jetting....why needles are overlooked


the thing is, all of us on TY read great posts like this one from mr. viper and we all understand/try to understand how our carbs work. most people i know have never read a letter on anything and couldn't even basicly explain how a carb works. most people don't even understand the concept of carb circuits. to them a fatter main jet means a richer mixture. to them main jet means "main jet" ie it is the main facter in jetting. you can try to explain things to these people but they just argue continue their ignorance. i tryed to explain how a bigger pilot could help with the high hanging idle on his sled and he went off into an explanation that really was just a bunch of nonsense. i can't believe the aversion to education that a lot of snowmobilers have.
 
X100 thanks again for all the important and informative posts, always love to here great info and thats what makes this site and members so important!!! THANKS DON!!!
 
mrviper700 said:
Numerous threads for why my sled wont run when its warm, burned my sled down,look at these pistons, etc. maybe a common sense approach explanation can take some guess workout of jetting your sled,and why you need to change those settings due to certain conditions. The thing is you see all these people who say "put a bigger mainjet in it", its too lean from not enough wash on the piston, but whats not explained is that unless the person burned the sled down in a certain distance at a certain throttle opening you cant simply have a answer of a bigger main jet because thats false. You need to run a sled wide open for a distance of 750-1000ft to be soley on the mainjet,because all carb circuits overlap.To put this in perspective a 1/4 mile is 1320 feet long!
The reason for most of these burndowns and problems arise from the midrange jetting being too lean,and too low of octane fuel. By richeng up the needle your working the area of the fuel curve where your spending 80-90% of your time if your trail riding, not on the mainjet!
lets give an example of typical trail riding: your riding along a groomed trail averaging 50-60mph, you hit a large straight away, you whack the throttle,the sled speedo hits 80-90 mph,straight is a good 700ft long, here comes a turn,you let off the gas,dive deep into the turn and give it the gas again coming out and getting back up to 80-90mph. Then you come to a field opening after a small turn, on and off the gas,you rip it across the field to the other side to enter the woods again,for a easy 600-800ft again,back into the woods,turn by turn your on and off the gas.
The thing is,in this example scenario,you were never on the mainjet, you were on a combo of the pilot jet and the needle,needle circuit and sometimes getting into the needle/mainjet overlap if the straight distance was 750+ feet long for a short blip.
The needle overlaps with the pilot circuit(lowspeed) and the mainjet(topspeed) circuits. The needle controls how much of the mainjet will be richning up the topend circuit,because of your needle is still down in the nozzle,no mainjet increase is going to save the engine because its being restricted by the needle not coming far enough up out of the nozzle to flow the extra fuel desired. If your lake racing,across vast distances, again, the needle is in play ,because the very instant you let off the gas from a wide open run, your shutting off the main jet, and a lean needle will instantly burndown a engine on decelaration, this is actually what happens alot of times. People will again,put bigger mainjets in and they are working on the wrong part of the fuel curve.
Every sled made yamaha,polaris,arcticcat,evinrude(lol), whatever comes richer then needed from the factory,even a 2000 srx using 146.3 mains. The service bulletin was made for riders running very cold climates to go up to the richer yet 148.8/150 mains if the sled was going to be run for extended wide open periods. The reason they went back to the 00 jetting in 02 was simply the add on of dcs was to take the risk away and cover the wide open running situations. However the leaner jetting also provides more performance.
Simply upping the octane makes the sled richer without ever touching jetting!! The more octane you have the cooler the burn,so keep this in mind as you buy fuel,going cheap on gas fill ups provides less protection with a given set of jets,just simply from the octane level. octane level seems to have come into play alot more in the last couple seasons, 2 stroke sleds are most affected by the lower quality fuels. You need to richen the sled but richen the sled thru the circuit where your spending alot of the time riding the sled, mainjets are not a coverall like some people here like to say, they surely wont protect your piped viper running at 7500-8000 rpm doing 60mph down a trail, its the needle circuit you need to pay attention to. Also, when dealing with a viper,they have leaner needles and nozzles then a srx does,so take that into account when setting up your sled.
Click to expand...

I'm sorry if i bring up an old post, but there is something here that i dont understand!?

The main jet is supplying the needle or not ? If thats true i guess a smaller main jet (like you want people to use) should give a leaner supply all over in the mid-range also?
 
While You Are Thinking Right That A Smaller Main Would Lean Out He Mid Range You Are Right It Will Play A Roll Bit Not That Much The Needle And Needle Jet Have To Work Together .............

The Needle Jet Also May Be Too Lean And Cause Everythign To Be Lean Mid Range To Wot .................

Most Don't Even Think About The Whole Tuning Of A Carb ......take A Vm Round Slide For Example We Have Spent Countless Hours On Our Drag Sleds To Make Things Perfect ........but Even In 500 Feet The Main Jet Is Just As Important As The Pilot, AIR SCREW, Tube And Needle Even The Slide C/a Plays A Role And So Much As A Half Shim On A Needle Can Mean Winning Or Losing A Race .....................i Could Write A Novel Explaining EverytHING But Olav's Carb Book Or The Sudco Tunning Manual Will Get You Farther But It Comes Down To Having Good Ear And Making The Right Adjustments To Get Peak Performance And Throttle Response
 
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Thanks for the bump. Also think I need to get a copy of both carb and clutch books by Aaen.
 
jetting....

I would like to get bigger needles so that as temp and altitude increase, I can swap in larger needles along the trail instead of changing jets. Does anyone know where I can find a chart for Mikuni TM carb needles? Mine is 2000 MM 600 triple with SLP pipes and TM31 carbs. Thanks in advance!
 
Yes and thank you. It does not show TM31 so I will have to pull mine and hopefully be able to read what size is in there.

Somewhere I read that it is recommended to put 1 size larger jet on PTO side of a triple, but the instructions from SLP makes no mention of this. How important is this? To be safe raising the needle a few slots would do virtually the same, no?

BTW I'm not a racer. Just a snow addict that wants good performance without mixture problems ruining the fun!
 
Slp jetting is what I would follow. Do the instructions say anything about ethanol? If you use ethanol free gas you give yourself a little more margin for error. I also use half a bottle nos race octane booster. Rode in -10f and had the dcs light flash on hard acceleration once.
 
I just read your post on Jetting - although the post is from 2010, it's still very informative...thank you for taking your time in writing.

I am a new (to me) sled owner with a 2003 Yamaha VMAX 600ER and an '89 Yamaha Phazer. I will be doing my first longer ride with the VMAX this weekend on state trails in northern Michigan, as well a little lake activity. The temperature will be down to 6 degrees F according to the weather forecast, and I am wondering if I need to adjust the jetting at the carbs. I have never done that, and am hoping I don't have to do any adjusting, but I don't want to put my sled at risk. I don't know if those temperatures are cold enough to worry about adjusting the jets from factory settings (middle clip position?). I certainly will be running premium!

I am wondering what the suggestions are - thank you!
 
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