OK, Ive grabbed this infor from snowmobille related sources so some info may pertain to "colder" conditions, Plus some of the info is contradictory, these 2 guys don't get along too good.
1. Not all round slide carbs run a "little plastic clear ring", (It's nylon). Usually a manufactuers modification for a particular application.
2.If the air mixture screw needs to be set less than 1/2 turn out, pilot jet needs to be larger(richer). More than 3 turns out, smaller(leaner).
3.A dead 0 - 1/3 throttle problem could be caused by many things, throttle valves being one of the last to change.
4.In a powerjet carb the powerjet circuit is used throughout the complete throttle range.
5. The temp. of the day to set your carb up should be within 10degF of what temp you usually ride at.
6.The Mikuni hex jet socket size is 6mm. The bottom plug is 17mm.
7. The needle jet is adjustable for a reason. You shouldn't have to change needles or needle jets unless you've made major engine modifications.
8. I've done a lot of carb tuning simply by adjusting the float level. Make sure it's set right first. You can alter it to make changes over the whole throttle range.
9.Make sure your carbs are in sync with vacuum guages , not just that the throttle slide goes all the way up and down.
10. The PTO side of the engine gets hotter and requires smaller jets than the MAG side. Especially on F/C engines where the fan cools the MAG cylinder first and then blows that warmed air over the PTO side.
-----Make certain your engine compression and all seals are good before fuel tuning----
and
round slide carbs are used alot on many snowmobiles.
not usually listed in parts list but is very important is a little plastic clear ring that goes underneath the e-clip on the main needle.. its VERY important as the carbs were set that way.
air bleeds should be set to 2 - 2 1/2 turns out if your engine doesnt run right that way change pilot jet
throttle valves should be changed if your 0 - 1/3 throttle is dead.
intermediate is controlled by main needle and needle jet.
3/4 - full is controlled by main jet( power jet carbs main controls 3/4 - 7/8 and powerjet 7/8 - full)
choose a nice non - windy day to setup your carb.. average temperature should be below -10 c
tools needed are a socket the size of your jets, 11/16 wrench for bottom plug, flatscrewdriver, a phillips, and a magnet(sometimes things just go ping...)a sparkplug socket and ratchet.
parts needed are extra jets pilot and mains, throttle slides,needle clips, sparkplugs,
find a nice flat 1 mile run space.
the throttle cable(s) should be set already.. ie with stop screw out the throttle should go all the way down and all the way up(check this by removing carb from engine and watching it on engine side to see if the valve goes up just flush with the top of carb bore. many aftermarket cables are short so you have to play with cabling sometimes. set idle stop to about 2 -3 turns in after it just hits the throttle valve. this is done by feel.
try to set your wide open throttle first.. make sure sled is warmed up. (if it has choke back out cable so its REALLY slack)
go for run hold open throttle and then kill engine but KEEP THROTTLE OPEN!
pull plugs and check color what you want to get is a nice brown color..usually when it runs great go one size bigger for main. Power jets same idea.
once main is done set your pilot jet..this is done at idle. if you have to get good sounding engine by turning the air bleed in increase pilot jet size and if more than 2 1/2 turns out smaller size.
once that is done set your throttle valve.. it should zoom up nicely from dead stop if it bogss go one size leaner throttle valve (ie 3.0 then go 3.5) if it pops go one size richer.
the main needle is done at half throttle go for spin and see how it operates if it seems to slow down when you hold throttle at half then your loading up change needle. try to find a needle that holds steady and produces a good color plug at 1/3 1/2 and 2/3 throttle. clip should be in middle notch if you have to go up to get this setting your needle jet is to rich change needle jet if you have to go down on needle then your too lean. remember check your plugs regularly for nice brown color.
this sets the carb itself the choke cable should be set at stopped engine.. and (like on fire series deeres) choke cable in normal operating position.. set this so that there is about 1/16 -1/8 play on lever.
if you have a primer system on sled make sure primer pump is good condition and not leaking fuel down the line. it should not leak extra fuel into system while running.
This is not a simple 1 hr job but may take all day or days so be patient when doing this. Pto side of engine is the "work" side and will require bigger jets than the mag side when doing dual or triple carbs. if you have to go larger than 2 jets sizes, between carbs(dual carb) ,difference then there may be another problem with engine.
good luck!
