View Full Version : Ironhead Unusual Engine Problem With My '78

16th October 2009, 05:40
I've let my '78 sit for a few months while I was busy riding the '77 XLT and my Honda CBX. Finally got around to installing a new clutch cable to replace the one that had just two wire strands holding it together. Went out for a cruise last Monday and after about 15 minutes I lost a cylinder, completely. No mechanical noise, just an occasional muffler pop but otherwise I was riding a 500cc single. Ran that way for about a mile back to my storage garage and towards the end both cylinders were running fine again. Didn't have time to mess with it then.

Today, checked the wiring (all good) and ran the bike over to my brother's house about 4 miles away. Lost the cylinder again when near his place, this time without even a pop from the dead cylinder's exhaust. Let the bike rest for a few minutes and it was running on 2 again. Headed home and within a few miles dropped the cylinder again, only to have it come and go several times in the short run home.

Checked the plugs: All in great shape. Checked pushrods: ditto. Checked electronic module (PowerArc single fire) and rotor: nothing loose or otherwise out of wack. Checked coil wiring: as new. Carb is working normally as well. Basically, everything checks out perfectly but I've suddenly developed this dropped cylinder syndrome. I have over 8K miles on the new top end, including the ignition, with nary a miss or problem along the way, including some long runs over the mountains in high temps.

I have a new MSD V Series ignition I'm going to swap in this weekend. I figure the most likely culprit is the PowerArc or the coil for the dropped cylinder (I have 2 coils for the dual plugged heads). Nothing else makes sense, especially with the lack of any mechanical noise whatsoever when running on 1 cylinder. Acts just like there's no spark getting through to the dead hole. Anyway, curious if anyone has experienced this before.


16th October 2009, 05:58
Definitely sounds like a coil dropping out and the engine just continues with its suck, squeeze, bang, blow. ...just with unburnt fuel??. I've seen an induction pickup you can wrap around a plug lead and this runs to an electronic tach. ..something like one of there would let you know if it dropped out when it heated up. . . ..bl00dy electrical faults. . . .like chasing unknown magical spells sometimes!!

16th October 2009, 07:28
Try this, as soon as you loose a cylinder turn your egnition switch off, pull over and pull the plugs and if the plug is wet then you know it's in your egnition, not fuel. hope this helps.

16th October 2009, 09:11
You run with a sigle fire ignition. I think the problem would be with the ignition module or coil (or the spark plug wire).
Bye, acb2

16th October 2009, 14:09
I'm going to go with a bad coil, I have had off and on problems with Dyna coils so I switched to the cheaper Rev Tech and no problems.


16th October 2009, 15:53
Try this, as soon as you loose a cylinder turn your egnition switch off, pull over and pull the plugs and if the plug is wet then you know it's in your egnition, not fuel. hope this helps.

The main thing is then you know which cylinder it is. Then swap your coils (you are running single fire aren't you) and test again. If the dead cylinder switches with the coils thats your problem, if not its your ignition module.

1976 XL
17th October 2009, 00:18
I would also bet on the coil.

Keep it running
17th October 2009, 17:56
I had a similar problem and traced it to a spark plug. R2 them and on the road again.

17th October 2009, 18:03
i concur on the sparkplug. i had one (new), that the ceramic insulation was broken all the way around the electrode, so it could slide up and down on the center electrode. when the engine had good cylinder pressure it ran great. as soon as it came to idle it would miss. turns out the ceramic was sliding down the electrode and masking the gap. drove me crazy for about a week hunting down a low speed misfire.:doh