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Ok so i know there aren't many snowmobile guys on here but its more so an motor based problem, and their are lots of motor experts on this.

So i have a 1998 polaris 488 indy super sport. Its a 2 cylinder 2 stroke motor(air cooled), no power valve system and about as basic as it gets. I got the sled not-running for free and the compression was too low to start, i wanted to keep it cheap so i put new rings in it and now the compression is back to where it should be. After the rebuild it fired right up and the first time i took it through the yard it barely had enough power to make it around. I ended up thinning out the gas mixture with the needle and clip in both carburetors, and it decided to run very well. I took it up and down the yard a few times and it ran perfectly. I had it running perfect for about 45 minutes and then i parked it. i came back a week later, i was about 10 minutes into my ride when it started bogging and running like crap again. Its hard to describe how it runs but i will attach a video to demonstrate. It is very boggy at low rpm, very little throttle response. But if you let the rpms rise and get to powerband, it sounds like it should and has good throttle response. keep in mind this is in a neutral position. it will not idle without me applying little pressure to the throttle and even then it runs boggy. When you try and drive the sled here is what happens, first it bogs down until you hit medium rpm range, then it hit power band and speeds up for about 5 seconds, then if you keep holding the throttle it bogs down until it will eventually kill. It reaches close to max rpm and powerband is decent. If you don't get it started right away it eventually floods itself, it will also not run or start with the choke on.

I've ridden many of these sleds with the identical motor so i have an idea of how they are supposed to run. I'm just looking for a head start of what might be wrong so i don't throw unnecessary parts on it.

here are the things i have done with it since then: Cleaned the carbs, adjusted fuel/air screw, cleaned fuel pump, new rings/seals/oil lines and filters/ spark plugs are brand new and gapped

Here is what i suspect is wrong:  To much gas is getting to it, reason being is i can see gas actually shoot backwards out of the carbs sometimes, wont start with choke

Let me know if you need more info or you have an idea, Thank you

I can’t put the video on this apparently but if you want it I can dm it*

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Posted (edited)
7 hours ago, DevinBianc88 said:

reason being is i can see gas actually shoot backwards out of the carbs sometimes

Reading this, worn/damaged reed valves immediately came to mind but looking up the parts fiche

as many 2T snowmobiles engines of that era this one seem to be of simpler 'piston port' design.

If so, you could suspect a bad center crank seal, each case 'pressurizing' into the other and forcing the intake charge out until rpms pick up.

 

Edited by mlatour

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Next time it happens stop and pull the plugs , see if one is dry or wet fouled with gas.  You can also pull 1 plug cap off at a time while it's running. It will keep running on 1 cylinder, if it doesn't you know which side has the problem. 

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