If you have good power thru out the band, then you don't need the white spacer mod.  do it if you have some surging in the mid range with a constant throttle.

all screws on the savage are JIS spec, slightly different screwdriver tip.
but if you give the screwdriver a lot of force into the screw, it'll work ok.
for your stripped screw, apply a little pen oil on the backside to get to the threads.  and use an impact driver if you still have a head.  you need one with a long tip.  if you can't get it to bite, you might try a torx tip.

if you got a muff with the baffle still intact, your jetting might be ok.
follow the jetting instructions in the tech section.

Dave, Thanks for the reply.  I rejected because the bike was surging during acceleration.  I replaced the stock main jet with a 147.5 and adjusted the air/fuel to 1 3/4 turns out.  the surging stopped, but the backfiring is pretty constant during downshifting.  I have a habit of raising the rpm's prior to downshifting. A habit from driving big rigs. Should I revert back to stock main jet? sorry for being ignorant on some of these issues, but this is my first bike and It's been somewhat of a learning curve for me.  Thanks again sir!

Thanks Kenny, Hope this will help. Model # is 65357-05

The surging while accelerating is an indication the bike is lean.  My bike had the lean surge just off idle.....like when you were leaving a STOP sign and were being watched by the Police.....or were just trying to ride slowly through a school zone. - at 1/2 to full throttle my bike ran smoothly, and it idled fine.  I re-jetted to a #150 main, #50 pilot, and then removed the white spacer on the needle and used 3 of the #4 machine screw washers.  I still had the lean surge, and the cure for my bike was to remove 1 more washer on the needle and only use 2 washers.

Setting the idle air screw is not something you do by just counting....it should be set by ear.  You can start at 1.5 turns out from lightly seated.  If you can turn the screw all the way in and the engine still runs well....you pilot jet is too big.  If you have to turn the screw out nearly 3 turns to get the idle smooth...the pilot jet is too small.  You never want to turn the screw out more than 3 turns - as the tapered portion of the needle is completely out of the hole and not doing anything.  Mikuni says the ideal pilot jet is the one that runs best at 1.5 turns out - I am OK with up to 2 turns out.....however the bike will likely run a bit better if you use a pilot jet that is one step bigger and requires you to have less than 2 turns out.....however you should be able to hear the bike begin to go lean as you get less than 1 turn from seated.

Here are a couple of YouTube videos that show how to set the idle fuel screw.  Our bike has a "fuel screw", and turning the screw out makes the mixture richer (some carbs have an air screw and turning the screw out makes the mixture leaner).

https://www.youtube.com/watch?v=YxpQjDQzX7g

https://www.youtube.com/watch?v=zm5mB3R8Ucw

Air leaks make backfires.
Check the header bolts and joints.