Just got mine dialed in really well this evening after work.
I had previously done the following;
-Disassemble and cleaned thoroughly.
-Rebuilt w/stock gasket and jets kit.
-Drilled extra slide hole and oversized the other (2x 1/8" or 3.2mm holes). Hand Drilled extra hole from the inside/top using the side channel as a guide.
-Trimmed 2 coils from spring, reduced to 4-1/8" long or a bit over 100mm.
-D'd the spacer so needle hangs freely, with wiggle room.
-2x 3mm washer spacers - (more later).
-Tapered stock needle; measured 45mm or 1-3/4" from top clip end, and tapered to roughly 1/16" or 1.5 mm point from there. (final 3/4" to point tapered).
Also removed snorkel from air-box, which likely aided in leaning out the top end a bit.
I'm no expert on this, and have not tuned a carburetor or even rode a motorcycle much in over 5 years. But I ignored these facts and dove right in...
The results were a far better running bike, it was as good or better than new, I'm sure! ...but it had ran poorly when I bought it, so my estimates are not based on actual seat time on a new DR. When I bought it, the bike had lean surges around parking lots, 41mpg best tank driving as efficiently as possible under 65mph, and 38mpg at 80. The problem was immediately identified as the original plastic spacer had worn to half its thickness and was nearly D'd shape clear through all by itself; effectively lowing the needle and starving the fuel supply at anything but wide open throttle. Though the top end was okay, pulling from 70-90 mph in 5th just fine.
This total carburetor rebuild and initial needle tapering, plus the addition of 2x washer shims ran well enough, with plenty of power and only needed 30 seconds of choke till it was ride-able even at 40-F degrees, which lead me to believe it was to rich and I could do better. Of course the mixture and idle screws were adjusted at every step, to no avail so I removed one washer to lower the needle, now the choke needed 1-2 mins to warm up, which was more realistic in these temps. But it lean surged with throttle barely cracked around town when fully warmed up after a long ride, and seemed to be a little flat until the middle rev range, with a hint and I mean a hint of bog at full throttle in high gear, where if one reduced to 7/8 throttle it actually made more power, or the same power but was smoother and still maintained speed.
All these symptoms lead me to think I needed to fatten up the bottom end, and lean out the top slightly, even more than simply removing the snorkel. Also wanted to remove the washers for good, to lean out the top end slightly. But this lowering of the needle would likely make for very lean partial throttle.
What I just finished this evening, was to take out the stock tapered needle, removed the remaining washer spacer and trimmed the needle to length. Shortening the stock needle roughly the white D-spacers thickness to compensate for hanging it off the shelf. I mounted it in a cordless drill and marked near the point using the white plastic spacers thickness as a gauge, then used a fine tooth hacksaw blade on a counter-top and lowered the spinning needle drill lathe-like to cut the tip off. Simply removed 2-3mm from tip of the needle.
Then once again tapered it aggressively, measured 1-3/4" or 45mm down from the clip end, marked it and tapered from there to the point. I sanded the point to less than 1/16" or <1.5mm. (Best way to taper needle I found - mount it in cordless drill, use medium or coarse emery cloth or roughly 150 grit sand paper and sand it to rough shape, focus on the tip and near the tip, use 220 grit or fine emery cloth to finalize the taper shape and reduce gouges, finish it with 600 grit water or oil whetted sand paper for near factory polished finish).
My final solution is not actually my own creation, as many of you know but dug it up on Auscanvet's epic BST magic thread where he compares needle lengths between stock, the shorter DynoJet, and the stock cut down to the same length as DJ then tapered. Auscanvet claimed the stock needle, trimmed to a shorter length and then tapered aggressively was the best needle found. It made sense to me immediately, since it effectively moves the needles tip back up to stock height prior to the D'd spacer lowering it. The spacer hanging off the shelf lowered the needle point, which leans out the bottom end, exactly the opposite of what tapering was supposed to accomplish in the first place.
Having read about it and considered doing it from the get-go, as it made dimensional sense when I first ran across it, but the information was not as well presented or clarified by others as I would prefer before irreparably hacking into my motorcycle, so I simply decided to take it one step at a time. And to check off what works or don't for myself and anyone else in a similar situation, So originally just tapered the stock needle, installed 2x washers to raise the needle back close to stock hight, like many people have successfully used in their bikes in the US. And it all worked fine on my bike - 2,000' elv in autumn weather, 40 some degrees. ...But not nearly as well as the complete (needle length trim + taper) job that I just completed several hours ago.
This thing RIPS and I mean it's FAST. Power everywhere and seems perfect! Yes, I test rode it in the cold, after dark, and with a damp road. But this thing will now lift the front wheel in first with just a snap on the throttle and very little coaxing or weight transfer. It will power slide the at will, and on damp roads you will be spinning away from every stoplight without careful attention to clutch or throttle application. And this is without a cut air-box or exhaust mods.
Note: ...Though it now seems to "hover" at slightly higher revs after chopping the throttle and pulling in the clutch, when it should go right to low idle speed, I do believe possibly cutting the slide spring, or an old stiff slide diaphragm might be preventing the slide from immediately seating at the lowest idle setting. The throttle cable has slack, and its closing with a nice snap. Also a possibility that cutting and tapering the needle combined is just giving it to much fuel and overpowering the idle mixture screw adjustment, but I don't believe this is the case, as if I dial the screw in it will eventually sputter and idle rough. I'm talking its only 300-500 revs higher than idle and it slowly comes down to idle at a stop, or if you leave it in gear while you slow to a crawl it will slow all the way and remain at low idle.
It was also more difficult to find the "right" idle speed and mixture screw settings taking about 5 or 6 tries to get it close, and some adjusting while riding around a parking lot. But based on my last spin of the night I think its nailed now! Excellent throttle response for any bike regardless of carb type. It snaps into the power as quickly as the throttle cable slack is taken up. Your wrist will be as close to telepathically connected to the rear wheel as a carbureted ride ever will be, I don't think you can get much better than this, possibly another 3-5% improvement for a pumper carb. And possibly 10% better response if you went all out for EFI. Where the bike not only responds but predicts your brains decision for more power and applies it before you ask.
My final recommendation for those riding in similar locales and conditions;
-Disassemble and clean carburetor
-Rebuild kit - $35 Moose or Shindy - For higher mile bikes (Stock D-spacer, needle, or needle valve may be worn and impossible to tune past 10k miles)
-Drill extra slide hole, enlarge original (2x 1/8" or 3.2mm holes). Drilling the extra hole from inside the slide, by using the side channel as a guide is easier.
-D sand the spacer so needle hangs freely off shelf, with some wiggle room.
-Cut & Tapered stock needle; Cut off about 3mm from tip, use the new white spacer thickness as a guide, then measure 45mm or 1-3/4" from clip-end, and taper from here to the point, sand to a 1/16" or 1.5 mm point or slightly less.
-You can also remove snorkel from air-box, its easily swapped in & out, makes some difference in power and tuning. (remove for more power/leaner mixture)
-Add extended fuel mixture screw, $20 well spent for fine tuning on the fly. You can use a fine flat-head driver bit by hand, but ware gloves! Its Hot!
I will not recommend cutting slide spring at this stage. Found the bike is hesitant to return to stock-idle after a long ride it just likes to "hover" a little higher than stock by maybe 500rpm after closing throttle. Even with throttle cable adjusted to provide extra slack, and ensuring throttle lever snapping back to the idle speed rest. Mixture screw and idle speed are set. So Just leave the spring stock it cant hurt, and likely would prevent this issue by forcing that slide down to idle setting in a more positive way. You can always trim 2 coils from it later if you desire an even quicker throttle response than the extra drilled hole provided. And you can record the difference if you notice a return-to-idle issue at that point and report on it.
I'll continue messing with the idle mixture screw, ill try 2-3 turns out and see if throwing fuel at it will get it to fall to idle more consistently, and there is a possibility the idle speed is just set to low, there is no Tachometer to set 1500rpm by, but it sounds nice and slow and smooth to me, and chugs along at 5mph idle in first.