NLM Pickup

The 3 1/2 forum
Post Reply
JonD
Posts: 37
Joined: 14 Jan 2014 10:40
Location: Wantage

NLM Pickup

Post by JonD »

I have just replaced my original second series red pickup with a NLM pickup after the rear pot section failed on the original. The bike, a 1979 3 1/2 Sport, will run on the new pickup after carefully setting the air gaps to 0.05mm (had to get some brass feeler gauges to do so). On checking the ignition timing the system is advancing but even on the most advanced setting it is about 10 degrees retarded at 6000rpm. The transducers are the original Ducati 323921 green spot transducers. I have swapped transducers with my spare and it makes no difference. I have checked the cam position and all the marks line up and I have checked the valve timing and that is correct too. The electrical contacts on the transducers have been thoroughly cleaned also. Refitting the old red pickup and the maximum advance is correct but the bike doesn't go too well on just one cylinder!

Has anybody else had this problem with the NLM pickup and if so how did you resolve it?

Thanks for any help with this frustrating problem.
JonD
Posts: 37
Joined: 14 Jan 2014 10:40
Location: Wantage

Re: NLM Pickup

Post by JonD »

An update on the pickup problem. I spoke to NLM when they opened and they hadn't come across the problem I was experiencing before. Suggested elongating the slots in the pickup backplate to resolve it. The rear pot coil prevented sufficient rotation of the backplate to achieve full advance so I fabricated a new rotor using the NLM one as a template only with about 5 degrees more advance on the rotor. Problem solved, backplate screws now in the centre of the slots and it's possible to adjust the ignition timing to the correct advance. The bike now runs very well and the idle is steadier than with the original red pickup. I found brass feeler gauges made setting the rotor / coil air gaps much easier than steel gauges as the steel gauges tend to stick to the magnets. Hope the modification continues to work.
fatnfast
Posts: 59
Joined: 31 Jan 2017 15:55
Location: Kent UK
Location: South East, UK

Re: NLM Pickup

Post by fatnfast »

Nice work Jon and thanks for posting. No doubt in the years ahead we will all be replacing the pick up at some stage.
I'm just curious as to why it never worked first time round without modifications? Good tip on brass feelers :D
JonD
Posts: 37
Joined: 14 Jan 2014 10:40
Location: Wantage

Re: NLM Pickup

Post by JonD »

I'm slightly mystified why it wouldn't achieve the full advance too. The stator coil is new, the pickup coils are new and the results are the same with any combination of my three transducers (one spare) - I checked the advance on the rear cylinder and it was the same as the front. This suggests there are no faults in the components electrically/electronically. I found a paper by Honeywell on variable reluctance sensors (VRS) which is the type used in the NLM pickup. The voltage wave form is influenced by the rotor shape quite markedly; not having an oscilloscope I can't see what is actually happening but I would expect a fairly sharp voltage rise from the examples in the paper There could be a compatibity issue between the pickup and the transducer but I would have thought it would be known about as the combination I am using will be quite common. Assuming there is no electrical / electronic incompatibility that only leaves a mechanical misalignment somewhere. I have checked everything I can and haven't found anything wrong yet. If I do I'll post an update. I have taken the bike out for a few runs over the last few days and it is going better than it has done for some time. Hard to tell why because I have replaced the plug leads, removed the spitfire spark enhancers and cleaned the carbs. as well as the ignition work. It does require a more enthusiatic kick to make it start but nothing compared to a Velocette! So far so good!
Post Reply