hi, hi, I'm trying to rebuild a 1978 RS100 for my wife.i was wondering if the DT100 had the same harness or anotheer bike.
i live in UK.

 from near Salisbury, Wiltshire.

Do you have a parts manual for your bike? Also, is there anything of the original harness left?

Thank you.
i don't have a parts manual.
the harness is intact but when riding the voltage spikes - i have been advised the harness has oxidised.

I'm no sparky, but my first port of call would be the voltage regulator.

Treat yourself (and the missus!) to a genuine Yamaha parts manual and a genuine Yamaha service manual. 

already changed the regulator.
i will look for those manuals thank yous

I had a look at the parts list on the CMSNL site and couldn't see a regulator listed.  Admittedly, this was the US spec model, but even so...

Could you post a photo of the part you fitted? Note that you'll need to use a third party host such as Imgur to post photos. Alternatively, was it this part you fitted (rectifier)

RECTIFIER ASSY for RS100 1976 USA - order at CMSNL

or something like this?

VOLTAGE REGULATOR ASSY for DT250 1979 USA - order at CMSNL

 

Make sure you have a good battery installed as it does provide some primitive "voltage regulation". 

Are you sure you replaced a regulator and not the rectifier?  Here in the USA 100cc bikes did not have a regulator until later on. 

Rectifier would have a red and a white wire.  Regulator would just have one yellow wire and case needs to be grounded.  Also, there are more than a few aftermarket one-wire AC regulators that don't work properly and will let the voltage spike.  I use Yamaha OEM...  often used ones are cheap on ebay if you look and the seller often doesn't know what it is.

I see Alan had the same idea I did. 

hi, firstly thank you for helping me.

Ihave taken my bike to an engineer who has changed everything except the harness - hence my desre for a new one.
the engineer is a motorcycle mechanic and enthusiast