Stornello - diodes and power to switch

I’ve been almost pulling my hair out trying to get a spark on my Stornello 160. I know that coil/flywheel/plug/plug cap/points are good. The wiring loom is in excellent overall condition and the bits in the headlamp are like new. I have already cleaned up and checked all connections

Power is coming up to the Aprilia connector block, but not reaching the other side of it where the wiring sends it back down to the points, hence no spark. There is a diode on the block, and I’m starting to suspect it may not be working but not really sure whether it should be checked with the power on, or off.

Can anyone help, or suggest anything?italianmotor2013-05-18 09:39:33

Diodes are very easy to check offload (without power).

All modern multimeters have a diode check function.

You select the diode function, place the black lead on the cathode, and the red lead on the anode, the multimeter will then display the millivolts dropped across the silicon junction, typically 400 to 700 millivolts, if you reverse the leads, it should read zero.

The cathode is the “stripey” end.

(as always with electronics, there are many different types of diode, all with fancy sounding names, they will read slightly differently, hence the range I’ve given you, largely it should conduct in one direction, and not in the other, bear in mind, and I hope I’m not teaching you to suck eggs, that the current actually flows from negative to positive).

What area of the country are you in, as hopefully someone may be local to you ?

The diode on mine gives a resistance of about 5 ohms one way (due to the resistor) and infinity the other way. I haven’t really got into the electrics to much and it is difficult to follow the circuit routes through the switch. I thought the diode was just to do with the battery charging. On the early models, when the key is removed, it earths out the coil power supply to kill the engine. Not sure if yours is the same. I have read the battery is nothing to do with the ignition circuit, the bike will run without the battery. A voltage must be produced within a coil in the rotor, up to the switch and back to the coil, then onto the points.I assume you have finished rebuilding the bike after the seize. It was working before taking it apart, so what have you disturbed? Coil wires round the wrong way?

In that diagram there is clearly another coil in the alternator to supply the ignition, not connected to the battery as you say.You have to allow for parts 8 and 9 being wrongly labelled though.

Thanks for all the suggestions, - I removed the diode and put another 6A one in, power is now flowing to where it should, but still no spark, and there’s something weird going on somewhere. I am wondering if the ignition block in the headlamp is shorting on the headlamp - there is a piece of card that is meant to isolate it but i think that a live terminal is touching somewhere. Thanks Don but that’s not the right wiring diagram for my bike, he’s also labelled it incorrectly, this is the one: Stornello 125/160

  • and I agree, though it’s a simple wiring circuit (which is why it’s even more frustrating, my S3 was much more complicated!), the diagrams are confusing. I never started the bike as it was seized when I got it, only since sorting the motor have I got on to the rest of it. italianmotor2013-05-19 19:41:55

Try hot wiring it, avoid all the switch mechanism, run a wire from the battery to the Ht coil and then to the points. That way you test the various parts and isolate the problemI have the same seizure problem with my Stornello. Spent ages today bashing at it with blocks of wood. It has moved 2mm so far! Trying to get hold of a piece of mahogany or oak to use as a drift as normal 2 x 2 rounded off just splits after about 10 big hits with the hammer.

Don-Spada2013-05-20 00:51:09

I didn’t realise your motor was seized - good luck! Mine took me ages and ages to get it sorted. I soaked for days on end it with diesel, then acetone/ATF. I’ve got a big gas torch, heated it and heated it. Bashed it with a purpose made oak drift. Ended up smashing piston but only way it came out. Someone said to me try Coca Cola too.

Adam, which diagram in the manual applies to your bike? Page 87 or 88? Looking at it further, both are basically the same. Are you sure there are points in the system, none are shown in either diagram.What I think you have is a sort of flywheel magneto, the coil in the flywheel puts out a pulse at the right time which is amplified by the ignition coil, much as happens with a CDI system. The HT coil acts as a transformer rather than a normal ignition coil with points. You need to be able to check if there are any volts coming out of that coil, on the primary input to the ignition coil, but also remove if you can, the cut out wire which goes from that terminal to the switch.Of course the engine has to be turning to get anything at all.I’m sure there is something about all this in the manual (p75?), but my knowledge of Italian is zero.If I’m right there will be a small magnet which passes the coil at a certain time, if this has demagnetised you won’t get a spark.

Brian UK2013-05-20 16:26:03

My wiring diagram is typical Guzzi mish mash of the schemes on both page 87 and page 88. The points are hidden behind the flywheel which is labelled ‘N - Volano Magnete’.
I have taken the flywheel from another bike that I know works. I’m convinced the problem is in the ignition switch…but I am starting to lose interest. It’s the most infuriating problem I’ve had on a bike for many years.

Disconnect the wire from the ignition coil to the switch, the engine should then run, or at least produce sparks. All that wire does is kill the engine.You should also be able to measure volts on that coil terminal when the engine is turned over.

You mean the coil behind the flywheel, not the coil with the HT lead coming out of it?

No, I mean the coil with the HT lead coming out of it. Remove the wire from there to the switch.

The engine on mine is seized solid and has been for many years. It was like it when I bought it and the same when Duffo had it before me! We are obviously working on similar lines, I have managed to scrounge an old oak table leg and have planed that down to a nice fit over the piston At least it has a flat top to work on. It has been soaking in penetrating oil for at least a year, I kept taking the plug out and squirting a bit more in every now and again, the barrel was full of it. It is now full of ATF, I daren’t pinch the wifes nail varnish remover as the smells from the garage percolate through the house! I might try some of the kids Pepsi-Max next, I’m sure it would be as good as coke!My thought on heat was that it would actually make it tighter as the aluminium piston will expand more than the steel liner. It is impossible to heat one without the other getting equally as hotI have a friend with a large hydraulic press and am thinking about splitting the cases, removing the crank with the piston and barrel attached and pushing the piston out. Just need to see if it all will fit in between the beams. That will be the topic of conversation down the local Harborough club night tomorrow!

I’ve always understood that heat is useful to help break any bond made by rust, dirt, old oil etc, and you’re right about the ally v steel, but I think heat will do no harm. It may be worth trying the press very gently with everything in situ first. Expect destruction of the piston whatever you do.

As long as it doesn’t also destroy the barrel too.

It seems to be on the move. So far it has shifted 12mm down the barrel, it is now moving about 1mm for every 10 hits on the nice oak drift I have made. Keep checking the distance top of the barrel to the piston crown with the vernier and it is moving, still a heck of a long way to go till it is out. There is still about half inch from the bottom of the barrel liner to the bottom of the piston skirt.Currently soaking it in carb cleaner to see what effect that has! The big press is always available as an alternative.Back to the original topic!! How are you getting on with the electrics?
Don-Spada2013-05-22 00:50:16

Sounds good,PM me your email address Don and I’ll send you a couple of pics. Still nowhere with the electrics, tried all of the above but no joy. Doesn’t help not having an accurate wiring diagram.

If you are not getting any volts on the wire to the HT coil from the generator, assuming you have removed all possibility of the switch shorting out) then it begins to look as if the coil in the generator which supplies the ignition is not working. Why is another matter.I can see one problem is that the engine has to be spinning over in order to produce volts, the battery is not involved, and it is difficult to do that and measure volts at the same time.

Yes, you need as many arms as an octopus to do it properly. I’m getting to the point I may have to take it to an ‘expert’.

Actually three experts plus you would give enough arms.I have to say these flywheel magnetos are a bit challenging.