Finally got My bike running... Mostly. Story to date:

First: Bike ran for crap since the day I got it, late April it developed an issue where it would after(or back, don't know for sure)fire FREAKING LOUD and die. Eventually left Me stranded on the side of the road awaiting a tow. Couldn't get to repairs quickly due to it raining nearly every day and other life-related fun.

What I did to it, round 1:

• New Raptor 660 petcock. Mounted perfect, no leaks. Capped off the vacuum line.

• Teardown and cleaning of carb. Carb was removed from bike, rubber bits removed, hosed with carb cleaner, reassembled.

• White spacer on needle removed and replaced with two #4 machine washers. Reassembled correctly, metal plate aligned with holes, dimple down, didn't forget any springs, clips, spacers or anything. Diaphragm, spring, and lid back on straight.

• Pilot jet left at stock 52.5, Main jet up'd to 150, pilot air jets at stock 67.5 #1, 230(2mm) #2.

• Fuel screw: Lost the original, replaced with one from a rebuild kit. Screw head appears longer, screw head is closer to the opening of the shaft it screws in to. Don't think the needle section that enters the carb and meters the fuel/air mix is any different.

• Tuning: Followed posted methods for tuning as best I could, there's conflicting methods.

Result round 1:
Bike ran like a raped ape, MY GOD THE POWER. No more dead spot in throttle just after You open it, smooth as silk, 0-to-25 range is actually ridable, not a bucking shuddering joke. Engine has wider range between gears, not as narrow and picky.
But it was rich as hell. Stank of burnt fuel, even though I only had the mixture screw out half a turn, any more than that rpm didn't seem to rise much audibly, but it reeked of gas. Could just about close the screw to 1/4 turn out before the engine started to choke. Not a single pop or afterfire except for one moderately loud pop decelerating gently (rolling off the throttle slowly) from 60 to 50.
Didn't want to run it rich to the point of stinking, as much as I loved the RAW FREAKING POWER, so I decided to rejet...

• Went down on the pilot jet, from stock 52.5 to 50.

Result round 2:
Bike is slightly tamer power wise, but still runs better than it used to. But now it pops loud when decelerating from 50 to 40, or from 40 to 30. It never used to do that in either it's crappy original state, or in the above 52.5 jetting. Mixture screw simply isn't giving the symphony of tones You guys state it should; Engine chokes at about 1/4 out, fastest audible idle is at only about 1 full turn out, maybe a bit more.
I've been out on a few test rides, and have tried the screw at 1, 1.5, and currently 2 full turns out, still get LOUD pop during decel in the 50-to-30 range.

To rule out vacuum leak, I'll re-snug bolts on the carb and hoses, was thinking of doing the old wd40 trick for a leak test.

I just want this pile of crap to run. The amount of trouble it seems to cause Me, along with how poorly designed it is, has convinced Me to get it in running shape (somehow), use it for the rest of the season, and sell the turd next year for something that wasn't engineered by screwheads.
But until that glorious day, You lot have any tips for getting it to run without doing shotgun impressions whenever I roll off the throttle in traffic? Being forced to clutch in and ride the brakes for a simple 5/10 mph speed adjustment and miss out on engine braking is silly.

My guess is your elevation is above 1000 ft.( should tell us when talking about your carb) ,you need to reinstall the 52.5 stock pilot jet ,and install a third washer on the needle jet . You went to a 50 pilot to lean your mix ,but that only leans it at idle( OR anytime you decal), but you only put two washers on the needle jet which made it rich and you've had to set the idle speed screw(under the choke nob) in to bring the idle up to normal ,bringing to much fuel in by way of the main jet/needle ,and that's why you have little reaction when tuning the idle mix screw.

I'm in central N.Y.  I live at about 1300 ft . but often travel to 450 ft . My setup is 52.5 /150 jets and 3 washers on the needle , I'm probably run a bit rich at home ,a bit lean when I'm ,say on the N.Y. thruway between Albany and Buffalo , I suffer no hard backfires  and rarely a soft poof at shutdown . You could try just changing back to the 52.5 pilot first, and tuning the mix screw ,that alone should kill the backfiring , if your still running rich after that ,then add the extra washer. It's always a good idea to make only one change at a time ,that way if things go south , you can just reverse your last change,  and try  something different.Each bike is a little different you may have to go to a 55 pilot but I don't think so. Be sure to ride the bike until it's fully warmed up ,and slow the idle(almost to a stall) so that you can hear the motor change speed when tuning the mix screw. then reset your idle to normal good luck!

A few things. The mixture screw only effects the idle circuit. By1/8 throttle you are off the idle circuit and on the needle. You’ll be riding the needle until about 3/4 throttle. If the bike is running rich it’s usually a problem of the needle, because that’s where you spend 90% of your riding. The afterfire is probably not a result of the jetting, so go back to the setup that gave you the best power and throttle response. These bikes with the OEM carbs afterfire. Get use to it and learn to smile when it happens. Excessive afterfire may be a lose exhaust or cracked flange/gasket at the head.

batman wrote on 06/10/19 at 07:45:29:
--------------------

Typo.  47.5 pilot which is only TWO sizes down from stock (a 45 would be 3, not 7 BTW).  I'm using a 3/4 spacer.  Half was too rich.  

Since I spent a lot of time tuning motocross bikes growing up, I actually know how to test jetting.  Everyone claims how lean this bike is from stock and it's just not working out that way for me.  As "lean" as I am now, my bike still runs better in cooler temperatures that it does in the dog days of summer.*  It starts up and is immediately ridable on half-choke in sub-40 degree weather.   I only begins to surge at steady throttle when it gets below 35 degrees.  

The bike runs and sounds completely different than it did when I got it.  The PO had a 55/155 combo in it.  It sounded cool, but it had a slow steady pull, no throttle response, topped out at a little over 70 mph, there was carbon all over the rear wheel, the plug was almost wet and it only got about 32 mpg.  

I get 50 mpg now, I get solid throttle response from anywhere and the bike will pull a quick 80mph before it begins to fall flat.  I can short shift it anywhere in the power band without falling on its face.  It's about all I think I could expect for what it is.

I'm a firm believer of using information to help make informed decisions, but ultimately you need to test the results for yourself.   My testing showed me what works best for my bike.   To each his own, but I wonder if people are overlooking some simple tuning clues.  

Until your reach extremes, a slightly rich condition and a slightly lean condition will produce a soft spot in the power band.  Both can produce backfires.   You can muck around with jetting all day long, but the absolute easiest way to determine with direction to go is to ride the bike during the coolest part of the day and compare it to the hottest part of the day.  I leave for work every day at 6 am and ride home at 4 pm pretty much every dry day that starts out above 35-40 degrees.

Dream on???  Okay.

*I'm really not even jetted lean.  One pilot jet leaner than the very common 50.  One main richer than stock which is only one main leaner than the commonly used 150.  White spacer 3/4 is richer than stock.

We use the Mikuni Roll-Off Method to check main size:  http://www.mikuni.com/tg_main_jet_size.html

The Roll-Off technique is the quickest and is almost as accurate as the Roll-On method. First, one gets the engine warm on the way to a safe roadway. If there is room, use fourth gear as this allows more time to assess the result.

Now, get the engine rpm high enough that it is on the cam and in its power band. This may need to be as high as 4000 rpm with some cam choices. Apply full throttle. Let the engine accelerate for a couple of seconds until it has settled in and is pulling hard. Quickly roll the throttle off to about the 7/8ths position. When you do this, the mixture richens slightly for a second or so.

If the engine gains power as you roll the throttle off, then the main jet is too small and you need to fit a larger one.

If the engine staggers slightly or has a hard hesitation, then the main jet is too large and you need to fit a smaller one.

Like I said each bike is different, Ohiomoto's pulls cleanly to 80 mine pulls hard to 90, Dave yours runs good ,but you are small and 150 lbs.and your bike is about 80 lbs lighter and your running higher compression and high octane gas . My bike may be heaver than stock running full Harley fenders and I'm riding two up most of the time ,I know I'm running a bit rich but it's a 95 without the anti-kickback gear so I run rich to protect the starter gears and side case from damage (I can't afford a heavy backfire at shut off) I run 52.5 /150 -3 washers but still gets 51.5 mpg on E87  and my plug looks good. My motor is stock but for a Dyna muffler , All three bike run well but all require a different state of tune. but we are all running at nearly the same elevation .
      Majorfatboy"s problem was that he was running rich at speed (which could be from having only two washers on the needle ,but could also be caused by the wrong air mix screw he used to from a rebuild kit) so I suggest he add a washer, his other problem was when he closed the throttle the bike went lean and backfired ,with a 50 pilot ( this lean condition could be caused by the wrong air mix screw ,but I thought that unlikely given that it was nearly closed off), so suggested he return to the stock pilot jet. In hind sight I should have suggested he should start by installing the proper air screw,  and be sure the TEV valve and passages are clear before a rejet.

You could also chase the rather small passage behind the "finger"of the TEV valve with a fine piece of copper wire ,and spray again. I would at this point,  order a new stock air screw ,spring washer and O-ring.  I wouldn't trust the one from the rebuild kit,  cost of all 4 being about $15.00
  Screw ,Pilot air -13269-37400
     Spring-13268-47070
   washer-13291-29900
      O-ring -13295-29900