RM370 RM400 Rotor swap ???

[DR]

Trying to sort out an ignition for the ol' girl and just wondering what issues I'd have using an RM370B rotor on a '78 400C  

[BAHNZY]

Doc,
Not sure about the rotor, but i swapped a 400C stator to a 370B without any dramas. It is my understanding that the 370 and 400 engines are the same, save for the bore/stroke.
Rod

[DR]

mmm, the engines are very very much the same Rod excepting the 3mm bigger bore. I figure it'd probably work so I might take a punt. The stator fortunately can be from pretty much any '77/78 model RM80/100/125/250/370/400 and it should work. The cdi unit which I'm also a hunting interchanged with RM100B/C so it makes me wonder if maybe a 125 cdi could work. The big risk factor here is not so much the cost but if it don't work, being only a smallish bloke I don't really like the idea of my knee smashing into my chin when/if it kicks back don't laugh, it's happened before and it even fractured my right ankle about 10 years ago from the same Now I tend to show a little respect but at the same time show no mercy either if that makes any sense..self preservation

[clutchslip]

Hi Doc,

the period aftermarket Mototek CDI unit which works with the RM370 is the CD-100 unit..I think theres a clue in the name there, it
was also listed as being suitable for the 78 250 as well. I think theres at least an argument that the same OEM unit might be useable in
RM100, RM250 RM370 and RM400...not that Ive tried it..Just a thought

[DR]

Not sure clutchslip but it could all be a smoke screen of types. I've pondered that even though the RM125
advance curve may be a little steep I'd highly doubt it'd matter on the big 400 at it's 8000 odd rpm redline as the 125
advance would only just be kicking in. Then I also pondered that may explain why the RM100 CDI was the
same part as the 400 as the advance my hit a little earlier I just dunno took a punt and bought the RM370 rotor today for a huge $24US delivered so it's just the cdi, a pipe, a few other bits and some polishing and I'll have myself another big banger for well under a grand another piece from here and piece from the story



http://public.bay.livefilestore.com/y1pCgzLPEkHp1bgJitGV6pZYd4t7MYRWVpxyoHmSJMzDZc5z7joqpeVzo0nokh_HOgc_9U_30hfizez7n09hLGtcg/400c.jpg
fullsize

[frostype400]

That is good news Doc I will keep that in mind with mine 370 motor although I'd like to find the right motor for it but if it is a racer it wouldn't matter.

[DR]

Frosty, consider that if you found another frame from say an RM370 or RM250A/B you could still use all the bits you have already (except the 400 frame and top triple clamp) and it would be pre'78 legal

[Lozza]

Most likely is CDI's a somwhat 'generic' across the range with the stator providing the trigger to tell the CDI to dump to the coil at different points. The 400 would have a static advance of 3-3.2mm and the 125 1.5mm btdc.If the stator rotor was from a 125 the risk factor is practicaly zero, no 400 will ping with 1.5mm of advance, it will just be easy to start and run like a dog untill you throw advance at it.