Sorry if this is in the wrong place, sure it can be moved if thats the case.
GENERATORS
Right then I've done this guide which should suit most basic small petrol single cylinder 4 stroke machines which I assume most people will be using. I've tried to put the checks in a logical order. Let me know if you require anything else.
General engine checks: Hard starting
OIL? You will not believe the amount of people who don't realise that there machine has a low oil shutdown switch fitted, make sure the oil level is always on maximum before starting, most machines have a max and min on the dipstick but still will not run unless the oil level is on max.
Spark plug - To check for spark, remove the spark plug, attach it to the spark plug cap and ground the side electrode to the cylinder head cover, turn the engine over to see if it sparks.
Fuel? Is there fuel in the tank? Is it getting to the carb? Is it getting to the chamber?
Try putting fuel directly into the carb, if that doesn't work then take the spark plug out and put a small amount in the combustion chamber. If the unit runs then dies once the fuels burnt off you know the fuels not getting through correctly.
Fuel filter clogged - Clean/replace
Carb out of adjustment - Readjust/clean
DO NOT ADJUST THE SPEED unless you have the correct equipment and you know what to set it to as this can and will cause the alternator to fail.
Governor out of adjustment - Readjust to correct setting/speeds
DO NOT ADJUST THE SPEED unless you have the correct equipment and you know what to set it to as this can and will cause the alternator to fail.
Compression - For a ruff guide just pull the starter rope and make sure it gets harder to pull at a certain point, if the engine turns quite freely then the compression could be low.
Valve clearance - Check/Readjust
Check cylinder head and gasket for correct fit and failure.
Check for worn piston rings and piston.
Check using compression tester
General alternator checks: low or no AC voltage
Circuit breaker: is it in? - Reset
Is the circuit breaker faulty? - Work out what power you are using and weather the trip should be tripping, replace if faulty.
Engine speed: is it too low? DO NOT ADJUST THE SPEED unless you have the correct equipment and you know what to set it to as this can and will cause the alternator to fail.
Wiring: Is all the wiring ok? Remove output panel to see if any wires have come off, shorted out, been trapped etc etc.
At this point you really need to know how the alternator is controlled, whether it uses a capacitor to control the output or an AVR (Auto voltage regulator).
A simple way to tell is to check for brushes and slip rings at the end of the alternator, if it has brushes then it will be AVR controlled.
The other method is a capacitor controlled method which basically uses different rated capacitors to give the required output.
AVR Controlled checks
Brushes and slip rings: Remove end panel to reveal the end of the alternator, you should be able to see the brushes and rings at the end of the alternator (opposite end to the engine) check for damage and check that the brushes are making good contact with the slip rings, sometime these just need cleaning up with some wet & dry.
AVR: This is quite difficult to test if you don't have the right equipment but if you can test everything else then you can start to eliminate things.
Disconnect the wires from the brushes which should go to the AVR, whilst the machine is running put 12 volts across the brushes (pos on one brush and neg on the other) do this whilst you have a volt meter in the output sockets, the voltage should rise slightly whilst the 12 volts is on it, if it does then the alternator windings should be ok, if not then they may be faulty which means a new alternator is required.
Capacitor controlled checks
Capacitor: Sometimes when the capacitor fails it will break up or crack so visually check it for signs of any damage. Some multimeters have a setting to check this.
The capacitor will have a microfarad rating on the side of it, this number is usually larger than the rest of the writing on it and will have the microfarad symbol after it, this looks a bit like fu.
This may need replacing, most are just two terminals which need disconnecting and fitting to the new one, no right or wrong way with these.
Make sure you replace the capacitor with one of the same rating!
Diode packs: This sometimes consists of a diode, varistor and small capacitor, there can be one set or two sets. These are usually fitted onto the side of the rotor (inner winding which spins) and you will more than likely have to strip the unit to bits to get at them.
Once you have done this you will need to remove them, they are usually soldiered in, be careful not to damage to wires from the windings!
Once these are removed you can test them with a meter, the simplest way to do this is to check the resistance both ways, and you should only get resistance in one direction. To be honest once you've stripped it down you may as well replace them as they cost very little.
Note: The diode has a certain direction, they will have a marking on once side, when replacing try to make sure that you get the new ones in the same way as the old ones and if there are two sets then they should run in parallel (if you look down at the rotor with the diodes in, both markings on the diodes should face be either on the left ot the right), this is not critical but may save you time.
Flashing: Alternators can sometimes loose excitation and will not produce the power from start up. This can be caused be the unit being switched, off whilst power is still being drawn from the machine.
When the diodes have been replaced and are in the wrong way round then this method will reverse them so that they work correctly.
With the machine running you need to put 12 volts across the capacitor terminals the same as with the AVR controlled unit on the brushes.
What you doing here is putting a direct dc current directly onto the rotor to re-excite it. These will more than like make a bit of a blue flash and a crack, don't worry, that's normal, check the output with a meter and you should have full power again.
Disclaimer
I haven't the first idea what the hell I'm doing and nobody should attempt to work using the above instructions, if you do you will die.
Seriously though, if your unsure about what your doing then don't tamper with it, a new alternators probably about £100 - £200 for a small cheap one
