Jetting Questions!

eazyduzit509 · Nov 20, 2009

Not sure what alt each jet is intended for (ex- 2000ft-4000ft), ive NEVER worked with jetting before but want to know what worked for you guys. I usually ride around 2500ft-3000ft, and mountain ride starting at 2500ft up to 8000ft. Im sure theres not a Jet for that wide of alt range, so mabey a Jet for home riding (2-3000ft) and one for mountain riding (5-8000ft?)??? Is it hard to swap Jets? Pretty much anything to do with Jetting im a newbe at. So, any info you have to offer is greatly appreciated. How to, difficulty, time length, what to use, etc... Im in college for the Automotive program and know a thing or two about cars and general engines, but when it comes to snowmobile Jetting and Clutching im still learning. Once again... THANKS!!!

powderdogx · Nov 20, 2009

do you still have the altitude compensater on your machine? its the cylinder shaped thing on the airbox with tubes running out to the carbs. if so you are good to 10k ft and don't need to change jetting. I ride the same machine and it works very well. if not then each main jet is good for 2-3k ft per size. in between you adjust your needle clip setting. Keep in mind at a certain point of changing the mains you will need to change the pilots. If you ride alot higher alt you may also need to change slide, the cutout on those is also a jet setting of sorts. It can be confusing, if you don't have the altitude compensater on it i recommend putting one on and jetting to the specs, then you never have to worry about it.

Skidoohmx · Nov 20, 2009

Your sled came with 55 pilots, I would go down to 50 pilots, this will help with easier starting. Your sled also came with 340 and 350 main jets staggered but I can't remeber which way. I would go down to 330 and 340 and it will make it more crisp. If your dpm is working correctly it should work good with this jetting at any elevation until you go way up over 8000 feet. I don't know how to check the dpm like you were trying to find out in your other post. You can make these changes pretty easy. Here is a link to a blowup of your carbs and the part numbers. http://www.shspowersports.com/fiche...=Snowmobiles&make=SKI-DOO&year=1999&fveh=2526

eazyduzit509 · Nov 20, 2009

Yes, i have DPM. I was reading up on it and this is what it said.

Compensation Mode
Two conditions must be met for the compensation mode to operate:
1. Engine must rev over 3000 RPM.
2. Air temperature must exceed - 20°C (- 4°F)
and/or atmospheric pressure must be lower
than 1000 mbar.
The compensation system brings both carburetor
bowls under vacuum (lower than atmospheric
pressure) in order to make the air/fuel mixture
leaner. The required vacuum is produced within
the needle jet air inlet.

Ive only ridin at -4 or colder like twice. I dont know about the atmospheric pressure deal though. I found a few ways to check and make sure the DPM is working, start the sled with the DPM sensor unplugged (the temp sensor on the left side of the head when sitting on the seat), its sappose to rattle/jiggle and it means its good, also removing the solenoids and putting a 12V battery on it should open it and stay open its working properly. I doubted it was working because every time i ride at higher elv's it dosnt have the pull it does down lower. I dont know what jets were in it when we bought it 2 years ago, or how to check...

Zachmandu · Nov 20, 2009

Skidoohmx said:
Your sled came with 55 pilots, I would go down to 50 pilots, this will help with easier starting. Your sled also came with 340 and 350 main jets staggered but I can't remeber which way. I would go down to 330 and 340 and it will make it more crisp. If your dpm is working correctly it should work good with this jetting at any elevation until you go way up over 8000 feet. I don't know how to check the dpm like you were trying to find out in your other post. You can make these changes pretty easy. Here is a link to a blowup of your carbs and the part numbers. http://www.shspowersports.com/fiche...=Snowmobiles&make=SKI-DOO&year=1999&fveh=2526

I agree. The factory jetting is for the idiot that races a cold sled across a lake wide-open when it is 5 degrees out. A little rich to cover thier butt. Leaning it out will really crispen the sled up, and make it snappier. Lean out the pilots some, and drop the mains one jet size. Do NOT get the jetting swapped between carbs. Meaning - make sure that you put the correct jets in the correct carbs. If you put the lean jet in the cylinder that requires the rich jet, then you will be in trouble. If the sled is unmolested, you will find colored dots on - the carb body, the chamber cap where the throttle cable attaches, and on the RV cover itself above the carb boots. IF the dots are still there, you line up the correct color dots. One side is all red, the other all blue. Keep the blue with the blue, the red with the red, and all will be well. At least that way you won't accidently swap carbs from side to side. Don't laugh, I've done it! (I no longer have any colored dots!) That way, you only have to remember which side the lean jet was in.

eazyduzit509 · Nov 20, 2009

Thanks guys, now for the obnoxious question... Which jet is the Pilot and which is the Main? Do i just buy 55 pilots, 330 & 340 Mains? Looks like i pull the carbs, remove the float bowl covers, remove floats and unscrew the jets? And vise versa... SOUNDS farely easy and straight forward. I appreciate the input and patience. Im gonna go look at exploded view for a few...

wildcard28 · Nov 20, 2009

The main jet set in the center of the inside of the carb, if you were to take the Brass plug off the bottom of the Float Bowl you`ll see the Main jet, You get to the Pilot jet by taking the bowl off and it sets up in the carb , you need a small flat screwdriver to remove it.

Hope this Helps you out. http://www.shspowersports.com/fiche...=Snowmobiles&make=SKI-DOO&year=1998&fveh=2492

#2 Pilot Jet
#4 Main Jet.

Good Luck, Wildcard28

eazyduzit509 · Nov 21, 2009

wildcard28 said:
The main jet set in the center of the inside of the carb, if you were to take the Brass plug off the bottom of the Float Bowl you`ll see the Main jet, You get to the Pilot jet by taking the bowl off and it sets up in the carb , you need a small flat screwdriver to remove it.

Hope this Helps you out. http://www.shspowersports.com/fiche...=Snowmobiles&make=SKI-DOO&year=1998&fveh=2492

#2 Pilot Jet
#4 Main Jet.

Good Luck, Wildcard28

Hey thanks, now, which jets are in which side? Which side is the PTO and which is the MAG? One of the two has the 350 main and the other the 340, so i assume i will swap them each down one size. Is a jet a jet? as in universal, or do i have to get one for my 670x? Never bought jets before so i guess i'll just go in and ask for 2 50's (Main), a 340 and a 330 (Pilot)...?

wildcard28 · Nov 21, 2009

Garret, I would tell them what sled you have an the mm of your carbs, which I beleive should be 44mm Round Slide Mikuni.

Most dealers stock a numerous supply of jets and pilots, I actually bought mine from the Local Cat and Polaris Dealer. Of course they looked up the Doo Carbs for me, they were in there Mikuni Catalogs

PTO is the Clutch side (power take off) and the Mag is the Stator side (Magneto).

Wildcard28

eazyduzit509 · Nov 21, 2009

wildcard28 said:
Garret, I would tell them what sled you have an the mm of your carbs, which I beleive should be 44mm Round Slide Mikuni.

Most dealers stock a numerous supply of jets and pilots, I actually bought mine from the Local Cat and Polaris Dealer. Of course they looked up the Doo Carbs for me, they were in there Mikuni Catalogs

PTO is the Clutch side (power take off) and the Mag is the Stator side (Magneto).

Wildcard28

Thanks again Wildcard! How do i know which side has the 350 or 340? How much they run for all of em? $10???

Zachmandu · Nov 21, 2009

They are just normal Mikuni hex main jets. A Mikuni VM-series carb is pretty generic from brand to brand. They all run the same jets. Go to any dealer, and you should be fine.

You do not need to buy a 340, you already have one. You only need to buy a 330 main. Replace the 350 with the 340 you already have, and then put a new 330 where the 340 was. Simple, no??

MAKE SURE, that you use a screwdriver that has a solid purchase on the pilot jet. If you use a screwdriver that is sloppy, and not a tight fit ALL THE WAY ACROSS the pilot jet's screwdriver slot, and the jet is tight, you risk stripping the slot out of the jet. If you do that, the carb body is just about toast. Unless you get lucky and can actually get the ruined pilot jet out, but that doesn't always happen.

And BTW...... NO question is obnoxious if you don't know the answer. How else are you going to learn, unless you ask??

wildcard28 · Nov 21, 2009

Zachmandu said:
They are just normal Mikuni hex main jets. A Mikuni VM-series carb is pretty generic from brand to brand. They all run the same jets. Go to any dealer, and you should be fine.

You do not need to buy a 340, you already have one. You only need to buy a 330 main. Replace the 350 with the 340 you already have, and then put a new 330 where the 340 was. Simple, no??

MAKE SURE, that you use a screwdriver that has a solid purchase on the pilot jet. If you use a screwdriver that is sloppy, and not a tight fit ALL THE WAY ACROSS the pilot jet's screwdriver slot, and the jet is tight, you risk stripping the slot out of the jet. If you do that, the carb body is just about toast. Unless you get lucky and can actually get the ruined pilot jet out, but that doesn't always happen.

And BTW...... NO question is obnoxious if you don't know the answer. How else are you going to learn, unless you ask??

X2 on your last comment Zachmandu, As my Dad was Teachin me Trasmissions he always told me "You`ll never learn nothin unless you Ask" ..

Wildcard28

780mod · Nov 21, 2009

another thing to remember when you ride up higher elavation to turn your clutch up so your sled will rev out properly just something else for a newbie to know:beer;

eazyduzit509 · Nov 21, 2009

780mod said:
another thing to remember when you ride up higher elavation to turn your clutch up so your sled will rev out properly just something else for a newbie to know:beer;

I will try that this year! haha, i never knew we had adjustable clutches until about a month ago.

780mod · Nov 21, 2009

ya its tells you on your clutch cover what to set it at for the altitude

eazyduzit509 · Nov 23, 2009

Im sure the can i have won't make anything much better, just a bit louder. Ive heard mixed storys about guttin gthe airbox (which i did), will that effect the 330 and 340 jets? On snow&mud.com alot of people were using a WIDE range of jets, like up past 400 and down to 300 on x's... Im not 2nd guessing, im just wonderin what they had done i guess (mabey twins?).

Zachmandu · Nov 23, 2009

I've got an old SnoTech magazine where they did a hop-up on a MXz 670 w/ twin pipes. They found that by gutting the box, or running pods, the BSFC on the dyno went WAY up, with little gain in power. They had to jet WAY up, to cover the change in carb signal from the gutted box. The only gains they found were AFTER twin-pipes, port-work, larger carbs, more radical RV timing, and more compression. What this means is, if you gutted your airbox, you WILL have to jet up. How much? You will have to do some testing to find out for sure. So what have you REALLY gained by gutting the airbox on an otherwise stock sled?? More induction noise, and worse fuel economy.... Save yourself alot of trouble and put a stock 'box back on, and re-jet accordingly.

ALOT of sleds see gains with the gutting of the airbox, however, the 670 is not one of them....

eazyduzit509 · Nov 24, 2009

Zachmandu said:
I've got an old SnoTech magazine where they did a hop-up on a MXz 670 w/ twin pipes. They found that by gutting the box, or running pods, the BSFC on the dyno went WAY up, with little gain in power. They had to jet WAY up, to cover the change in carb signal from the gutted box. The only gains they found were AFTER twin-pipes, port-work, larger carbs, more radical RV timing, and more compression. What this means is, if you gutted your airbox, you WILL have to jet up. How much? You will have to do some testing to find out for sure. So what have you REALLY gained by gutting the airbox on an otherwise stock sled?? More induction noise, and worse fuel economy.... Save yourself alot of trouble and put a stock 'box back on, and re-jet accordingly.

ALOT of sleds see gains with the gutting of the airbox, however, the 670 is not one of them....

Ive been told... Its in 1000 peices now. i dont have the center piece with the holes in it... Hmm, what to do...

weetee · Nov 24, 2009

I have a stock air box off a 98 summit x 670. PM me.

Zachmandu · Nov 24, 2009

I also have a couple airboxes laying around.

Not trying to make any money with my above statements.... You pay for shipping and one of them is yours......

Jetting Questions!

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