Rebuilt Top End...Not Running Properly

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XxXskidooXxX

New member
This is my second thread on this issue. I rebuilt the top end of my 1998 Ski-Doo MXZ 500 (left piston rings melted to piston/scored piston). I did this with some guys that knew what they were doing.. Here's the problem: When it was all back together, we tried to start it and it would not even fire (tried starting fluid), kept pulling and pulling but no luck so I thought I would just take it in. A few days later I decided to try to start it and it fired in one pull, ran for 10-20 seconds then died.. (it will only start when it is able to sit, but won't stay running). My thought is that it is flooding itself out, because the plugs come out soaked. I replaced the gas but that made no difference. Had a mechanic come look at it but he didn't know what to do other than try new gas. It has spark, 115 compression in each cylinder, Yes the pistons are put in the right way, we did replace the gaskets..
When the sled does start, it will stay running if I keep on the throttle, but it sounds like absolute crap, and before the rebuild it ran like a clock. I'm almost scared to start it because I feel like I'm hurting it (that's how bad the sound is). I don't know what to do! Any suggestions would be appreciated.
Thanks
 
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stormrider3

New member
Make sure the exhaust is clear( nothing built a nest in there)
I would re-check your spark might be intermittent because of poor ground.
You can test ground with a jumper cable
Good luck
 
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G

Guest
Did you ever figure out why it burned down to begin with? Did you clean the carbs before you started riding it this season? 9 times out of 10 a burndown is a carb issue. Sure there can be bad seals or a cracked boot but most of the time these things happen from some kind of an obstruction in the carb. Now that it appears to be flooding out it could be a variety of things. Stuck needle and seat - stuck choke plungers - lots of stuff. You got rid of the old gas - good. That could be full of water from condensation. There are ways to check that before you throw the old gas out. Water in the gas will also burn down your sled. Back in the day before I became addicted to Seafoam it was standard procedure to clean the carbs and get rid of the old gas before the first ride of the season.
 
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XxXskidooXxX

New member
grub said:
Did you ever figure out why it burned down to begin with? Did you clean the carbs before you started riding it this season? 9 times out of 10 a burndown is a carb issue. Sure there can be bad seals or a cracked boot but most of the time these things happen from some kind of an obstruction in the carb. Now that it appears to be flooding out it could be a variety of things. Stuck needle and seat - stuck choke plungers - lots of stuff. You got rid of the old gas - good. That could be full of water from condensation. There are ways to check that before you throw the old gas out. Water in the gas will also burn down your sled. Back in the day before I became addicted to Seafoam it was standard procedure to clean the carbs and get rid of the old gas before the first ride of the season.
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Thanks Grub, I actually just bought it from a mechanic I knew last year, he took really good care of it..I didn't soak the carbs This year. I am really new to this which is why I rebuilt it with two other guys that knew what they were doing, but does the oil come through the carburetors? Like I said, the left piston was just fried but the right one was perfect (could have even re-used it) so if the oil does come through the carbs then I would guess there is something wrong with the left one. But..wouldn't the sled still at least start/run..?
 
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G

Guest
It will smoke a lot if it is being overfueled. It does not matter if you bought your sled from a good mechanic or even the Lord Jesus himself - fuel deteriates over time. Not saying your friend screwed you or anything like that. Also will not revisit the whole ethanol thing. You mentioned that you were able to rebuild the top end without removing the carbs so I am assuming you didn't remove the carbs. You need to remove the carbs and take them apart and clean out all the little tiny orifices and check to make sure there is no green slime lurking around. You can't just take them off and throw them in a bucket of solvent. I have not worked on a 'Doo for a long time but I am sure the oil injection lines are routed much the same as other sleds that being the oil enters via a little line between the carb and the motor. From what you describe a lack of fuel and not a lack of oil burned you down. The fact that only one side burned is kind of proof of that. If you are lacking in oil even nastier things happen. I get the feeling that you want to sort this out for yourself and good for you. I was there once too and it can be fun to do things for yourself. But sometimes you get stumped. Not to slam you in any way but maybe it is time to take your sled to your local Ski Doo dealer and let him get you figured out. Winter is a wasting!! You want to be riding not fixing.
 
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XxXskidooXxX

New member
Thanks grub, I removed, cleaned, and soaked each carb. I put them back on and like usual, it would start after fighting it for a while, but when it did fire, the thing went wide open by itself (luckily it was on a stand). If I primed it a bunch the rpm would come down but then it would die! What else would cause that?
 
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rmk4ever

New member
Leaking needle & seat... Stop asking for help if you wont try what has already been stated. Needle & seat dumping fuel into combustion chamber, will make your motor "run away" If you have the carb in your hand, out of machine hook a small fuel line up to the fuel inlet of carb(s) now with the carb pretty level (in your hand) blow thru the fuel line... you should be able to blow thru there without restriction, now tilt the carb (while blowing) out of level (up or down) you should be restricted on the amount of air it will accept. If theres no change then your needle & seat need to be replaced. & dont cheap out on them get a genuine mikuni... Where you at in Minny, could maybe help with this. P.M. me It is also known that the fuel pump goes bad sometimes they dump fuel straight into the crankcase.
 
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XxXskidooXxX

New member
Thanks rmk4ever. Believe me, I'm trying what people suggest, I just did what you suggested..on the left carb, I could not blow At All (not even with it level) but on the right one, I could blow through it but it was still pretty hard and it didn't get any harder when I tilted it.. Does this tell you anything?
I don't want anyone to go out of their way to try to help me, I really appreciate the advice and have learned a lot :)
 
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rmk4ever

New member
Sorry carbs should be dry when performing this. However I'm not familiar with this model, can you swap sides with carbs, or are they racks. If you can swap them this will tell you if problem is "carb" related if you see the "wet" conditions in the other cyl. I hate to say it but when it did fire & "runaway" was very hard on rings. Again not sure on this model, but most sled motors have a "drain plug" for each cyl. on crankcase if you keep flooding out 1 cyl, you can drain that excess fuel from the drain plug usually a 10mm or a 8mm
 
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XxXskidooXxX

New member
rmk4ever said:
Sorry carbs should be dry when performing this. However I'm not familiar with this model, can you swap sides with carbs, or are they racks. If you can swap them this will tell you if problem is "carb" related if you see the "wet" conditions in the other cyl. I hate to say it but when it did fire & "runaway" was very hard on rings. Again not sure on this model, but most sled motors have a "drain plug" for each cyl. on crankcase if you keep flooding out 1 cyl, you can drain that excess fuel from the drain plug usually a 10mm or a 8mm
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These models do not have drain plugs, I'm still a little confused as to why I couldn't.blow air through the left carb like what you recommended..especially since the left piston is the one that fried
 
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G

Guest
rmk - If the needle and seat is stuck open on one carb or the other it will fill the lower case full of fuel and potentially hydrolock it. It shouldn't fill the combustion chamber unless the sled is standing on its front end. If the carbs are put together with the slide(s) in backwards it will result in a wide open situation. I have dug up some micros on this sled and it is not overly complicated. In fact is is a really good sled for its age and engine size. I think he is missing something basic. I still have to believe that all the problems originated in the carbs. Take the sled to a Ski Doo dealer. Tell him the entire story and when he fixes it ask him to tell you what he did to remedy the problem. That way you will learn something. There is only so much that can be done over the internet.
 
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G

Guest
xxxxx - You might be blowing though a vent line. This would be confusing to anyone.
 
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G

Guest
Stormrider - you bring up a good point about exhaust blockage. It can happen. However if it has run wide open it can breathe. The compression numbers seen a little low. This can be a result of washed down cylinders or the rings could be installed incorrectly. As simple as this engine is I would take the top end off again - since it is so easy to do - and check to make sure the pistons are installed correctly and the rings are installed correctly. If the pistons are backwards it might actually start but the port timing will be so screwed up that it will not run properly. It is possible to put the rings on wrong. Hard to do but possible. Back up and start with the basics. After that all it can possibly be is carbs. He never split the cases so he could not have screwed with the timing. This has got to be something pretty simple.
 
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XxXskidooXxX

New member
I was able to get it running! After cleaning the carbs, i also cleaned the fuel pump, and we got it started. We ran it in the garage on the stand for a while but it was late so I thought I would break it in this morning. So I get it started (like nothing was ever wrong with it) and I'm doing laps in my field and after a few minutes, it would 'bog' at high speeds/would not go over 35 mph. I get back to my house before something happens, shut it down and now it won't start again. I'm getting closer, hopefully it bogging at high speeds should say something..
When I removed the plugs, they were dark and shiny (not cardboard brown like they should be).
 
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motor_slut

New member
How old is your gas? Water will sink to the bottom where your main jets are, if you have water or phase separated ethanol it will cause high end running issues. If your carbs are completely full of water it won't run at all. I have been through this on a Arctic Cat that had a cracked gas cap and would get just enough water in it to fill the carbs after a long ride.
 
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snowman72

New member
FYI

If you are working on the carbs, make sure you do not switch the PTO and MAG side carbs. Ski-doo runs different jets in each carb. There should be a red or blue colored dot on the carb that matches the same colored dot on the intake side of the engine. If you switch carb locations it can burn a cylinder down. I had a friend that did not know this on his '98 500. He cleaned carbs at the start of the season and burned down one cylinder on a very cold day. My '99 583 is the same way. IF you are not sure, look up the carb diagrams or ask the local Ski doo dealer to find out which jet size is for PTO/MAG.
 
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XxXskidooXxX

New member
snowman72 said:
FYI

If you are working on the carbs, make sure you do not switch the PTO and MAG side carbs. Ski-doo runs different jets in each carb. There should be a red or blue colored dot on the carb that matches the same colored dot on the intake side of the engine. If you switch carb locations it can burn a cylinder down. I had a friend that did not know this on his '98 500. He cleaned carbs at the start of the season and burned down one cylinder on a very cold day. My '99 583 is the same way. IF you are not sure, look up the carb diagrams or ask the local Ski doo dealer to find out which jet size is for PTO/MAG.
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Thank you, I did not know this...When your friend burned down, was it still running good with the carbs switched (before the burn down)?
 
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snowman72

New member
XxXskidooXxX said:
Thank you, I did not know this...When your friend burned down, was it still running good with the carbs switched (before the burn down)?
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Yes, ran fine prior to burn down. It was very cold out, I think low single digits or near 0. One side of Rotax motors runs hotter than the other on the twins, that's why they run a larger main jet to add more fuel to help compensate.
 
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