Are there any workshop manuals in the tech library for a 1975 MX400 - I can’t seem to find one. 

Will the spark plug be the same as the DT400? (B9ES). 

Thanks

I cheated & looked on partzilla say BR8EV. B8ES would be ok & a B9ES of course. Depends a lot on how hard ya beating it. In the forest & slow--ish riding in trees you'd be ok with a B7ES so long as mixture is 'not' set too lean.
Wish i had an MX400B 1975. Had a 250 same year new in "75.
www.partzilla.com/catalog/yamaha/motorcy...linder-head-cylinder

Thank you RT325

Hello, 
I have a couple MX400’s and I have not seen an actual service manual, I do have an original owners manual and I think it kinda tries to act as a service/owners manual in one as it is much more detailed than the usual Yamaha owners manual but not quite as extensive as a Yamaha service manual. I looked up the plug and it shows to be a NGK B-8EV gapped at 0.023, I remember they were quite hard to find a few years back at the time I bought a handful of them, I’m sure there are probably many substitutes, when the picture feature is working I will post some pictures, in the meantime if you need any information from the manual let me know.

Thank you LIZEEK that’s very helpful. Please can you let me know the timing specification to?

The above are the MX400B specs as shown in the Yamaha Technical Service Bulletin M4-105

Gold Palladium electrode plugs (V suffix) are not very common these days... 

I'd say you could use B8ES but Autolite is one of the few that still make non-resistor plugs...  Autolite equivalent for B8ES is 4054

Oh, and for the stuff not in the Owner's-Service manual you'd most likely use the DT400 service manual...

(I don't know if the MX400b owner's-service manual directs you to a DT service manual for things not covered in the abbreviated service manual included in the owner's manual, but I've seen that note in other MX/YZ/IT owner's-service manuals. Yamaha started doing that with the race bikes about that time.)

MarkT beat me to it, it’s the last entry on the picture he submitted, I think I have a handful of the NGK  B-8EV spark plugs, I think I bought them from a auto parts store that probably had them sitting on the shelf for decades, if you need a few let me know.

Thank you LIZEEK & MarkT - I’m in the UK so will have a hunt for someone selling the plugs and if I get stuck will get back to you :-)

Just became the owner of a 1975 MX 400. Thanks for answering a boat load of questions. I will be asking more.