For quite some time now I've been using a dynaS ignition using 3 ohm coils that originally came from a later model Kaw and the reason for that was because the bikes original 5 ohm coils had junk wires and I wanted coils that I could actually replace the high tension wires but was too cheap to purchase new 5 ohm coils which wires could be replaced.
Well anyhow all has been well for over a year now but after reading a recent thread about the use of later factory coils that were intended for use with factory CDI ignitions I decided that maybe my choice of coils wasn't such a good idea after all so I grabbed a set of 5 ohm coils off my shelf that came off a 77 750 to give it a try. The bike fired right up and during that first test ride the bike actually seemed to run better that is until I got a mile down the road and it blew a main fuse. I popped another in except when it did fire up it was only hitting on maybe 2 cyls and ran for maybe 10 feet and blew that fuse. With no cell phone on me and nobody home anyhow I had to push it back home on a wavy up and down road which about killed me.
A couple days later after cleaning my fuse holder and inspecting for any bad connections I popped a new fuse in, it fired on all 4 cyls this time and all seemed well so off on another test ride. Again the bike seemed to run better than it did with the other 3 ohm coils but about 5 miles down the road and thinking I had the problem fixed it blew the main again except this time once I replaced the fuse it would blow the moment I turned the ignition on. I had cell phone handy this time and wife was home so she picked me and the bike up in my truck with ramps in back just in case it failed again.
The problem was definitely coming from the ignition circuit because it didn't matter if the headlight was on or not, the instance I'd turn the switch on the handle bar control on it would blow the main and keep in mind that up to now I had never had an electrical issue with the bike. Of course the only change I made was the coils so off came the 5 ohm coils and on went the 3 ohm later model coils and so far all is well again in regards that it's not blowing the main fuse anymore.
As of now I definitely plan on getting a set of 5 ohm dyna coils and replace the glass type fuse holder to a more modern pin type but before I spend the $ on new coils I want to make sure all is well with my wiring because it didn't make sense to me that the 5 ohm coils were pulling more juice and blowing the main where as the 3 ohm coils aren't so my question is can anyone explain what exactly happened and or what could have caused this?
