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Old 01-06-2013, 12:13 PM   #1
Shawn420
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Default need help fine tuning rmk 800

i was wondering what i can check to get my 800 runnning as good as it should be, rebuilt it this year (new shortblock) and it's running pretty good, but i dont think its quite as good as it could be.

one ride it kinda sounded like it was sputtering a tiny bit at wot on the trail, almost like it was missing just a little bit, hits about 7500-7600

also noticed on some big pulls the other day in the steep and deep it wasnt quite hitting 8000.

so where should i start looking first?? how do i check if my timing is perfect on this motor, i lined up the stAtor plate how it was on the old engine and the mechanic i talked to said this will work 99% of the time, but how do i make sure 100% its spot on

also when im riding at low elevation (2000- 3000 feet) the sled loads up and backfires at low speed, this happens more when its cold, but also happened at the end of the day coming back down the trail at low speed (let out a big pop right as i rode past a big group of people loading their sleds)

maybe i need to go through the carbs again?

what kind of jetting should i be running? i do most of my riding 5000-8000 ft, gutted airbox, bosen reeds, aftermarket can

sorry if thats alot of info all at once, and thanks in advance for your quick responses
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Old 01-07-2013, 09:58 PM   #2
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A change in altitude from 8000 ft to 1000 ft will have a pretty good impact on the fuel mixture. At 8000, you'll be running awesome. When you get back down to the lower levels, you're way rich because the air is more dense.

You cleaned the carbs. Did you sync them? Did you check the plug caps on the plugs and where they screw onto the wires to be sure the connections are solid?

Have you cleaned / rebuilt your clutches?

How is the chain, chaincase in general, and the chaincase fluid?

Did you replace the belt? How is the deflection and offset?
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Old 01-08-2013, 11:33 AM   #3
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If the setup is spot on at high elevation, you will be running way LEAN at low elevations. Jetting requirements are always larger for low elevations and for colder weather. Jetting for 8000 and running at 5000 wide open across a meadow will melt holes in your pistons. Been there. Have the piston on my desk as a reminder.
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Old 01-08-2013, 11:47 AM   #4
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Yeah, I definitely got that one backwards. Thanks for catching and correcting that, Dan.
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Old 01-08-2013, 03:16 PM   #5
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no problem, sometimes we don't type what we mean... the fingers get crossed or something
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Old 01-08-2013, 03:25 PM   #6
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Yeah... I had half of going the right direction where I was talking about air density, then I somehow twisted the rest around backwards.
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Old 01-09-2013, 12:09 PM   #7
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carbs sync, no, what do i need to do to get that right? just make sure they open evenly or is there something you do with a vacuum gauge

clutches were went through last year
seem to be aligned good, far as i could tell with the alignment tool i have tho my belt isnt riding up an 1/8inch in the secondary, belt is from last year, have a new 1 i could slap on though

what should i look for in the chaincase?? its got fresh oil, im running 80w90 in it, cant remember how much adjustment was left in the chain, how tight should it be?

i'll check out the plug wires closly or just order new ones since they're 12 years old

once i do all this how do i figure out where my jetting is at, and is it worth screwing around with? what do you jet for when you ride in the mountains and unload the sleds at 2000-2500 ft at around 0Celsius , and then ride up, hit the clearcuts on your way up to the alpine, and keep going up right to the 8-9000 ft ridges wheres iits -15, i suposse the fact that the air is getting colder as your going up is in our favor
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Old 01-09-2013, 12:27 PM   #8
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To determine the jetting, you have to pull the carbs and remove the bowls. Take the jets out and read what's stamped on them. You want to jet for the lower elevations. Yes, the temp dropping with elevation increasing works in your favor to keep it from getting "too rich". But, if you jet for the higher elevations / lower temps, you're going to burn the motor down at the lower elevations.

If you have fresh oil in the chaincase, check the adjustment on the tensioner. Put the rear end up in the air, remove the drive belt, loosen the jam nut on the tensioner, and unscrew the tensioner a couple of turns. Screw the tensioner back in by hand as far as you can, spin the track or secondary a little in each direction as you continue to snug the tensioner. Loosen the tensioner 1/4 turn, hold in place, and tighten the jam nut.

Put the new belt on, warm it up with some gentle riding for a few minutes (similar to how you would warm it up when you head off for a ride and the sled is cold), then check the offset and deflection.

The "proper" way to sync the carbs is with a sync tool. However, since the only reason that the vacuum that they develop would really be different is if there was an air leak, you can do it totally without a sync tool. In fact, doing it without the tool actually makes it more likely for you to identify a difference of conditions between the cylinders.

Syncing the carbs:

- You need to adjust the free play in the throttle so that there is about 2-3mm of play. This is necessary for the safety switches to function properly.
- With the kill switch engaged and the key off, zip tie the throttle wide open (or have a buddy hold it).
- Remove the airbox so that you can easily see and access the intake side of the carbs.
- Ensure that the slide is installed correctly with the cable inserted correctly into it.
- Ensure that the slide cap is screwed on all the way.
- Ensure that the top edge of the carb slide is -just- above the bottom edge of the carb intake throat. If it is not, adjust the cable on the slide cap until it is.
- Repeat for each carb.
- Remove the zip tie / release the throttle lever.
- Rest the solid part of the drill bit in the carb opening "the long way" and adjust the idle screw until the slide -just- comes in contact with the bit. This will give you a reasonable starting point for the idle.
- Once all of the carbs have been adjusted, slowly squeeze the throttle and ensure that all of the slides begin to move at exactly the same time and go up exactly the same amount.
- Adjust the air screws according to the manufacturer's setting.

Now, adjust the choke plungers.

- Get a thin gauge piece of solid wire and bend it at a 90 degree angle about 1" from the end
- Remove the airbox so you can easily access the intake side of the carbs
- Apply the choke
- Slip the bent section of wire into the enrichment hole on one carb and release the choke - make sure that you can not easily pull the wire out
- If the wire comes out easily, you need to screw the plunger adjuster into the carb body until you feel it contacting the wire and keeping it from moving
- Screw the adjuster in another half turn from that point and tighten the jam nut
- If the wire does NOT come out easily, pull on the choke lever until you feel tension in the cable - you should only have to move the handle a small amount after you feel tension before you can remove the wire
- If, after lightly applying the choke, you still can't remove the wire, you will need to unscrew the plunger adjuster from carb body
- Unscrew it until, with the choke off, you are able to remove the wire then use the process above to adjust it correctly

- Re-install the airbox

You're now ready to start the sled and tweak the air screw and idle settings until it's where it needs to be.
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Old 01-09-2013, 03:34 PM   #9
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I'm pretty sure he's got rack Mikuni TM40's on that sled, unless someone messed with it a lot, it won't be out of sync. There may be air screws and fuel screws that can be adjusted, those should be equal and adjusted to eliminate any bog or hesitation on take-off.
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Old 01-11-2013, 10:13 PM   #10
Shawn420
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Jets were 310 on clutch side and 330 on mag side. why different sizes?

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