100 Mile Snow Safari Forum

Alright guys- I need a little tech guidance from those who have some experience with this stuff.

Just picked up a 2002 MXZ 800X. The sled does not have the original motor in it- and the motor that was put into it has been rebuilt (new top end, as far as I know). After reassembly, the previous owner was unable to get it to light off; and he ran out of patience, time, and $ to fiddle with it so I took over.
I have verified the following:
1) It has a boatload of compression. This leads me to believe the guts of the motor are good and I shouldn't have to go after anything inside.
2) It sparks on both sides. Previous owner checked this and I confirmed it just today. His suspicion was that the thing isn't getting any gas. I haven't gone that far yet- that'll be one of my next steps.
Here's what I'm not too sure about. I don't know if any of the electronics were changed/swapped in the process of exchanging engines around. It stands to reason that any of the following could have happened-
a) the DESS key and MPEM on the machine are original
b) one or the other was changed
C) both were changed

Since it sparked and had good compression- I gave the thing a little shot of gas right into the cylinders and attempted to start it- just to see if it would burn that drink out. Nothing. My thought is that if the DESS/MPEM were not properly matched to each other, the plugs wouldn't even be sparking, right? And modern machines don't have even any provisions for any sort of spark timing adjustment, what with all the electronic controls, so there shouldn't be any reason it won't fire right up once I have a constant supply of gas.
I've been trying to find some info on dootalk- and I ran across a couple threads about programming a particular DESS key /MPEM to each other. I suppose it's possible that it's as simple as that, but again I ask why would it even spark the plugs if it didn't see a proper security combo?

The first person to offer me up the winning suggestion receives lunch @ the DoubleBuck in Laona during one of my trips this coming season. :redgrin:

Thx in advance. :thumbup:

Well normally if you have fuel air and a spark it would run, that is in most cases. You said it has spark and compression so its ok to say them causes are out. Now to move onto the fuel you could be getting to little or even to much, causing it to flood out. I would look at the carbs check all the settings on them, needles and seats tend to go bad and allow leaking or flooding. Look at your fuel pump to check to see if its working correctly. As far as spark goes it could be out of time. Maybe its not matched to the piston position, possbily sparking way to early or to late. I would look at all you electronics for that. This could be caused by the bad electronics. They might have took a surge at somepoint. If im correct on some machine there are timing marks on the recoil side under the cover but im not sure on the newer sleds.

Double check your Dess, we had one that wouldn't fire. Some times the dess is touchy.

02 800's were notorious for getting a vapour lock in the gas lines. To correct the problem, make sure that the carb heaters are OFF. For some reason that was causing the problem


Kinna lengthy but thats some of the things that could be wrong.

My 1st step would be the dess. take your lanyard off and reinstall it and try starting it. Move it around a bit while trying to start it. i

If that dont work also look into possbile reed issues, very easy to pop the reeds out and check for cracks. Along with your fuel pump/lines

When you checked compression did you use a tester. I know sometimes you can break a skirt and still have fair compression.

if all else fails the MPEM


If you cant figure it out let me know. If im correct you live in little chute, I had a duplex over there last yr. Id be happy to make a ride over to help out or what not.

Ya, you'd think so. Yanked the carbs this morning and pulled it through- fuel pump DOES put out, so once I go thru the carbs I'm confident it is capable of getting a proper mixture into the cylinders.
But I still don't understand why it wouldn't light off when I gave it a drink- it didn't even sputter. I'm gonna talk to the service mgr at Appleton Powersports and see what he says about spark in spite of of a potential DESS/MPEM mismatch. I did reposition the DESS a few times on the theory maybe it wasn't making good contact. But AGAIN I ask why would it spark? That's like the 10th time I've asked myself that question.
I'm no modern expert but I'm pretty sure the ignition is triggered by a hall effect type CKP sensor/switch. I reckon that's another thing I could ask the svc mgr. At least I didn't pay a fortune for the machine, even though it is roughed up a bit.
I'm going to pull the reeds because I want to look at the pistons. The previous dude rebuilt this motor, claiming new jugs. When I look down the spark plug holes, the tops do indeed appear new, and it simply feels good and tight when pulling it through. Did I mention this thing has compression out the wazoo? I haven't gauged it yet- might do that just for the heck of it but there's no doubt it puts the squeeze on the intake charge.

I know if my DESS (key) is not seated correctly my sled will still start. You just can't give it any gas and it beeps at you. I beleive it is the same if you put the wrong DESS on the sled. So I don't think it is that stopping it from running. Sorry I can't be of more help.

Good thing I pulled the reeds out- there was some nasty lookin debris on the PTO side, some of which was metallic. It was both on the reed cage and along the intake porting. I bet that was partial remnants of when the motor blew up last time. Pistons are definitely new- they're clean as can be and still have all the machining ridges on them. Schpritzed a healthy dose of fogging oil down both sides of the crankcase since I had access to it already and it likely wasn't done when the motor was reassembled LAST YEAR.
Carbs were virtually spotless inside. I pulled the jets out and blasted them good with compressed air anyway, same for pilots and all the carb passageways. I reckon I can put the whole intake tract back together at this point- I don't see anything suspect at all. Altho I still have to split the airbox open and clean all the crapola outta there- some idiot completely shredded a belt and just kept riding and riding until the thing disintegrated. I found about 2 dozen chunks and ribs all packed up under the left side footwell- don't people know how to clean stuff up and take care of it?
So, I'm pretty well right where I started, except now I'm fairly sure it is not a fuel related problem.
Anybody have a service manual for 2002 models and can do a little investigation as to what triggers the ignition system? Lunch in Laona is still on me- IF you provide a correct diagnosis. :rolleyes:
Check out this garbage on the PTO side reed.... jeez

Your problem is either ignition timing or lack of compression. you say it has compression, but unless you put a guage on it, you have no idea. Put a few squirts of oil and a drop of gas in the cylinder to seal the rings and give it a pull. Many skidoos I owned suffered dry ring syndrome. Maybe the rebuilder broke the ring upon installation. If it's not that, check to see when it fires, if the piston on the upstroke and is almost at TDC, you are close to correct timing.

If compression isn't in the 150psi neighboorhood, I'll eat my dang shirt- WITHOUT salt, pepper, or ketchup. In fact this one is (marginally) harder to pull through than the other 800 I have, which also has one reasonably fresh piston. I'm no newbie to engines- I am confident that the guts are good.

As for ignition timing- that's pretty well out of my hands, is it not? Gone are the days of condensers, points, etc. Unless I'm gravely mistaken, the ignition system is nonadjustable (see previous post re: trigger type) and I'll have no effect on WHEN it sparks. This remains to be confirmed.

I haven't thrown out the idea that maybe the coil is weak, but that's fairly low on the probability scale with less than 5000 miles on the sled. One of my uncles has an 87 Phazer with well over 15000 miles on the clock- on the original components (save for the cooling fan, which went tango uniform last season)! Another uncle is approaching 9000 miles (95 Polaris XLT) on the original belt yet. So I figure the coil, while not out of the realm of possibility, probably isn't it either. You never know though- just because you see sparks when the plugs are laying on top of the cylinders doesn't mean that's what's going on when they're down inside, either.

:shrug:

check the spark in the dark, to see if it is blue, and not a weak yellow color,.... new plugs, I assume...dirty ( fowled) plugs will spark, but not strong enough to fire.
switched plug wires???

see you at the doublebuck.

Ya, I did that last night. Nice and blue. New plugs. They were gapped a bit wide (inspected closely today) so I mooshed em down a little bit. Plug wires are idiotproof- the magneto side won't reach all the way to the pto side. lol fowled plugs- what'd they do..... fly south for the winter?

I'm gonna modify my lunch statement- for every incorrect answer somebody has to buy ME lunch. Let's see...... that's about 3 of them so far. :redgrin:
Dang, now I'm inspired to go out and spark it around again. I'll be back......

I've taken this tour before! Doncha hate it when everything you check is GOOD and yet something is WRONG? Reading these posts is way better than prime time TV. Hope you nail it soon!:thumbup: