Why does that happen?

[Geoff Hastings]

To free up some space for my next project I reluctantly decided to sell my C95. The guy, who is a friend of a friend, who is interested in the bike rides a brand new Triumph with lots of performance and brakes to match, the exact opposite of the little Honda. I was reluctant to sell to him thinking he would be so disappointed he’d think I’d sold him a duff bike. Between him viewing it and me delivering the bike I’d ridden about a100 miles so was at least confident that it was as good as it could be. The very next day he phones up, he’s stranded as the battery had gone flat after 10 miles. I take another battery out to him and the bike starts straight away but is definitely not charging. The bridge rectifier had gone short! A couple of days later I ride the bike back to my place and it’s only running on 1 1/2 cylinders. Nice fat spark, I swop the plugs left to right and fault stays on the same cylinder, single carb with no manifold joints to leak so it’s not that. I did a compression test and both cylinders are good. Single coil, single points. I put a strobe on the leads and the faulty cylinder is definitely missing the odd spark or two. I’ve ordered a new coil and plug caps. but why, I can only imagine the bike doesn’t want to leave home.

[deuce_7]

I put a strobe on the leads and the faulty cylinder is definitely missing the odd spark or two. I’ve ordered a new coil and plug caps.

Let us know how it works out. Sounds as if you've done the correct troubleshooting so far.

[Michael Stoic]

Only the lead and the plug cap left and you'll have a new cap on that side soon. While waiting for the new one, can you swap the caps? Maybe you'll have one more data point. I agree with Deuce - you're going through this methodically.

[Geoff Hastings]

Hi guys,
The new coil and caps arrived so I took the original coil off. Sod’s Law that the replacement coil has different lead fittings so I changed the new fittings to match the original. While I was at it I ran a quick check on the plug caps, the originals read 8k ohms and 11k ohms the new ones have no resistance. I can’t remember at this stage if they should be resistive caps or not but for now I’ll just run them. The bike started first push of the button and first signs are good, both cylinders running smooth and even so once the England Fiji rugby match finishes I’ll take it for a buzz down the road.

[Geoff Hastings]

Ok, big relief as England won, just!
Anyway, back to the bike. I loosely hung the coil where the air filter was and went for a short ride. The bike felt fine with both exhausts getting hot, however….. if I put my hand in the exhaust gas at the tail pipes the left is definitely cooler yet it’s definitely firing on both cylinders. I must admit I don’t usually feel the exhaust gas temperature so perhaps I’m now looking for a fault that’s not really there. I’ll take it for a longer run with everything back in position and check again, perhaps tomorrow if the weather behaves.

[Geoff Hastings]

So, after considering the fault overnight I decided to put the original coil back on along with the new plug caps. In reality I should have just changed the caps as it’s never good to change two things at the same time when fault finding. Anyway, with the original coil and the new caps fitted I went for a longer ride this morning and all seems well plus the bike definitely idles smoother. Only problem now is the new caps are shorter than stock so the left cylinder with the more recessed plug is at full stretch so I’ll have to order two slightly longer reach caps.

[Michael Stoic]

Since you did detect some temp variance between the sides, at least once, maybe an IR thermometer could be useful.

Are your spark plug resistor types? I would try confirmed 'non-resistor' plugs with your original leads and caps - and 'resistor' plugs with the new caps.