Idles Great, Backfires Under Throttle
 

Bike started easily, ran great for about 50 miles with two stops along the way. Then started to miss, sputter and backfire. Seemed like low on fuel, but plenty when I stopped. Idled GREAT, but any throttle applied it misses, sputters and backfires. I have a theory about what is causing this, but wanted other thoughts without contaminating your thinking. Does this periodically, then starts and runs good after sitting awhile.

i dont have an answer, but i experience the same thing. lol. only when its cold though. after it warms up, about 20 mins, it runs good.
im running a c.v.carb. i have other issues as well but dont want to steal yer post.
i will moniter this one...

Sounds like a bad condensor.

Spark Plugs
Plug Wires/Connectors

Was thinking perhaps of a torn or pinched diaphram in the carb. Had that once and bike ran like crap under acceleration as is was sucking air - but -don't think that this is your issue if it runs fine, then runs like crap.

Sounds like a no/interminnant spark issue, then, the unburned fuel in the exhaust pipe causes the backfire.

if it backfires thru the carb your running lean if you backfire thru the exhaust its a loose exhaust manifold or exhaust leak

48 FL I'll put some thought into this Did you drop the carb float bowl to see if water might be around ? Have you done any re wiring lately ?

Haven't done any re-wiring and have not checked float bowl. Will do. It was washed recently, but put probably 50 miles on it since before the problem developed. Have a mechanical advance distributor and was thinking that maybe it was malfunctioning, but I would think it would tend to stick in advance mode versus retard? I looked at it and no springs are broken and it did not seem to be stuck. I stated in original post that it idleed well but sputtered and backfired with any throttle. Actually, I can rev it up in neutral and it seems fine. Only when under a load does it backfire. I plan to double check the timing, points and condensor as well. Thanks for responding.

Did you ever find the solution to this problem? My 1978 FLH has started experiencing the same symptoms. I checked the plugs though and they are wet and black. I'm thinking an air leak at the intake rubbers?

I forgot to mention I have an S&S Super E and brand new points ignition. Bike idles fine and spits and chokes through the carb when accelerating but only does it when the engine warms up. Help!

I did figure out my problem, after double-checking timing, carb, points, etc figured it had to be the coil. When I removed the cover to test the coil, discovered the nut was gone that secured the wire to the coil. That's why it only happened under a load. Vibration made the wire jiggle around while riding. If I did not have a coil cover would have discovered it much sooner. Runs great now.

No expert by any means but wet sooty plugs sounds like too rich fuel mixture to me.