continuation post from https://www.hdforums.com/m_2656283/tm.htm#

We are basically at a loss as to why it is backfiring and not idling (stalls). The bike from what I can tell has VH pips straight without baffles and a SE A/C.
He said the backfiring is coming from the rear part of the engine (Top exhaust pipe). So the front part is not backfiring. It still stalls even when it is warm, and even when we came back from a 5 mile jog yesterday. Backfiring and stalling out is pretty much what it was doing. If he didn't keep the throttle open a bit, it would stall immediately. If he opened it wide and rode it like he stole it, it sounded very nice...until he let up off the throttle. If he is just cruising (keeping enough throttle to keep a load on the engine) sounds like crap.

He recently took a trip to Mexico on the bike from Washington State. On the way back from Mexico (in California) it started backfiring a little bit. It progressively got worse as he came home. Now it is just terrible sounding. When he got back from Mexico, he changed out the plugs and the problem went away at first, then came back like a lion. The old plugs were black. The bike still backfires and stalls. His bike has 13,000+ miles on it. He is not the original owner. There is no pinging at all.

What we will do next is:

I was thinking that we would take this a step at a time and go through the plug wires, wiring to battery and charging...make sure everything is real tight like.
We did manage to get a fix my hog DVD for his bike. I got one for my bike as well and it is worth every penny.
I have every tool in the book for cars and my bike (motorcycle jack too), so I think we can get this thing working in time for our Canada trip next summer. I really want to help him out. He is my neighbor too.

Pull the plugs he has in now and check the color against the http://www.dansmc.com/spark_plugs/sp...s_catalog.html link. The old plugs were number 15 on the link above.

Check oil consumption.

Check gas mileage.

 

Have you checked the compression?

You said the plugs were black - both of them? If they are both black, then the carb is jetted too rich - change the jet(s) and see if it improves. Plenty of info in the Do It Yourself section on how to tune a carb.

If only the rear plug is black, then you may have oil leaking into the cylinder (as was previously commented), or you have lost compression on the rear cylinder, which is causing it to not burn the fuel properly. Had a similar problem on mine a while back, though not to the point of stalling. Rear plug was getting "sooty" and bike seemed to be sluggish. Turned out that one of the head bolts had loosened on me and waslosing compression on the rear cylinder. Definitely get a compression tester and check both cylinders. Saved me a lot of time and money.

I agree with the earlier comments about doing it yourself. It's not that hard - maybe time consuming, but not hard. Get a service manual for the bike and start tinkering. None of us on the forum gained any knowlegeor experience by writing checks to someone else.