Fault Diagnosis-Fails to Start

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.

Before going through any of the other diagnosis topics, check for fuel flooding evidence as detailed below.

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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.

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Spark Problems

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:

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Timing Problems

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.

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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.

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Fuel and Air

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:-

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