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Discussion Starter · #1 ·
Hey everyone.. New to snowmobiles here.

I just got this skidoo summit x 670. I’ve been on a couple 20 to 30 mile ride, and it seemed to run really well initially but now chokes out and dies quite often especially at lower rpm though sometimes at full speed also. I am going to start by changing out the in-line fuel filter, but when I took the airbox off, I also noticed something interesting on the carburetors. Maybe someone can help with suggestions. It has aftermarket CPI racing pipes, and written on each of the carburetors with a sharpie are the numbers 390 and 400 any idea what that might mean? Also, any suggestions on getting to the in-tank Fuel filter? I really couldn’t tell while I was driving if it was flooding or not getting enough gas.

cheers!
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The larger jets are necessary for the pipes. The DPM system should be compensating for atmospheric conditions. The DPM manifold applies vacuum to the carbs to make them act like they have leaner jets than they do when conditions warrant it (warm temps, high altitude). The baseline carb jets are usually set for -20F and sea level. That means if the DPM manifold fails, it will revert to those rich settings. The needles are also set richer than needs be for the same reason. It's been over a decade since I had a Summit X, so this is what little I can remember. My DPM manifold failed and I just set up my carbs to be jetted manually. Best I can recall I went down several sizes.. like to 340's (don't quote me on that). The DPM manifold is that silver double tube above your carbs. If you find a shop manual, it will outline how to test the manifold.

If you haven't yet ABSOLUTELY put fresh fuel in it, especially if you live at low altitude. They run a high compression head and will detonate with bad gas and low octane. Make sure your bogs and stall outs are not lean condition created by bad carb boots or crank seals.
 

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Here is a spec book to get your library started.

Looks like stock jetting for the X was 350/340. Being new to sleds that means a 350 main jet in the left/pto/clutch side carb and 340 main jet in the right/mag/muffler side carb.

So, they are running pretty fat/rich for the pipes if the 390/400 is accurate and not sure why they have the mag side richer than the pto side.

I might start by getting smart on carbs and pull them apart and set them up properly. Here is the shop manual for reading up on carbs.

Also, pull the plugs and take a picture of them (the business end) and post it here. This may tell us many things.

Keep in mind, severe engine damage can occur running your sled with bad gas or not enough gas so sorting out a potential lean condition is important.

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Discussion Starter · #7 ·
thank you! The manual is helpful! Just bought a pdf last night unfortunately haha.
I’m tempted to buy a rebuild kit and return the carbs to 340/350, but as per the comment above would that be a problem given the pipes are aftermarket? I might have a guy that will trade stock pipes for my aftermarket ones… but would it work without trading?

I have new high octane ethanol free gas in there, I’ll take a pic of the plugs today and post.
Also, would lowering the carbs back down to stock level require I change anything else other than the pipes (ie dpm etc) …

really appreciate you guys I’m always impressed when forums respond with thoughtful replies …🤝

Here is a spec book to get your library started.

Looks like stock jetting for the X was 350/340. Being new to sleds that means a 350 main jet in the left/pto/clutch side carb and 340 main jet in the right/mag/muffler side carb.

So, they are running pretty fat/rich for the pipes if the 390/400 is accurate and not sure why they have the mag side richer than the pto side.

I might start by getting smart on carbs and pull them apart and set them up properly. Here is the shop manual for reading up on carbs.

Also, pull the plugs and take a picture of them (the business end) and post it here. This may tell us many things.

Keep in mind, severe engine damage can occur running your sled with bad gas or not enough gas so sorting out a potential lean condition is important.

Welcome to dootalk.
 

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Let's see your plugs before making any big changes. Too rich can be problematic but it's safe. Too lean is dangerous.

At this point, because the sled is new to you and you really don't know what's inside I would tear the carbs down and verify what's inside and make sure all is clean. Before doing this however I would read the carb section of the manual 3 or 4 times to really get comfortable with what's required to properly set them up upon installation.

Youtube has many good and not so good videos on cleaning and adjusting Mikuni carbs so watching a bunch of them can be helpful.

You'll want to note the size of the main and pilot jets, needle clip position and airscrew setting.

There may have been a change at some point that I'm unaware of but your mag side carb should be jetted 1 size smaller than the pto side. Not the other way around as indicated by the 390/400 markings on the carb caps.

You'll want to get this task right the first time so if the problems persist you can be confident it's not a carb related problem.

any suggestions on getting to the in-tank Fuel filter?
Yeah, this is important as well. Often times the pickup line in the tank comes apart and the sled picks up dirt from the bottom of the tank so your newly cleaned carbs get pumped full of dirt and you're back to square one.

For a quick inspection of the pickup line and filter I use a coat hanger through the fill up port and fish the line up so I can see the filter on the end. The line lays on the bottom of the tank on the right side. It's about 14" long. You can see it with a flash light if the tank is empty or near empty.

On the 97 carbs there is a small screen filter in or near the fuel feed nipple where the fuel line from the pump hooks up to the carb. I removed mine back in the day and threw them away. At a minimum you need to inspect and clean them along with the rest of the carb. #32
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Not sure how old those plugs are but they don't look lean. I might do the carbs as suggested swapping the current mains and adjust and sync properly and see where you stand. New plugs as well.
 

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Discussion Starter · #11 ·
Not sure how old those plugs are but they don't look lean. I might do the carbs as suggested swapping the current mains and adjust and sync properly and see where you stand. New plugs as well.

You think swapping the jets on the carbs even though I have those cpi pipes will be ok? (figured I’d confirm before diving in)
 

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Yes. Your carbs are fattened up because of the pipes. They are just installed backwards. The 400 ( if in fact that's what's in there) should be on the pto side. 390 on the right.

Here's the deal on carbs. Both of yours are the same but jetting is likely different. If you knew your carbs were clean and the jetting was as marked you could simply swap them out. But it's a new to you sled and no one knows what's inside them. So, do yourself a favor and tear them down, all the way and verify everything to include cleanliness. Put them back together correctly, adjust and sync and you know that system is working for you.

Good luck and let us know how it goes.
 
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