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Carb upgrade, Dyno Charging upgrade & Electronic Ignition upgrade for my 1957 NSU Prim D


Steve D

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Help!

I've got two '57 NSU Prima D's that are in need some TLC in a few areas and would value any help, guidance or just info that you can share.

  • The original Bing 1/20/32 & 1/20/37 carburetors just won't hold a consistent tune.  I'm ready to bin them.
    •  I've seen a photo where someone fitting a Dellorto PHBG and was planning to do something similar
    • What spec and jetting works best for a smooth, touring ride characteristic with a good amount of low down torque - I was thinking about using the PHBG R2912 - 21DS or PHBG R2696 21DS 
    • Will I need to change the exhausts, to get the best performance out of a carb change? Any recommendations for sensible pipes that won't result in a need to chop panels or foot boards?

 

  • Both scooters will start from a freshly charged battery ( a pair of new 6V 11aH Gel Batteries have been fitted, in series, to each scooter), but neither scooter is recharging the battery during running and eventually, they stall, when there is no more charge left in the battery to produce the spark.
  • Both Scooters have the old-fashioned points, condenser & coil based ignition with an auto advance/retard mechanism 
    • Am I correct to think these will be Marelli or Filso original systems (like Lambretta fitted), or did NSU go the Bosch route and develop something unique for their scooter?
    • Can someone recommend a good, robust, upgrade for everything from 'Stator to the Spark Plug' that delivers reliable charging capability, along with reliable, Ducati Type, CDI controlled, Electronic Ignition similar to those available as aftermarket kits for the later Lambretta Li's, TV's & GP's
    • Do I need to be looking towards the vari-tronic spec's?
    • Is it safe to assume that something advertised as suitable for a Lambretta D/LD 150 that originally had 'electric start' will fit the NSU? Or am I just opening a whole box of hurt?

 

  • I'm planning to replace bearings and seals throughout, probably fit fresh cranks and conrods and have the barrels honed ready for fitting fresh Pistons & Rings to bring the compression back up to where it should be - currently they're only running at about 60% of where they should be (4.2 : 1 rather than 6.1 : 1) 
    • Has anyone got recommendations for cranks, rods and a particular brand of three ring piston, preferably based on actual experience?

That's enough for now, i guess. Obviously there will be a photo diary of the journey to bring these 64 years old machines back to life.

 

Cheers

SteveD 

 

 

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Back in the day, I had a good little earner in high school. I'd be given Siba dynastart armatures from Primas by Whiting and Waltho - the local agents.

Invariably with over-revving they'd shed a segment or two off the commutator. Epoxy them back on - clamped with a hoseclip - and remachine them round. resolder connections and test. Five quid in my pocket and W&W had an exchange armature.

Because of the Siba you're stuck I think with a DC generator. Modern voltage control would probably help. If they'll start, the srmature is OK - but if it's not charging, it could be the control system not changing from power in to power out. A points replacement module - like a Pertronic - would probably suit to clean up that side. I can't remember if it's a downdraft or sidedraft carb. If a sidedraft setup, I'd simply go for a VM Mikuni.

Crank and rods I'd measure and look for alternatives if OE isn't available. OE may be available - i was surprised recently to be able to get a BMW Isetta rod kit. Search the net and scooter forums. Wouldn't surprise me if most stuff is still available.

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A few more neurons collided in the memory section. The dynastart runs on 12V - the starter switching uses both 6V batteries ganged to get 12V. But once the switch is released and it's running, it charges at 6V. For the era, it's a fiendishly complicated way of doing it - but very NSU. 

We found out about the armatures when the family Prima did one. I may or may not have been complicit in over-revving it. When the old man (AMIMechE) pulled it down and found out from his old mate Ian Whiting what a new armature cost, i stepped up and offered to have a go at it. I'd been heavily involved in slot car racing and was up to the point of building/rewinding my own motors - so knew what I was looking at. And it worked. Once it was proven on ours, the old man bragged about it to Ian - and I was given a box of the bloody things to fix.

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  • 2 years later...

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