My '74 FLH is backfiring and shooting flames out of the pipes sometimes. It has an S&S carb. It was running just fine for over a year now. I parked it for about two weeks and washed it before I went to ride it. I was careful not to get anything wet that I shouldn't have (as I always do). Well, I went to ride it and it started to backfire and shoot flames out of the pipes. I cleaned the plugs and they looked fine. I cleaned the points/condenser and it still backfires. Any ideas?

 

most likely the timing changed.could be a slipped ignition plate or broken spring or something in the nosecone/dist. could be a leaking/worn valve or just running way to rich...could just be a wet plug wire or coil,or soaked air filter.the cylinder loads up before misfiring,then it shoots flames. its easy to splash water in the wrong spot even if your trying not to...

 

Sounds like misfiring from something getting wet then shorting out . Plug wires and the coil is the first place I'd look a bit of moisture and old wires can play hell with you plus check your circuit breakers if you still have them . They do get weak with age and will flake out when wet or hot , very overlooked problem that can drive you nuts trouble shooting because it can cause symptoms like a number of other problems . Good luck bud and please keep us posted .

 

Okay....I checked the plug wires with an ohm meter and they're good. I eventually changed plugs after cleaning the old ones didn't do anything. I have the new plugs gapped at .30. I went for a short ride and it was still backfiring, just not as much. I then road a bit more and higher speeds and she quit backfiring so much. It does still backfire a little bit, mostly in first and second gear when I'm starting out at a stop. I road her at speeds of 35-65 and she didn't backfire until I let off the throttle and whipped it back up to speed, then she'd back fire about once or twice. But still backfires a little in first and second gears. I checked the points/condensor...I cleaned them. I don't know how to check the timing, but I'll look that up and see what I can do. I'm also running 70 weight HD oil because it's hotter than hell here in Az. I went from 50, then 60 weight when it was getting above 90-95 degree out. The lifters were chattering, so I went to 70 weight (I know a lot of people think 70 is too think, but out here that's what a lot of people run in the summer months) and she ran great all summer until now. It's still about 95-100 degrees out. Could the weight of oil have anything to do with this? I had just changed the oil again a couple weeks before I parked it, but it was running great. Am I missing something? Thanks in advance....

 

70 wt is fine for an older worn shovel & Az. heat I used to run it in Mo. during the summers there . I'm still leaning to your coil getting flakey on you and causing mis fires sounds like something is shorting bud and the ohm meter isn't telling you **** about the wires . When they get old they lose the insulation properties and will arc , solid core or graphite wires and do they feel stiff & look kind of dull or feel very soft and flexible like oil softened rubber ? Both will short out on you .

 

Thanks for the reply; The wires feel normal, not soft and not stiff. I'll change the wires this weekend anyway just to be on the safe side. I also had my bike idling for a bit, then I sprayed some carb cleaner on the intake area (both sides one at a time) and the bike stumbled/almost stalled. I think I have an intake leak. I did tighten the clamps, but that didn't do much when I sprayed the intake again. How do I know if I have o-ring or the other type of intake seals? I have an S&S carb, so I would guess that the intake was replaced when the carb was replaced (before I bought the bike). The intake looks like the one I had on my Sporty when I switched over to an S&S carb on that bike (kind of aluminum looking). Thanks in advance....

 

Yours should have O-ring heads where the clamps hold on bare metal , rubber band heads ( 78 & later ) you'll see the band squeezing out the sides of the clamps . Make sure when you do replace them you have both supports for the carb too one goes from the case to then lower manifold bolt and the other from the from rockerbox to the air cleaner . They will develop a leak fast without them .

 

just for ***** and grins, check the advanced timing spring (behind your points cover). a broken spring, and your bike wouldn't even start. yours does start and run but's the symptoms are similar so maybe your springs are on the way out. worth checking anyway. let me know.

 

Okay, I removed and replaced the O-rings....twice. Still have a vacuum leak at the intake. I thought I had them on correctly. I got her started and idling, then I sprayed some carb cleaner on the intake on both sides, and she almost conked out. I road her and she backfired in first and second gear a little (but still too much) and then she ran just fine in third and fourth gear. However, when I slow down a little in third/fourth gear and then hit the throttle, she'll backfire/pop once or twice until she gets up to speed. Before I replaced the O-rings, she ran like crap no matter what gear I was in. I don't get it. She was running absolutley fine for over a year and then I parked her for two weeks, then this crap started (and she was in the garage the entire time). I'll check the timing spring and I guess re-do the O-ring thing since there is obviously still a leak. I hate taking her to the shop, but I'm running out of ideas. I like to fix my bike myself, even though I'm not a wrench. I'll give it another try. Should I mess with the settings on the carb? I just find it hard to believe anything went out of adjustment or broke since it ran fine before I parked it. I even put all new gas in it two days ago. I road it up to the gas station and she wold pop/backfire in first and second gear, then she ran fine in third/fourth gear. The gas station is about two miles away one way.

 

Sam, if you still have a problem with a leaking manifold, as you mentioned, then you need to fix that first before moving on to look for a different problem to fix.

Clamps too tight or not tight enough?

Any chance the manifold is cracked or maybe has a pinhole leak in the body?