My 2012 Fatboy has this hesitation when rolling throttle just coming off idle rpm when the engine is warm. No such issue when it is cold.
I also noticed that the rpm not stable at idle, kind of searching around the 1k mark.
Check the plugs looks fine, gap is set to .040.
I tried swapping the NGK iridium with a set of old stock plugs, same hesitation off idle.
Any ideas?

 

I would first check/clean the IAC (Idle Air Control) on the induction module...

You can search this forum and/or YouTube and find numerous "how to clean the IAC" threads/videos...

It's free, other than some good MAF cleaner..

 

Thank you for the suggestion.
I went & clean the IAC using throttle cleaner. I rode for 20 minutes or so after that and there was no hesitation off idle. The bike was smooth and power was responsive.
Just when I thought all is good, it came back. The bike struggle to move off, below 2000rpm it feels like misfire. Above 2000rpm seems to be fine.
Would a manual idle control be the solution?

 

Do you have any diagnostic codes.....?

 

From my experience if you improve the problem by cleaning it , and only last about 2 to 3 months and gives you the same problem replace it . I put up with it for 3 to 4 cleaning , and just replaced it .

Here’s a little thing that I had to do to it . When it’s installed you will see that it leaves a small open hole on the top and the bottom where it attaches . To keep the dirt and water out , I cover / closed the hole on the top with a small amount of apoxy .

 

It's a long shot, but any chance your coil is shorting out? My metric v-twin was doing the same thing. Rough at idle, stumbled on acceleration from a stop, ran fine once the RPMs were up. The problem was less noticeable when the bike was cold. I chased the problems for over a year.

On my bike, it turned out to be the tiny (22ga) solid wire feeding 12v to the coil. The insulation had worn off and the wire was shorting out on the cylinder head cover. The engine vibrated more at idle and on acceleration, allowing the wire to randomly touch the cover. I fixed it with a piece of electrical tape and zip tie.

 

The part that I replace is the unit right above the number 2 , which is what your talking about the ( IAC ) my bad sorry.

 

Thank you. I'll check on that.

 

Did you ever check for codes....????

Barring any codes, I agree with Iron lHorse, if cleaning the IAC temporarily fixed the problem, but a short time later the issue came back, I would suspect the IAC may be failing... I'm not a fan of throwing parts at a problem, especially when a new IAC is like $65.... but electrical parts can have intermittent issues before total failure... Your call...

A diagnostic code may support the IAC beginning to fail assumption, or could point in a totally different direction, such as a plug, plug wire, or coil like Imagineer mentioned....

Keep us posted.

 

Update!

I found out that there were a few codes P131, P151, P507, P661, P1457, B2151 even though there wasn't any check light.
Send the bike to workshop and the mechanic did a fuel pressure test. Fuel pressure low.
Open up the fuel tank to check, found a small crack on the plastic housing. The mechanic told me this is quite common problem with old bike. Replace it together with new fuel filter.
Now the bike is smooth, responsive as it should be.
For anyone experiencing similar issue, get your fuel pressure checked. Maybe it is the same issue.