This guide is aimed at systematically and correctly diagnosing why an engine fails to start. It does not go into detail regarding fault correction, the relevant Briggs and Stratton engine repair manual covers servicing the various problem areas in depth and should be referred to once the problem has been isolated. Manuals can be obtained through www.Briggsbits.co.uk.

Important

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Fuel Flooding Checks Before carrying out any other checks ensure that there is no evidence of fuel flooding. Has the fuel tank emptied. Has the oil level increased and/or the oil viscosity reduced(thinned). Is there evidence of fuel coming from the exhaust. Remove the spark plug(s), is it excessively wet. If any of these symptoms are apparent, do not attempt to start the engine. Service the carburettor paying particular attention to the float/inlet valve. Drain and refill the oil. Leave the spark plug out and allow the engine to "dry out". Where possible, fit an in-line fuel tap to prevent future occurrences. What an Engine Needs for starting In order for a four stroke engine to operate it requires the following: The starting mechanism to turn the engine at the required speed. A strong spark at the correct timing The right mixture of good fuel and air correctly compressed in the cylinder. If any one of these conditions is not met, the engine will not run. Alternatively, if all these conditions are correct, the engine will start.

Starters

• Manual Starters

  Check that the starter mechanism is engaging the engine and enabling it to pull over. As the starter is pulled, engine engagement should be felt during the first few inches of movement. After that the engine should be felt to be turning over. If no resistance is felt then the starter is not engaging.
  If the cord cannot be pulled, check that any brake/stop levers etc. are properly activated. If the engine still cannot be turned, remove the spark plug and check for blade obstruction etc. taking care only to tip the machine so that the front of the engine is lifted.
  If there is nothing found to be externally stopping the engine turning, remove the starter and check its operation off the engine. Service the starter if it found to be at fault. If not, it would appear that your engine is seized. Read the "seized engine" section of this page.

• Electric Starters

  Check that the starter motor is turning the engine fairly quickly. If the motor sounds as if it is struggling, rapidly slows down or fails to turn the engine at all then the electric starter system will need checking. Given the many different systems available this is best carried out using the repair manual to identify what you have and check through it accordingly. However, by far the most common problem is the battery. Make absolutely sure that the battery is providing sufficient power before looking at anything else.
  also be aware that an engine turning slowly may be due to engine resistance. check this by removing the cowling, disconnecting any braking mechanism or load from belts etc., remove the spark plug and turn the engine by hand. It should turn freely.

Always try a new plug first.
To check for an efficient spark, remove the spark plug, attach a spark tester to the plug lead terminal(a tester plug can be made by opening the gap on a known good plug to about 3 mm), ensure any stop switches or mechanism are not active, restrain the metal body of the test plug against the engines metal surface away from the spark plug hole, pull the engine over and look for a good spark cracking across the oversized gap.

If there is no spark check the ignition as follows:

• Electronic ignition(Magnatron etc.)

  Remove the engine cowling to access the ignition armature. Pull off the small stop wire connector from the body of the ignition unit. Re-fit the cowling and repeat the spark test. If there is still no spark then the armature unit needs replacing. If there is now a spark, the problem lies with the stop wire system.

• Points ignition

  Disconnect the stop wire from the points at its cut out post(usually with the throttle bracketry). Repeat the spark test. If there is still no spark then the problem is either with the points/condenser set or the coil. The points can be replaced but the long term repair would be to replace the old coil with a new electronic ignition armature. On no account consider cleaning the points, this is at best a very short term solution.
  If after disconnecting the stop wite a good spark returns, the problem is with the stop system.

Briggs and Stratton engines have fixed timing. The flywheel position is set by means of a keyway.
Generally, a spark timing issue will reveal itself by the engine kicking back and /or popping when trying to start it and almost always follows some sort of sudden stopping of the engine. Normally through the blade of a mower hitting something.

Before checking the engine itself, check that what it is driving is solidly fitted to the engine crankshaft extension. A loose blade or broken key on the output shaft will cause the engine to missfire as if out of timing. Has the blade been removed and refitted recently is often the clue.

Otherwise remove the cowling and check the flywheel key for damage. If it is broken, clean up the flywheel location carefully and reassemble with a new key. Note that engines with the older square clutch starter drive, this acts as the flywheel nut and unscrews anticlockwise. Do not pry the flywheel to get it off, a sharp tap usually does the trick.

Engine compression is essential for correct operation. Not only does it allow for the fuel/air mix to be combusted, but a leak in compression will almost certainly prevent fuel and air being sucked into the cylinder. It is therefore important to check for compression before looking at carburation.

The basic compression test for Briggs and Stratton engines is carried out by turning the engine backwards. This ensures that the engine's compression release mechanism does not faulsify the diagnosis.

First detach the spark plug lead from the plug to prevent accidental starting. If the engine has a clutched starter mechanism which would interfere with turning the engine over backwards, remove the cowling and starter assy. Ensure that any engine brake or other resistance is disengaged.
Spin the engine backwards by hand. It should bounce back again at the point of compression. If it does, compression is not the starting problem. With larger engines which are difficult to spin, try to turn it as fast as possible and feel for a definate springy compressed air resistance.

If compression is low, follow the relevant repair manual section to service the compressive components.

This section deals with supplying the engine with the correct mixture of good fresh fuel and air.

First, is the fuel in the engine good, fresh unleaded petrol?. If the engine has not been used for a long while, then chances are that the fuel in it is stale and non combustable. Just filling it with new fuel will not cure this as the residue in the carburetor will still be stale. The carburettor will need to be stripped and cleaned through.

Having ascertained that good combustable fuel is at the carburetor and compression is good(section above), there are now two possible mixture problems:-

• Not Enough Fuel

  Insufficient fuel entering the engine is caused either by a fuel line/jet blockage, an inlet air leak or throttle valve failing to open.
  Remove the necessary covers and check the inlet manifold for possible cracks ar gasket leaks. Rectify if found. Manually check the throttle butterfly valve moves freely and is fully open at starting.
  Where the fuel is delivered to the carburettor via a hose, check for good fuel supply by detaching it at the carburettor. any reluctance of the fuel to flow is normally due to a blocked filter, tank outlet pipe or tank cap breather. Does the fuel flow better with the cap off?
  
  

• Too Much Fuel

  Too much fuel(flooding) is due to either a blocked air filter or faulty float/pump diaphragm operation.
  Check and replace the filter if found to be badly soiled.
  Service the carburettor. With float carburettors, check that the float does not contain liquid(leaking) and that when held upside down, the valve stops you blowing through the inlet. Change any carburettor diaphrams.
  The addition of an in line valve on fuel line fed engines helps to minimise flooding problems.