"1990 fxstc harley davidson... its backfiring from carb and pipes... I did the timing now I'm lost on the carb I put a new fuel pump thing (diaphram)... and it idels and starts like a camp but you give it gas or put a load on it and it spits and backfires the..

I've got a 1990 FLTCU and it does the same thing , when it's cold. I found the Evo aluminum motor takes a fair while to properly warm up. Until the engine has warmed and the aluminum has expanded to hold the head and barrels to the crankcase with enough force it will spit through the carbie if I blip the throttle. Using the choke set at a quarter and reducing to an eighth helps but on cold days it's a problem. I prefer to Idle the bike till I'm happy with it then ride it slowly through the gears until I figure it's warmed up enough. I got a throttle stop screw from the HD dealer that lets me set the throttle at fast idle so it helps warm it quicker. Let's me walk away from the bike when it's cold. I had to disassemble the throttle housing to fit it.

I'm running over a 215 main used a drill in it and a big pilot I forget how big but it's pretty big. The main problem is the idle jet on the 40mm Kehlin it's pressed in and I can't replace it. I reckon it should be much bigger. I still think the main can be a bit bigger and I'm at about a 1000 feet above sea level.

If you have a Kehlin check the rubber manifold gasket between the carbie and the manifold is not rotted they can get holes in them. Check your EVOS tube is connected to the carbie and not rotted.

But mine will do this backfire thing until it's nice an warm and happy then I can flog it as much as I wan no problem.

Also cold air sucking into the exhaust flanges can cause the exhaust to pop check that where the exhaust pipe joins are that they are tight and sealed and your headers are tight in the head that your exhaust gaskets are OK and not blown.