Misfire – cause and effect.

[cyclobutch]

Just a note here on an issue I've had – now sorted, just as a pointer for others, albeit with a coulpe of questions at the end;

First up a description of possible cause. When I first bought the bike it would noticeably miss a beat from time to time. This always coincided with my hitting a bump in the road. And I also noticed that the rev counter (electronic) needle would plummet on each occasion also. Being intermittent I just couldn't trace it. But it got worse, to the point that on one occasion – heading for the European mainland, I figured I wasn't going to make it and turned back for home. It did make it but was then dead when I tried another start up. Result. The fault was a wearing wire for the nose cone sensor that was being rubbed against the frame at the back of the oil tank. Spliced in a rerouted wire and we were all back in business.

Time passes and the bike is kind of running OK, but will sometimes run a little snotty. I replace the coil and leads and it seems a bit better. But maybe it still ain't right. So I pull the carb down a few times for cleaning and some different jets. Now my carb is an odd one – it's an OE fit Keihen (but MoCo branded), but with no accel pump. So I buy one that has that. But this creates more problems that it fixes (needed the 88 Sportster needle I now think) so I revert. But it's still maybe not quite right, so I go with another coil and leads – this all happens over a few years.

But I've also got an ongoing starter issue (see separate thread), so it ends up getting parked up for a while. Now as it happens the very last ride out when I switch the lights on the bike gets a real bad misfire. Switch them off and it's kind of alright again. But I have other bikes and time passes. Eventually I have so many problems with others on the fleet that I figure I'll never get round to looking at that starter so I bite the bullet and hand it over to the local independent to sort out. So along with sorting that starter I kind of flippantly mention the misfire with the lights – I figure they might as well find the bad earth whilst they're about it.

Starter turns out to be fubarred, so I'm in for labour plus the price of a new replacement. The misfire turns into a bigger deal. They just cannot find it. Surley Steve - their main man spends a whole day going through the loom from end to end. Nothing. They can only conclude that the ign module is playing up – would I like to stump for a replacement and see if they are right? So we go with it and I have a transformed bike – all at some cost (though Surley is a decent sort and waives a chunk of investigation time). 

So.

Seems that the misfire issue that finally became so bad when the lights were on was probably a longer running deterioration in the ign module. Now being solid state I would have expected that to either work or not. And are these prone to failure? Could that earlier issue with the shorting wire be the original cause?

So anyway, if you're chasing your tail with something similar maybe you need to go the same route. 

And I'm back running the old girl on the daily commute and maybe back on track for hard tailing come next winter.

[Panzer]

Well it seems that you went the right route, from start to finish.
I don't see where you mentioned the year and model, I'd like to know for reference.
Glad you got it sorted out and she's back on the road and reliable again. 

[cyclobutch]

Ah - thought it was in my signature line somewhere.

Early one - '89 XLH1200