) as a provisional immediate measure blow or knock out with compressed air
    • exchange in the long run

A clogged carburetor


In addition to dirt that is carried on into the carburetor by defective air filters, other foreign substances can also mean that this heart of every simple gasoline engine no longer works reliably. If the ratio of gasoline to air fluctuates, the speed also fluctuates and the entire system of intake, combustion and exhaust emissions does not run smoothly. There are many options to choose best riding mower for hills for a clean yard. But still they comes with slight issues that need to cater such as,

  • Causes: Dirt, dust or oil in the carburetor chamber
    • as a result, the function of the float and needle is impaired
  • Remedy: Remove and clean the carburetor

Tip: A carburettor that is severely or repeatedly clogged is usually due to another cause. In addition to cleaning the actual component, you should therefore also check whether the air and petrol filters are intact and clean and whether the fuel filled is dirty.

Wrong attitudes

A lawn mower will run out of round even if the carburetor settings are not correct. Normally, the gasoline-air mixture can be adjusted so that it meets the respective requirements. If incorrect specifications are made here, it is possible that there is either too little fuel or too little air for a smooth run. As a result, the engine does not stop immediately, but it can make itself felt through large fluctuations in speed.

  • Causes: Incorrect carburetor setting
  • Remedy: Make the setting according to the manual

Technical defects in the mechanics

The classic carburetor is regulated mechanically. Regardless of whether it is the connection between the throttle lever and the carburetor, or its internal mechanics, the lawnmower does not run smoothly when bent, stiff or broken components no longer function reliably.

  • Causes: technical defects or damage to transport or operation
  • Remedy: replace or repair defective parts
    • In the event of severe contamination and buildup, first clean and check for damage

Lack of fuel

If the gasoline is running low, the flow from the tank into the carburetor may be temporarily reduced by gasoline sloshing in the tank and the formation of air bubbles. The result is an irregular run until the fuel runs out completely.

  • Causes: Too low fuel level in the tank
  • Remedy: refuel

The spark plugs

The proverbial spark is the alpha and omega of every combustion engine. If the spark plug can no longer reliably do this, the regular ignition sequence is disrupted and the engine literally gets out of step.

  • Causes: worn, heavily soiled or damaged spark plugs
  • Remedy: Clean the spark plug and replace if necessary

Obstructions to the run of the knife

Block knife

With lawnmowers, the blade is in direct connection with the engine. The motor runs out of round when the knives do it. The reasons for this can be defects, but also simply wear and tear. As soon as a knife no longer cuts the grass cleanly, it is braked by the increased force required when tearing the grass, which ultimately also affects the speed of the motor.

  • Causes: blunt knives, damage to the knife shaft
  • Remedy: Regrind or replace knife, remedy detected defects

Even the most accomplished hobby gardener will not be able to identify and eliminate every cause of a fluctuating speed of the lawnmower. When the point has come that all obvious causes have already been ruled out, it is worth going to a specialist. Although there is a certain probability of finding a fault by trying it out and searching further, on the other hand there is at least as great a risk that improper repair attempts will cause further damage instead of eliminating the existing problems.

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