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Discussion Starter · #1 ·
Hey guys, I just rebuilt my 05 mxz 800, it had low compression on one cylinder before the rebuild and about 6000 miles so I decided to do an overhaul.
My problem now is after the rebuild the compression is so high I can't pull it to turn it over more than once. If I pull the plugs it turns over nice and smooth no issues. All I could think of right now is the cylinder gasket, I got 3 different ones in my gasket kit, I put the closest one to the old gasket in there. I also gapped the rings and rubbed two stroke oil in the cylinders during the rebuild. Wondering whether you guys have any ideas on how to solve my problem. My next step is a squish test but I can't see how the cylinder gasket could make such an issue when I put what seemed to be the same size in there.
 

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Discussion Starter · #3 ·
Did you have the clutch bolted on? If not, install it and try again.
You're probably smarter than me, but just curious on how the clutch being on could make such a difference? I did not have it on and I will give it a try though.
 

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Also, which gasket did you use? The kit I got had 3 also, all marked with small holes between the cylinders. 5, 6, and 8 holes. I ended up putting in the one with 8 holes, thinking it was for 800's but later found out it was the gaskets thickness.
 

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Some generalizations when choosing the base gasket. The 8 hole will have less cranking compression than the 5 hole. A replated cylinders will have will have higher cranking compression than one that is only honed. The higher cranking compression, the greater the power. The higher the elevation, the higher compression ratio it can handle on pump gas. The muffler is important to check for any damage to the baffles. Any restrictions in the muffler looses power and holds a higher risk of engine failure.
 

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By the way, I think everyone at one time or another has experienced hard cranking with the clutch off. It's all about the momentum that is provided by the mass and velocity of the clutch.
 

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Discussion Starter · #8 ·
I appreciate all the help guys, I would have never thought the clutch helped that much. It completely fixed the issue, I feel like an idiot.

Now my issue is I don't have spark. Never had any spark issues before the rebuild, I had to remove the stator and trigger coils in order to rebuild but I put them back on the same way they came off, torque specs and everything. Ive tested the stator and trigger coils, both tested good. Im wondering if I'm missing a plug somewhere but I've ran through the whole engine and everything seems to be plugged in fine. Is it possible to plug anything in wrong on these engines? Anyone have and ideas on what to do next for this? I'm going to test the DES and Kill switch tomorrow. Would be weird if they were the culprit though.
 

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There are two similar plugs that could have been mixed-up.
 

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I checked the trigger coils, they are good. I still have no spark, I also can't seem to find any plugs that could be mismatched
They are talking about the trigger coils/pickups. They are easily switched with each other and then your engine will only start if pull started in the reverse direction(counter clockwise). Easy to test by wrapping your emergency pull cord around the clutch the opposite direction as normal so that you pull the crankshaft in a counterclockwise direction. If it starts then you know you swapped the triggers.
 

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Discussion Starter · #13 ·
Thanks for the help guys. You've been awesome, another weird question I have is the trigger coils. The manual I have says they should have DC voltage coming out of them, but I don't get any dc voltage. However when I switch to AC the voltage is spot on spec (.200 to .350). Is the manual mistaken on the dc part? I'm trying to rule out the trigger coils as the culprit.

It also says to test DC voltage out of the mpem to ignition coil, 220 to 275 vdc, why would that much voltage be coming from the mpem, I thought the ignition coil would be the only thing with that much voltage coming out of it. Anyhow I have no voltage coming from the two prongs off the mpem when cranking. I'm going crazy, any ideas from anyone?
 

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I think it's the CTS and CPS sensor plugs that can be mixed up. That's the Coolant Temperature Sensor and Crank Position Sensor. I took pictures of those two plugs the last time I worked on an 800HO, but they are buried in a restored folder. You obviously have the Shop Manual, so just match the wire colors.

For the trigger coils, if you are not sure they are in the right order with the trigger having the blue wire at the bottom, then the emergency pull cord can be used to pull start in reverse.
 

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Discussion Starter · #15 ·
I think it's the CTS and CPS sensor plugs that can be mixed up. That's the Coolant Temperature Sensor and Crank Position Sensor. I took pictures of those two plugs the last time I worked on an 800HO, but they are buried in a restored folder. You obviously have the Shop Manual, so just match the wire colors.

For the trigger coils, if you are not sure they are in the right order with the trigger having the blue wire at the bottom, then the emergency pull cord can be used to pull start in reverse.
I am for sure that the trigger coils are in right, I pulled the flywheel and checked, green on top, blue on bottom. I have no spark cranking forward or reverse anyhow. I tested the stator and trigger coils and the stator tested good, but the manual says to test trigger coils in DC, i get nothing in DC but when I test them in AC I get the proper values for both, so I'm not sure if the manual screwed up.

Right now I think its my MPEM but I just can't believe I'm so unlucky that it broke from sitting there for few weeks. I've checked all plugs and wires female and male ends and can't find any issues. Killstart and dess test good as well.

I am a diesel generator mechanic which are so much more complicated, but man this thing is throwing me for a loop, electronics just want to give me issues for no reason lol.
 

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Give me the year of the Shop Manual that you are using, the section, the page number on the bottom right of the page if it has one, and the pdf page number if you are viewing it in soft copy.
 

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You have done the right thing to check both AC and DC. Knowing the shop manual and page you are looking at, I can dig deeper because you probably want to know, but I am not at all concerned. Check one trigger coil, then the other. If both measure about the same then all is good for that test. That is about as far as I need to concerned.

By the way, it's good to hear that you are a diesel generator mechanic, or any experience of that nature that delves into the electrical. I see that you weren't afraid to question the Shop Manual. I consider that experience, because there are errors to contend with. I have built my own document to identify the ones that I find over time. It can be frustrating.
 

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Check the connection down in the front where you plug into the regulator. If it’s not bottomed in the plug, you won’t have spark. And make sure the ground strap in front of the engine down low is hooked up. On mine it bolts to the starter mount bolt.
 

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Discussion Starter · #19 ·
Give me the year of the Shop Manual that you are using, the section, the page number on the bottom right of the page if it has one, and the pdf page number if you are viewing it in soft copy.
Its page 280 of the 2006 rev series shop manual. I have attached a screen shot of it. If my trigger coils test good going into the mpem but there is no voltage from the mpem to the ignition coil when cranking, would that be a bad mpem?
 
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