Only firing on one cylinder

Matt o

XS400 New Member
Reaction score
Cornwall, UK
Morning all,

I've been chasing this issue for some time now and haven't really progressed much so I'm here to ask advice.

After washing the bike it was fine to ride to work, (8 miles or so) but on the return journey in the evening it became lumpy and only just got home with increased revs. After a quick look, the right cylinder isn't firing. It also did this last winter in the rain but it resolved itself without my intervention so I put it down to wet HT leads.

What I've tried so far.
New plugs because they needed doing anyway
Swapped the cables over feeding the ignition coils. The problem moved, so i assume that the coil, HT lead and plugs are fine.
The bike has the ignition system without points which all looks fine on a visual. The 3 cables to the pickup have a resistance of just under 700 ohms and are both similar readings.
Removed the TCI unit and tested the cables to here, the cables to the coils have continuity at 0 ohms and the pickup cables have the same 700 ohms that I tested at the pickup itself.
The TCI unit looks perfect inside, no obvious dry joints, no cracked solder, no burnt areas or expanded capacitors.

Whilst poking around I found the inline diode was burnt out and traced it to the two metal cased safety relays. I believe that one of these is for the headlight and one prevents the starter motor from being energised once the bike is running. These relays were in a bad way, one corroded inside and the other had a piece of cardboard wedged inside to keep it made. I have temporarily linked out the starter one to get the bike to crank over, the other one i have just removed for now. Presumably what I've done here wont affect anything?

I just want to say that the battery is weak but during this testing I've had a car battery hooked up to it with jump leads and it has been turning over fine so I didn't think it was related to that.

Thanks for reading and thanks in advance for any advice.