Snowmobile jetting specs

Only of you have ran the engine out of oil...Sounds like the carbs may not be set correctly.. .Did a professional do the rebuild?..What exactly was "rebuilt".. were any parts replaced?
Sounds like the low speed (pilot jets/air screw passages may not be clean as they need to be??)
Please check the compression on each cylinder..remove the spark plugs... hold the throttle (carbs) wide open when you check this.. spin the motor for 5-8 seconds... you should get 120psi. minimum..(and less than 10psi difference).. if less, it might be time to actually rebuild the top end..(new pistons/rings/hone cyl.).. only bore it if absolutely necessary.. get it measured..

There are only 3 parts in a two-cycle carb that ever need to be replaced,,(unless they are actually broken) The needle and seat assy., the jet needle and the needle jet. ..see pic... parts # 41 = needle and seat assy. #6 = jet needle, #37 = needle jet.



The needle and seat regulate the flow of gas into the carb as the floats rise and fall..,, if they are damaged or leaking the carb will overflow with gas., the needle jet is a orifice with a specific size..The jet needle has a specific size, taper, and length..== The jet needle and needle jet work in unison to regulate the flow of gas from right off idle all the way to WFO..the needle jet/jet needle WILL wear over time causing the fuel/air mixture to become richer and richer and start fouling spark plugs..(this is more pronounced on 4-cycle ATV's)...
I hope this has shed some light on your carb situation...
Good luck...
exploded view of mikuni carb Google Search

You should really have the workshop manual if you are working on the carb. Not only will this tell you the correct main jet size, you will also get the pilot jet size, mixture screw, and jet needle positions. All of these should be checked if you're taking apart the carb yea?

You need to do research to find out what size you need given your elevation, humidity, and mods (if any) Don't trust anyone who just says to "go one size up" or whatever. Other things to check are the float level height and the jet needle clip position (if no clips, it could be improperly shimmed). Both of these will have a big effect on fueling. Check your FSM for the appropriate specs.

Here is specs for those carbs
- AISAN AS27VW
I.D. No. - 04A10
Bore - 27 mm
Idle r/min - 1300+/-100 r/min.
Fuel level - 9.0+/-0.5 mm
Float height - 20.5+/-1.0 mm
Main jet - #94
Main air jet - 1.2 mm
Jet needle - 5B10-3rd
Needle jet - 2.60 mm
Throtlle valve - 11.0
Pilot jet - #34
By-pass - 0.8,0.8,0.8 mm
Pilot outlet - 10.8 mm
Valve seat - 2.5 mm
Starter jet - GS1 #60, GS2 #54
Pilot screw - PRE-SET (2 turn out)
Pilot air jet - 1.35 mm
Throttle cable play - 0.5 - 1.0 mm
Choke cable play - 0.5 - 1.0 mm

Use a DYNAJET kit & go with the directions. Or check with the Hypercharger manufacture for correct jetting. But you will still need different needles & main jets for those pipes.

Front cylinder carburetor:

·
112.5 Main Jet

·
17.5 Pilot Jet

·
(1) 0.020" thick shim washer under the needle

·
PMS setting apparently varies from bike to bike... but it should be 2.5 turns

Rear cylinder carburetor:

·
110 Main Jet

·
17.5 Pilot Jet

·
(1) 0.020" thick shim washer under the needle

·
PMS setting apparently varies from bike to bike... but it should be 2.5 turns

You have a mikuni carb. there are curcuits to the carb. the first that works from idle to 1/4 throttle is the pilot cicuit, from there you transition to the needle jet/and jet needle. That controls the 1/4 to 3/4 throttle and then the main' jet which controls WOT. you may have a air screw that would be before the throttle plate if it is after it is a fuel screw find the factory specs for the jet sizes and make sure all is correct. if not you isolate where the problem is and correct in a rich or lean adjustment' I race yamaha 2 stroke road racing bikes, it is not an easy task sorry to complicate you so much

I would jet the lean side up one jet size if the sled is running extremely lean. You can also adjust the needle up if you can without having to increase jet sizes.

check your jetting, Clip postion on needle jet. floats could be sticking. Carbs dirty.

than it was the day before you can actually go back to your sea level jetting a 260 main jet! If you don't rejet you can kiss your assets goodbye when you rebuild the seized engine. Air temperature makes that much difference! Our final example will deal with something often overlooked. We are still up in the hills enjoying our NEW riding area when we notice the old fuel supply getting shorter. No biggie; there's a little store/gas station just down the road. A short trip a few bucks change hands and we are ready to go again. Out on the trail the bikes are running funny sometimes pinging and running HOT. What happened?!?! When we changed jets to compensate for altitude and temperature we were still using SEA LEVEL gasoline. Gasoline sold at higher elevations have a different blend of additives to compensate for the altitude. Generally high elevation gasoline is less dense to compensate for less available air going into the engine and to aid starting. The lighter specific gravity of the high elevation fuel actually leaned out our mixture! A 1 to 2 sizes bigger main jet will get us back into the hunt. If you ride in vastly different areas try to bring enough or your normal fuel along to last the entire ride. It will save you hassles and grey hair in the long run! PILOTS NEEDLES, MAINS: So far we have only talked about main jet changes to compensate for altitude, temperature and fuel density. As most of you know there is a pile of jets in a carb. While main jets are the most critical for ensuring full power operation and engine longevity, the other jets are equally as important for a good running engine. Let's run through them quickly. Pilot jets control the low-speed and idle mixtures. Many times an adjustable jet is used in conjunction with the pilot jet. The adjustable jet allows a precise setting of the idle mixture. If the adjustable jet is located to the rear of the carb and usually on one side it is a AIR adjustment. It controls the amount of air that mixes with the fuel coming from the pilot jet. If the adjustable jet is to the front of the carb, on the side or bottom, it controls the amount of air/fuel mixture going into the engine. In either case if adjusting the mixture screw won't improve the low-end running speed it's time for a different pilot jet. Throttle valves (the slide) control the off idle, to 1-quarter open ,mixture. Some aftermarket carbs have replacement slides available with different cutaways. Changing the cutaway changes the mixture. More cutaway is lean, less cutaway is rich. Some carbs do not have different slides available, so you have to compensate by changing the mixture on the idle circuit or needle circuit. Partial throttle hesitation or rough running can be caused by the slide cutaway. Needle jets control the amount of fuel going by the needle and into the engine at low to mid throttle. There are 2 types of needle jets used in a carb. One is a primary type that has a very precise hole hole drilled through the middle of it, along it's length. The size of the hole relative to the size of the needle determines how much fuel goes into the engine. The other type of needle jet is constructed essentially the same except for a bunch of holes drilled into the side of the jet. These holes allow air to mix with the fuel before it's metered into the engine. Either type of needle jet works well in most cases but there is power to be gained on HIGH PERFORMANCE 4-STROKES by going to the needle with the holes in the side. These are called bleed type needle jets and produce more midrange power in a 4stroke. In any engine going to a leaner (smaller) needle jet is the easiest way to rejet the midrange running when going to higher elevations. Changing the needle jet leans out the mixture evenly at all the midrange throttle settings moving the needle clip doesn't. Jet needles more commonly know as the needle control the fuel mixture throughout the midrange. The shape or taper of the needle dictates how much fuel goes into the engine at a given throttle opening. The needle must work in conjunction with the fuelling requirements of the engine relative to slide position. If you have an engine with a strong hit in the midrange the needle will probable have a noticeable reduction in size the the slide is half open. Remember it takes fuel to make power and when the engine makes power it needs fuel NOW! If it doesn't get the right amount of fuel it pings or misses. You many have cleared up a little midrange pinging by moving the needle up a notch but at the same time you may have overrichened some other areas. If the problem isn't too bad you won't even notice the rich condition. If the machine stutters before it comes on the power that part of the needle's taper is too small and the only way to cure it is to get a needle with a different taper. Finding the right needle can be difficult so hopefully moving the clip will do the job. Finally the good old mainjet comes into play at 3-quarters open to full throttle conditions. Most of you already know a bigger mainjet has a bigger hole so it lets more gas into the engine! Pretty simple!! As simple as it is the mainjet is absolutely CRITICAL to high-speed engine operation. Not only does it meter the gas into the engine, it can aid in cooling the engine as well. A properly sized mainjet will let the engine make good power for a long time. A one size smaller mainjet may make greater power for a while. A slightly rich mixture burns cooler than a lean one so be sure the mainjet is big enough! One final note on jets. All of them and the carburetion functions then perform tend to overlap into some other jet's territory. If you mess with one jet, you may have to mess with a few of the others. My best advice is to not change more than one jet at a time. Slowly work out the correct jetting and keep notes on what you are doing. If you get totally fouled up at least you can go back to where you started. SIGN, SYMPTOMS AND CAUSES: How would you know if there was something wrong with your jetting? If you listen, your engine will tell you! All you need is an interpreter. Since I speak and understand several different engine dialects, I will give you a hand. Let's start with lean conditions because they can cause the most damage. In a lean condition the engine will surge and sometimes ping under acceleration. The engine will also be cold-blooded (hard to start and keep running) but will run better when hot. The spark plug will look bone white or burned in extreme cases. The engine may spit back or sneeze thought the carb once in awhile too. If the engine is running rich the throttle response will be fuzzy and not too quick. The engine will burble, miss and blow black smoke. It will start easy but will run funny when fully warmed up. The plug will be dark, wet or fouled (possible all 3!). Ok so what do you do first to cure the problem? The very first thing is to check and adjust the float level. If it's off one way or another it can throw the jetting off too. Set the float to the specs and retest the running. The next item is to determine a rich or lean condition. Let's say the engine gets hot and doesn't pull well. This is a lean condition so the engine wants more fuel. Stick in at least a two size bigger main jet and try it again. If it's better but still not right go even bigger on the jet. and try it again. Bear in mind that drastic or sudden changes in jetting usually mean an air leak has developed somewhere in the engine. Find it and FIX IT!! When the engine burbles on the top end come down 1 jet size at a time until it winds all the way down. Don't drop and more sizes! If the engine seems sluggish and lumpy or want to load up on the bottom end the mixture is TOO RICH. Adjusting the low speed mixture screw helps a little but doesn't cure the problem completely. What you need now is a new pilot jet. Go 1 size smaller and try the adjustment again. When the engine runs smooth with the adjustment screw about 1 and a half turns out from the seat you have it!! IS THAT ALL THERE IS TO JETTING??: There's a lot more to jetting than just stuffing jets in holes and hoping the problem goes away. If you can understand what your engine is trying to tell you when it runs funny you will have a better chance or correcting the problem than someone who doesn't have a clue. When you rejet, go slowly and carefully until the problem is solved. As a final thought let me remind you that jetting is a lot like life, if you have a choice it's always better to be a little rich!!,,,