Lawn mower engine problems.

Clive Macintoshin Machinery & Mechanics

Lawn mower engine problems.

By Clive Macintosh


Most mowers are now working flat out cutting grass however, even when running, they can often begin to have mechanical problems. A problem that often occurs is the engine beginning to run poorly and lacking power. This may be due to a number of reasons:

mower-auto-cetees.jpg Faulty or unsuitable sparking plug:

Sparking plugs can break down during use, a build up of carbon on the electrodes can reduce the efficiency of the spark. The porcelain part of the spark plug can also crack which may affect the spark of the plug. The wrong plug may have been used. It is important to check the plugs on a regular basis, ensuring that the electrode is correctly gapped to manufacturer's recommendations.

2 The High Tension (HT) lead to the plug may be damaged or shorting out, resulting in an intermittent spark being produced and causing the mower engine to labour or run poorly. Check for damage or breaks in the lead. Check connecting to sparking plug.

3 Loss of compression due to:

  • A leak through cylinder head gasket. Tighten cylinder head, nuts or bolts. If the gasket is damaged you will need to replace with a new one.
  • Valves not seating due to insufficient tappet clearance. Reset tappet clearance to manufacturer's recommendations.
  • Valves not seating correctly. Requires lapping in to reseat.
  • Valve sticking in guide. Remove valve and clean off gum or carbon from stems.

4 Excessive carbon deposit on cylinder head. Remove cylinder head and scrape off carbon. Silencer should also be cleaned.

5 Incorrect fuel mixture. Check and set carburettor fuel mixture.

6 Air filter blocked. One of the most common reasons for poorly running mowers. The air filter often becomes totally blocked with dust and debris, preventing the correct amount of air being available to the carburettor and resulting in the mower running too rich. Keep air filters clean and replace after recommended hours of use.

After all checks are made and should the problem continue, seek technical advice from Clive McInotsh, e-mail

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Machinery & mechanics