My 1986 fxrd what are the best upgrades I should do to this bike? I was thinking about a 42 mm flat slide Mikuni with open air filter and maybe bigger cam what is your opinions on this?

 

42 mm is overkill for an 80 inch Evo. Also, on a bike of that age, you need to consider the effect those changes will have on the existing stock parts that will interact with them.
For example, a "bigger" cam will likely trash '86 lifters in short order. "Bigger Cam" doesn't describe much. Duration, lift, rpm power band are key things to know what you expect BEFORE you start.
Simply dumping more gas with a cam won't necessarily make that bike better.

 

I had a 96 FXDWG and you can really wake yours up. Start with a Single Fire Ignition (more aggressive spark advance and adjustable rev limiter. Fine Tune you CV-40 with a Yost Performance kit, money well spent vs new carb. Arlen Ness Big Sucker air cleaner kit which comes with a K&N filter. Have cylinders bored to the next over size and drop in some Wisco Forged pistons with 0.040 head gaskets. Andrews EV-27 cam will give toy a Torque Monster.

 

you just can't throw a bigger cam in an 84-86 evo. pistons may or may not have reliefs cut in the top of the pistons for the bigger cam and the carb. if it's stock is not a CV. i would ride it like it is and just enjoy it for what it is. if it ain't broke don't fix it.

 

Since the bike doesn't look stock I suggest you find out what's already been done to it before you make any grand plans, unless you are the original owner chances are it's already been camed.