I have had this issue since day one. I have replaced the exhaust with a true duel system and notice a little soot on the frame at the back. I have heard that an exhaust leak can cause a wicked backfire and this sounds like it. However the bike will miss on that cylinder at first when the backfiring starts cold and dissipates when its hot. things get hot and expand and perhaps reseal but what is causing the miss. Is this perhaps a coil instead. the back fireing occurred with stock and true duels. the intake is sealed good. Bike runs strong when hot and always starts. 1974- flh Dyna S, S&S carb. and a complete rebuild.

Any suggestions.

randy

 

Swap the plugs front to rear and see if anything changes, if not could be loose connection somewhere , doesn't sound like the coil.

 

sounds like rear exhaust gasket, not the coil, to me , too.

 

Mufflers?

 

Yes mufflers and fishtails. After it warms up its not bad although now and then even hot it will backfire when it first starts. A big one lol.

randy

 

Ok heres the scoop. I fixed the exhaust leak and it didn't change it a bit. So its not that although there was a leak so that had to be fixed. I swapped the plugs and guess what...rear cylinder backfires. I changed the plugs and rear cylinder backfires. So given that this is a dual fire, I changed the wires on the coil. Top one always went to the rear, bottom to the front. Changed them.....Front cylinder backfires though the muffler but not quite as bad but still pops in the pipe. So I guess that pretty much says whats going on. I have a Dyna S ingnition moduale and an Accell coil. What is it guys...The coil or the dyna S. Im thinking coil. Can someone recommend one for me?
By the way, what colour should the spark be, mine is yellow, not blue.
randy

 

make sure you have a 5 ohm coil. I got the dynatek one.

 

I doubt the Coil has any influence on this..Single fire or dual fire maybe..If single fire it is something that happens ..Not usually with Duals tho...
Tough one...Slap your Points plate in and Run it...Best troubleshooting device I have.
Depending on the primary resistance of yer coil. Should be 4-Ohms+ for Points...the Dyna S can handle 3-Ohms

 

Dyna S says to use 5 ohm coil.

 

This is what I did to determine a coil issue. The bike backfires severly out the rear cylinder. I have true duals so I know its that cylinder. I fixed any exhaust leaks I can find and that didn't work. I changed plugs then replace them without success. Finally I took the wire off the top part of the coil and swapped it out with the one on the bottom and vs vrs. I can now here backfiring in the front cylinder pipe although not as loud primarily I assume because the pipe is longer. That has to be the coil. It is specific to the top port on the coil. I have had two completey different exhaust systems on it and it has always backfired out the rear cylinder but this is the first time I swapped the wires on the coil. Its a dual fire Accel coil. What else can it be?

randy