Electrical Gremlins are visiting! (Aparently Toad would know this one).

[Hillbilly Deluxe]

Do'nt know if this will help,or not.Have you tried running it with the "points" cover off ?
If the gasket,has been replaced,or ommitted,you can get arcing,very common fault on 70's,80's stuff,especially XS 250/400's.
Try running it in the dark,you will see more clearly if you have arcing due to bared wire.

Petrol cap venting ?
Carb diaphragm ?
Petrol tap diaphragm ?
Perforated float ?
Worn float needle ?

[Mr_G]

OK so I just came in from the garden after spending all evening working on the bike. Tonight I have :-

Taken the fuel tap off and apart (no faults found there) Re assembled and fitted.
Tested that the fuel cap is not at fault, Apparently it vents through the key hole. I can blow through it and after I refitted it and did something I heard a sucking noise coming from the cap so I know that's working.
Re made all earth connections and added a couple to ensure that that's not a problem.
Had a furkle with some wiring.

I then took it out on the road and tried it. I then discovered that only the left exhaust was hot so I wheeled it back and stuck my strobe on the right HT and found that when the problem occurs the Right plug is not firing. After a few moments scratching my head and deliberating I called my Boss and had a chat with him about my findings. We came to the conclusion that the chap at Bitzabikes was more than likely correct in that he said it sounded like an electrical fault. We discussed that I had replaced the coils and swapped the CDI unit but the fault was still occurring. We then talked through the ignition system and ended at the conclusion that the Signal Generator was most likely at fault. I then endeavoured to investigate, I then found that the brown signal wire PVC coating was warn away in places where it had been rubbing against the ATU and causing an intermittent earth fault, i.e. the small signal voltage created by the SG was going to earth and not to the CDI, I swapped this for my spare SG and tried it again, this time the bike would not even start. I guessed that my spare SG was dead so I wrapped the chaffed wire on the original with insulating tape and put it all back together (I also noted that my spare SG had a wire retaining clasp screwed to it and the original one did not so I nabbed it and stuck it on). Tested it up and down the road a couple of times, tidyed up and then went for a 10 mile ride. The fault seems to have reduced now but it is still happening.

So I suppose that I am getting somewhere with it now, but am wondering if I need to replace the ATU and SG as this is the second time that I have bodged the SG back together.

Thanks for all suggestions and please keep them coming, If I don't need to spend £3k on a new bike I would rather not but time is running out rapidly.

Thanks again Mr_G

[BikerGran]

If one bit of wiring is chafed it may be worth checking as much of the wiring as you can cos it could be happening elsewhere as well.  Electrickery is funny stuff, it drains away or ends up in the wrong place if your wires aren't all proofed!