Substitution with 'known good' components is sometimes the only way to diagnose something. I've lent a working twin cylinder pickup to a couple of people that proved that was the issue though their unit looked fine.
Not many 125 Hs around in the UK though as you say. I nearly bought one off evguru for my lad to use for 'racing' May have dodged a bullet.
Beer is good, but I find a decent Italian red plonk more efficacious in this weather.
It may well be, but its easy to collect lots of useful or useless ornaments in the form of components that did not cure the problem.
The 125H was never officially imported into the UK so knowledge and spare parts is rare I made a rookie mistake when I bought mine, the sump was full of oily sludge and the fuel tank with the remains of old lawnmower fuel. Instead of draining it all down and trying to start it I embarked on a restoration project, if I had done some research and tested the engine it would have been back on the auction site before the restoration was started.
So I now have a very pretty Italian lightweight that does not run as it should and having just about exhausted all expert advise and spent a load of cash on testing only to be told what I already knew its difficult to know where to go from here. I have a few last ditch things to try but am not hopeful unless I can find a complete new system that will fit. There are options of course involving some quite serious and expensive conversation to a modern adjustable advance system but it would probably require machining of a new rotor to fit the crankshaft and how much money is it worth throwing at a 125H.
I would agree about the Italian red plonk, I have a nice Chianti warming by the fire.
I used Google Lens on a smart phone to translate the article https://www.motomoriniclub.nl/zuendung.pdf to English and I can see that this gives a good description of the importance of the 1.38V trigger point voltage and how it occurs earlier as engine rpm increases. You can use Google Lens to give a voice-over description too. I quite fancy a 125H but the problems you guys are having has put me off a bit. Ian
The 125H problem is not just within the UK I have made quite a few contacts in Germany and the Netherlands with very similar issues, although some owners have no problems at all with them.
One German contact is looking at using a modern Ducati single ignition/generator unit and I await with some interest as to how he gets on with that. Personally I have wasted time and money having my ignition tested by so called experts with no result, so yes you probably did miss a bullet by not buying into one, although I doubt very much that Paul would sell you a crap bike that did not run, its not worth his reputation.
Personally I think the problem may be one of assembly that is to say setting the static or base timing, according to the Germans this is achieved by setting the piston at TDC on the compression stroke both vales closed PMS mark aligned with crankcase mark, then turn the engine back to the ANT mark or A mark in the case of an early bike. Hold that position and then move the stator plate to align the two scribe marks on the rotor with the two lines on the black pick up, the engine should then run - hopefully! Final adjustment is made with a strobe light. I did a quick check with a timing disc and this appears to give 32 degrees BTDC. This is of course in theory, problems may exist in the pick up not aligning with the two magnetic triggers in the rotor, I was told that sometimes if a non - standard pickup was fitted that it may require shimming to get the right switching for advance, again more theory. My 125H is under wraps its too cold for an old bugger like me to be out in the garage, but once things warm up I am certainly going to look at the static timing to see if it will run, if not more investigation will be required.
May have dodged a bullet.