Took my 99 Sportster 1200 custom build out today; took it to the highway and got up to speed, clutched to upshift, and the motor cut out immediately. Upon trying to restart, it coughs briefly or occasionally shoots a nice fireball out the pipes, with no success of turning over. After walking her 2 miles back home I initially inspected vacuum lines thinking it could be a pressure issue; all were connected and in good shape. Then fearing it could be a thrown rod or something catastrophic, I inspected oil level which was at capacity, fairly clean, and without any glitter or shards whatsoever. This is only my third time riding her since build; first two rides were quick & under 50mph for testing the handbuilt frame & chassis. Normally I wouldn’t take to forums so quickly without further in-depth investigation, but after a whole winter of wrenching & welding I’m eager to just be on the road ASAP. If anyone might have any leads, it’d be much appreciated!

 

Remove your "13" cover on the right and check to see if your Ignition Rotor spun or sheared the stamped key.
Wouldn't hurt to check the rubber on your vacuum slide piston at the top of the carb.
Did you check and see if your getting spark to both plugs?
Plus, when you say "with no success of turning over", do you mean dead battery? Cause a shot battery will also render the bike useless. You need to load test it after it's charged to make sure it didn't take a dump. After all the mentioned above and they check out. It will be time to check the Stator and Regulator.

 

Tested everything right before her first ride, new plugs, wires, etc. Upon letting her sit for about an hour and a half, I decided for ***** n giggles to see if she’d start up cold, and lo and behold she fired right up before even cranking a full rotation. IGN rotor looks fine; I’ve heard failing ignition modules can cause motor cutout once a motors warmed up, and only start again after a cooling period. How would one test the module?

 

That would be so effin' cool if there was a button on your controls that you could push to shoot flames out your pipes when you're riding. I'm just sayin'.

 

The coil will also do that when hot. Sorry man but I'm kinda busy to be thumbing through the service manual right now. Maybe someone else will chime in.

 

Old trick for testing ignition modules, take a hair dryer or paint remover gun and blow hot air over module. If it fires up while cold, blow the hot air over it before starting up. Then as you crank it see if any spark. If there is do same to coil but you can check that with ohm meter

 

yeah the coil is still testing fine, great idea with the heat gun, thanks!

 

Have you look for some debris on carb? I've just repair a 99 night train with same symptom, and the problem was the inner tank sealant was peeling off and this debris clog the carb.