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Sled is running like crap.

Tuning

 
 
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  #1  
Old 12-01-2010
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Default Sled is running like crap.

So my sleds have been started up the last few days. RMK runs great and rips around the yard just fine. XC ran crappy before I jetted for MN, now it runs real crappy. Since the RMK runs great, I don't think it's a fuel problem. Sleds were filled up with the same gas on the same day, were treated with the same fuel treatment, and were stored on the same trailer. The XC is usually pretty tempermental before it warms up, but yesterday I rode it until it was fully warmed up and it still ran bad.

At idle, the sled runs rough. It sounds like a race car with a bumpy cam. Also, it backfires a lot. Just small backfires, but they are sort of continuous. Flames shoot out the exhaust, which is more cool than LED's, but I'd rather have a sled that runs smooth than looks cool. I adjusted the fuel screws in and out half a turn from where they were, and neither adjustment made any difference. The sled flat out RIPS when it is above quarter throttle! I put some radical new clutch weights and springs in and they make this sled a beast. I think traction will be a real problem with this setup.

So I know people will say clean the carbs and put fresh gas in. I already had the carbs out, and I'm sick of messing with them. The other sled runs fine, so I think it's something else. How does timing get messed up? I think it seems like a timing issue. Motor seems to fire at the wrong time which dumps unburnt gas into the pipe. Maybe I'll make a video of it. I watched a video of a sled on SF, and I could tell by sound that it was a lean bog. Maybe if I made video , someone would recognize what is wrong?
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  #2  
Old 12-01-2010
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Your description does point to a pilot jet problem, although, it could also be sticking chokes.
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  #3  
Old 12-01-2010
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When I rebuilt a chainsaw once the flywheel was loose which messed up the timing and shot flames out of the exhaust. Also some of the other symptoms sound the same, not sure how that applies to your snowmobile but it is something to think about.
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  #4  
Old 12-01-2010
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if you removed the pilots and made sure they were clean, and made sure the chokes were moving fine once is usually enough.

did you lift the needles back up for low level riding?
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  #5  
Old 12-01-2010
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Quote:
Originally Posted by OCR View Post
Your description does point to a pilot jet problem
This is the first thing I thought. You said you had the carbs apart, when? If it were in spring you may want to tackle that project again.
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  #6  
Old 12-01-2010
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I just rejetted it for Minnesota elevation yesterday. I put the right main jets in, and raised the needle one notch. I didn't disassemble anything else, because the carbs were clean and the gas smelled fine.
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  #7  
Old 12-01-2010
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You need to check the pilot jets, not the main jets. Sounds like one or both are plugged.
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  #8  
Old 12-01-2010
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I had to change the main jets. I didn't check anything.
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  #9  
Old 12-01-2010
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So check the pilots and clean them now.
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  #10  
Old 12-01-2010
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Plugs.
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  #11  
Old 12-01-2010
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How about exhaust valves. Are they clean, and operating. I saying this not knowing if the sled even has them.
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  #12  
Old 12-01-2010
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i would remove and clean the pilots.
while you have the carbs off you should pull the reeds just to check them out while you are there.
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  #13  
Old 12-01-2010
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I like IBS's idea the best. It doesn't involve pulling carbs off. I'm in no mood to be working on sleds.
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  #14  
Old 12-01-2010
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I've had plugs make my machines do all kinds of crazy shit, that's why I constantly replace them.
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  #15  
Old 12-03-2010
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So I cleaned the carbs. Not a fullblown awesome cleaning, but I got the pilot jets plenty clean. The carbs were pretty spotless, so I didn't go all-out on them. The chokes are working fine. I don't think carbs are the problem.

The reeds don't look so great. Out of the eight petals on the top, three of them don't close too well and have gaps. The reeds on the bottoms looked fine.

You can see in the video that it struggles at idle and coming off idle. Once it revs up, it snorts really nice. It isn't really too rideable, because when you're going down the road and you let up on the throttle it backfires LOUD! Neighbors seemed pretty stoked about that actually.

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  #16  
Old 12-03-2010
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Uneven gap of the reeds would contribute to the issue, just might be enough to solve the problem.
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  #17  
Old 12-03-2010
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its an exhaust backfire. like its trying to go into perc reverse?
time to pull flywheel and inspect it for loose magnents? do a possible stator test? try a cdi swap?

if its any issue why does it run fine at higher rpms?

had a sled here last year, they were riding and put gas in, they didnt get very far from the station and the sled quit. other sleds that filled up there at the same time didnt have any troubles. we swaped stators, flywheels, double checked all gaskets, and crank seals, finally got to the point of drained the gas, syphoned the tank dry. put new gas in it and the sled ran like a top.

it would run but, start normal but to keep it idleing or low speed you had to play with the primer, at wide open it ran crisp and normal but slowly lost power and would die.
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If the boner police are here I need a lawyer.

in a time of deciet telling the truth is a revolutionary act. george orwell.

aint no body ripin like me. M to the a-d-c-o-w. rock it hard for my fly ladies. I rock it yes in deed. homies still roll with me. money dont change me


nothing goes like 3 holes.

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  #18  
Old 12-03-2010
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So you're saying Madcow, that reeds wouldn't cause this?
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  #19  
Old 12-03-2010
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reeds are supposed to be closed tight but it might not be your only issue, ive never seen reeds make something pop when bad usually just flood it.
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  #20  
Old 12-03-2010
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I'm just thinking out loud here, but if the reeds don't close would it cause a rich condition? The carbs were wet with gas inside the airbox, so something there isn't really right. If some of the reeds are open when they're supposed to be shut, wouldn't air/fuel mixture be pushed back into the carbs? Then, instead of getting fresh air that the carbs should be adding fuel to, fuel mixture comes in instead of clear air. The carb doesn't know better, so it adds more fuel because, well, that's what carbs do. This super rich mixture dumps unburnt fuel into the pipe causing backfires and flames. At higher RPM, the reeds open a lot further, so the engine isn't as effected by this scenario. The higher velocity and pressure of air in the airbox holds the fuel mixture back so it doesn't go back into the carbs like at idle.

Right now, I like OCR's answer the best. (Once again because it is the least work.) I liked IBS's sparkplug answer too for the same reason, but that didn't get me far. If I go with Madcow's answer, basically the sled goes to the dealer. I don't have a flywheel puller, and I have no test equipment to test a stator. I don't have CDI's laying around to try out either. Who knows, my crank could be out of phase too, and I have no way of testing that. Since it's still an RPM specific problem, I'm thinking it might be reeds. One thing I haven't checked yet is the valves like Ratstang said, but I had torn bellows last year and it didn't act at all like this.
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  #21  
Old 12-03-2010
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i think your on the right track there, get some reeds and give it a shot, i mena itneeds them either way right if some of them are loose!
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  #22  
Old 12-03-2010
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Can you flip the reeds? I've done it and it sometimes works lol. If the reeds aren't closed then yes replace them, that could quite possibly be the problem.
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Nobody on this site likes Ski Doo except Dirty Harry, and he's better at making babies than buying or fixing sleds, so you don't want to listen to him.
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  #23  
Old 12-03-2010
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You've got fuel goin back into the airbox?is the sled hard to start after it sits for a while as if its flooded? If so its probably a bad needle. Easy test is to shut the fuel off over night and try it again the next day.
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  #24  
Old 12-03-2010
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Reeds slightly open are really not a big deal. As soon as there is the slightest amount of positive pressure in the bottom of the crankcase, they seal shut. More than a half a millimeter is a problem. Then if the reeds are frayed or bent, that's another issue that would make a mess.

Like Madcow said it's an exhaust backfire. The pipe is loading up and BAM. I'm thinking leaking needle and seat, but possibly ignition misfiring. Check connections, plug wires, trim 1/4 inch off the boot and reinstall both ends, etc etc. The other possibility is a loose or leaking carb boot. My old TX had a loose boot (where it bolted to the engine) that would do exactly what yours is. As the RPM increased it eventually had enough suction to suck the carb boot against the engine and it screamed.

Rubi your reasoning for the rich condition seems right until you think of the positive pressure created during the downstroke - which closes the reeds and pushed the charge up the ports and into the combustion chamber. If the reeds are open the charge goes out the carbs and nothing makes it to the combustion chamber for combustion.
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  #25  
Old 12-03-2010
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Are the reeds worn to the point that they are full of tiny holes?

My rotoracer did this mid-summer.
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