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Covers... All 50cc Piaggio and Gilera 2-stroke engines plus some geared 50cc (e.g. Derbi) which use the same ignition. It's assumed you have and know how to work a multimeter!
Testing for a spark
The best option is to use a spark tester which plugs into the spark plug lead (instead of the plug) and is clipped to an earth point on the moped engine. It will have an adjustable gap which the spark can jump across and the gap can be adjusted.
At its simplest you can check for a spark by removing the plug and resting its threaded part on a good earth point on the engine and either turn the engine over on the starter or kick it over with the ignition on. A better option is to connect the threaded part of the plug to earth with a jump lead to make sure there is a good connection and that the plug doesn't keep falling off.
*Unfortunately it is not enough just to see the spark plug sparking*. This is an important point- the distance the spark can jump directly reflects the voltage of the spark. If the spark is weak it's perfectly possible for the spark to be strong enough to jump the 1mm spark plug gap out of the bike and appear good, but not to actually spark at all when the plug is fitted.
Part of the reason for this is that a spark cannot travel so far in the compressed air atmosphere of the cylinder. This is especially true in more highly tuned higher compression moped engines. Also the spark simply may not be strong enough to ignite the mixture correctly.
For this reason it's necessary to check the spark voltage (how strong the spark is) and the easiest way to do this is by checking how far the spark can travel in open air. If you have a spark tester just keep increasing the gap until you reach the maximum distance the spark will consistently jump without missing, if it can jump minimum 7mm this represents over 15,000 volts and this will be fine for Piaggio/ Gilera engines but anything less is likely to cause running problems.
If you don't have a spark tester you'll have to improvise, you can screw two screws into a block of wood approx 7-10mm apart, connect one to the terminal up inside the spark plug cap with a wire and the other to engine earth.
Things to remember when testing...
There is no spark, what next?
The Piaggio/ Gilera ignition is competely independent to the rest of the bike electrics. This means that it is unaffected by any faults in the charging system, battery condition, or any other electical faults and makes it pretty straight forward to fault find.
Click to enlarge picture
First to test the pickup (CDI red wire)
The pickup is located on the side of the flywheel and generates a small voltage everytime a nobble on the flywheel passes it. From the timing of this voltage 'pulse' the Piaggio CDI works out when to make the spark. Without this signal the CDI will never spark.
First we need to disconnect the CDI block connector as a faulty CDI can mess up the results.
Click to enlarge picture
Connect a multimeter set to read resistance (ohms) between the red wire and a good earth point (see pic)
Piaggio state this reading should be 80-90ohms, but it is nearly always 120-130ohms. You should get a reading between 85 to 140 ohms if all is well.
We've not yet seen a faulty one which has given readings within this range. The only other thing to check is the air gap between the pickup and the nobble on the flywheel. Rotate the flywheel by hand until the nobble passes the pickup and check the gap, it should be approx 0.8mm. Anything more than 1mm loosen the 2 mounting screws and move it closer. If there is no adjustment left you may have to bend it very carefully to get the airgap correct.
If you get this reading and the pickup gap is ok then the pickup is almost certainly ok and you can move on to the next section
If you don't get the correct reading either the pickup is faulty or there is a problem with the wiring between the pickup and the coil/cdi block connector. To check the wiring disconnect the flywheel/pickup/stator block connector and retest the red wire from there instead (see pic below for block connector location)
Click to enlarge picture
If you still get a bad reading from between the red wire here and earth then the pickup is faulty and the Piaggio Stator and Pickup assembly will need to be replaced.
If the reading is now good but was not good before there is a fault in the wiring, almost certainly in the flywheel block connector which tend to get damaged or corroded. This must be rectified before proceeding.
Testing the CDI earth (CDI white wire)
Disconnect the CDI connect block again and making sure the flywheel block connector is reconnected check the resistance between the white cdi wire and a good earth (or battery -(negative) terminal)
You should have a reading of 0 Ohms as the white wire should be connected directly to earth.
if so you can move on to the next section... otherwise read on
If not disconnect the flywheel block connector and check the white wire again from there. If you now get a good reading but didn't before then there is a problem with the white wire between the flywheel block connector and the cdi block connector, it will almost certainly be the flywheel block connector which can sometimes become corroded or damaged. This must be rectified before proceeding.
Testing the Piaggio stator output (CDI green wire)
If the pickup reading is fine at the CDI connector block and the white wire is earthed ok then we can move on to the green wire. The ignition key must be in the 'ON' postion for these tests. The green wire carries AC voltage (approx 80volts) from the stator to the CDI which is generated by the stator as a power supply for the CDI. This is completely independent to the other stator coils (not shown on our diagram) which generate power to charge the battery and operate the lights and are not involved in the ignition circuit.
QUICK TEST:- with both block connectors reconnected test the voltage between the green wire at the CDI and earth- if you get more than 50V AC when cranking the engine you can move on to the next section... otherwise read on
With the CDI block connector disconnected (but make sure the flywheel block connector is connected if you disconnected it previously) connect the multimeter between the green wire (at the CDI block connector) and a good earth point and set to ohms. You should get a reading between 600-1000 ohms. If you get an open circuit (infinite resistance) or short circuit (0 ohms) there is a definite problem (but carry on with the following test which will confirm it).
Next change the multimeter setting to volts AC and crank the engine over. If you get a reading of less than 50 volts AC (should be around 80V AC) there is a problem and you will need to repeat the test by disconnecting the flywheel block connector and retesting the green wire from there as we did before with the pickup.
If you can't get over 50V AC directly from the flywheel block connector you'll need to replace the Piaggio Stator and Pickup assembly.
If the reading is ok at the flywheel connector but not at the CDI connector there are 2 possibilities. The most likely is that there is a problem with the green wire between the flywheel connector and the CDI connector (almost certainly the flywheel block connector will be damaged or corroded)
If you look again at our diagram (click here) you'll see that the green wire also goes off to the ignition switch and the rev counter. To see if there is a problem here test the resistance between the green wire at the CDI block connector and earth with both the CDI block connector and the flywheel block connector disconnected. You should get 0 ohms with the ignition key in the 'OFF' position and infinite resistance (greater than 20M ohms) with the key in the 'ON' position. Anything inbetween these values demonstrates a problem with the key switch or tacho.
If all is well so far we have one more test. On Piaggio's it's still possible for the stator coil to be faulty even if it's giving us a healthy voltage when tested at the CDI block connector, so we need to do one more test to check that it isn't failing when placed under load.
Reconnector both block connectors and again test the voltage between the green wire at the CDI and earth. With the CDI plugged back in you should still have at least 50V AC when cranking the engine. If so all is well, if not and you have completed these tests correctly in order you'll need to replace the Piaggio Stator and Pickup assembly.
Done all that, still no spark- what next?
Ok, we've now confirmed that... you have more than 50V AC between the green wire at the CDI and earth when cranking the engine AND the white wire at the CDI has good continuity to earth AND there is between 80 and 140 ohms resistance at the cdi plug between the red wire and earth (with the cdi disconnected).
This means you have eliminated everything except the cdi itslef and you'll need to replace the Piaggio CDI/ Coil Assembly. The standard Piaggio/ Gilera CDI is restricted to approx 10,500-11,000rpm which is pretty high and won't bother most people even with a sports 70cc cylinder kit and exhaust fitted. But if you need higher revs or are thinking of tuning your bike any higher in the future it's probably better to get a CDI with unlimited RPM (no rev restriction) such as the Athena De-restricted Piaggio CDI.