Over the past year I've had sporadic problems with a random miss that occurs usually only in 6th gear, at very light throttle input (17-20% TP), and generally around 2100-2400 rpm. It is more prone to happen when the bike ('07 FLHX) is under some load, like riding against a headwind. I'm going to try to be as detailed as possible, so bear with me.

The first time this happened last May I replaced the Autolite spark plugs (one heat-range colder than stock) with another set of Autolite and the problem didn't recur for another few months. During the summer months I didn't notice the problem at all, but as more moderate weather moved in the problem resurfaced. It also has never happened when riding in colder weather (below 70°) and always happens after the bike has warmed-up, never when cold, although it is rare for me to jump to highway speeds when the engine is cold. When it does happen it usually lasts only for a few miles, then goes away.

The worst case encountered was starting out on a 2000-mi., four-day trip to AR last Oct. when it began misfiring early-on, quite bad for 40 miles, decreasing for the next 20, and disappearing after that. At my first stop 175 mi. down the road I changed the plugs (a used set of stock HD on of my friends had) and the problem didn't happen again during the entire trip and didn't resurface until about February. On an unseasonably warm day (80's) in Feb. I felt two quick misfires within about a mile of each other and no more. I changed the spark plugs again, this time to HD Gold, and the problem didn't happen until the other day when I felt a few misfires in a span of 10 miles or so, then all was well again after that. In every case, changing spark plugs causes the problem to disappear for at least a few months. Plugs are gapped to .040" and at no time has the ECM ever thrown a code except once for a failed running light.

Last Sunday I changed the plug wires (SE) and yesterday took a 240-mile ride, and the bike couldn't have run better, with no issues at all. OTOH I'm not naive enough to think this is the end of it, as it will often go for extended periods with no problems, then it'll crop-up again with no warning. IMO if it is ignition-related I would think it would be throwing codes, but it isn't.

I thought it might be the fuel pump or lines, perhaps a leak somewhere inside the tank. OTOH the misfire has happened after filling with gas, so I would think that might eliminate this as a possible cause. I don't consider the coil to be a leading suspect since if it was misbehaving it should throw a code, the same for plugs or plug wires, and since there are no codes I would think the plugs are firing during these misfiring events.

The misfires only occur at very slight load and almost always in 5th or 6th gear, usually only 6th, and only at relatively low RPM. It never occurs under heavy throttle input (>20% TP). Until recently (Feb.) I was running a PCV with a map switch where I could toggle between lean and rich maps on-the-fly. The misfires occurred regardless of whether the map was rich (13.5:1) or lean (14.6:1). I even made some changes to the ignition-advance tables in this region, but that didn't help either. I changed to the Power Vision in Feb. and imported my leaner PCV map on top of my stock ECM tune, which should've given no change in operation, and it didn't. The bike runs very well with the exception of these rare incidents.

I've done some datalogs with the PV and have noticed that the rear cylinder will occasionally show a knock-retard event in this exact region (TP, RPM, and gear) where I'll get a quick retard (set to 4° max correction but usually doesn't retard this much), tapering down in subsequent seconds even when the conditions don't change. This is random, and most of the time the retard event doesn't occur in the same conditions. Retarding the timing in that area does not help, and the retard events still happen. The misfire happens in the same environment, but not nearly as often as the knock-retard events. I never hear any audible detonation at any time.

The only thing that has ever stopped the problem is changing spark plugs, and that only for a few months. The plugs always look perfect, almost new, when I inspect them and never show any signs of wear or abnormal deposits. I had an accident in Oct. 2009, when I hit a dog and the bike incurred some damange, and these problems started occurring a few months after getting the bike back from repair. They installed a new tank but reused the old fuel pump and other fuel and ignition components. I changed the cams (to SE255) in Feb. 2009 a year before the first misfire incident and replaced the the intake seals last summer. Since I'm running open-loop I would think a leaky intake seal would show-up at idle, not just where the misfiring occurs.

Any ideas will be appreciated, as this has me stumped.

 

I am working on a similar bike with a similar issue, I believe the trouble is arising from Harley's knock system that automatically retards the timing. I am not sure if it another senor that is affecting the way the ECM is processing the timing or possibly to much resistance in the plugs but I will keep you informed if i track down a cure.

 

If a flash-based tuner is installed (PV, TTS, SEPST, etc.) we could turn-off knock-retard, but I'm not ready to try that just yet. I can't believe that 2° of retard for a second or two every so often would cause a misfire, but who knows. Please keep me posted on your progress and I'll do the same.

 

One thing I just noticed he has NGK Iridium plugs in the bike and they were gaped at 30th, Ordered another set and the new ones were also gaped at 30, Harley and NGK want 40th, Haven't had a chance to check yet but maybe we will get lucky. If not I will keep on trying.

 

Well, if it helps it won't help me. I've been using conventional plugs gapped at .040".

 

sounds like an ing miss for sure, if you install new plgs and goes away it could mean bad wires too and a possable weak coil could be the cause as when new parts are installed it's easier for the weak part to work than with old parts. You can have a bad wire and install a new plug and have the problem go away like you have had happen.

 

As mentioned in the first post, I changed the plug wires last week and the problem hasn't resurfaced. That doesn't mean anything, necessarily, as these events can occur with days or weeks passing between them, then pop-up again without warning. Time will tell.

I'm also entertaining the possibility the injectors are/were partially clogged from using discount gas. I will be using name-brand gas at least part of the time to make sure the injectors are kept clean. So far so good since changing the plug wires, but I saw no visible problem with the old wires except one area that had been rubbed a bit (slight dent), but not worn through.

 

I would think changing the wires and or the plugs would change the ion sensor
constants ? It would also tend to spark at a lower voltage than old wires/plugs
which might disguise a failing coil ?

 

What you describe sounds like a secondary ing problem which is coil,plugs and wires. A fuel inj would not do what you describe if one was bad or clogged then it would skip all the time, they don't self heal. Sorry I see you changed the wires and I would think that is the problem. If you put new plugs in with weak wires the problem will go away for a short period of time.

 

Try gapping the plugs to 0.035.

0.040 is for the very lean stock running condition, which you don't have. and 0.030 is just too small to get things going good.

It does sound like a spark issue. Could still be the wires or coil but this is a easy place to start.

Check the battery ground.

With a stock setup the battery ground is only at one place, to the frame close by, on my FLSTS. I added another to a transmissing bolt head to give that area some help..