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Discussion Starter #1
This snowmobile was given to me along with an 88 400. The 88 is running good now. The 86 is all together after a top end rebuild and it has no compression on either cylinder. The engine has not fired once yet. What would cause no compression in both cylinders? It has maybe 5psi in either side. Would the crank seals be so bad that it would have this little compression?

When it was given to me, I was told that a screw had been taken out of the oil pump of the 86 to put in the running 1988 400 but I cannot see what screw is missing on the 86. Would this cause the loss in compression? Thanks!
 

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Rick
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no that won't cause the loss of it. look on the top of the oil pump it think that will be the one they are talking about. make sure if you do get it running to mix the gas in that sled. also did you do the rebuild? take the plugs out and look in each side to make sure both pistons are move and don't have holes in them. you can just turn the clutch by hand to do this.
 

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Rick
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also did you use a compression tool to check it?
 

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Discussion Starter #4
I didn't do the rebuild and the oil pump looks fine from what I can see. I took the head off and everything the pistons are brand new, look great. Compression tests between 15-30psi a side.
 

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Rick
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bad crank seals shouldn't do it also. and the pistons are the right size? no play at all?
 

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Discussion Starter #7
There is no play in the pistons. The sled was rebuild with .010 over pistons. I don't get it!
 

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Rick
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i'm lost then.
 

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Rick
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i had broken skirts on one piston and it still had 130 psi.
 

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Discussion Starter #10
I cannot see where there would be that big of an air leak somewhere. The crank seals could very well be toast but to barely make 30 psi? The rings haven't even seated yet because it hasn't been run.
 

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Discussion Starter #11
I took the rings off of the pistons. There are no marks anywhere to distinguish the top from the bottom of the ring. I know the beveled edge goes to the top but it's so hard to tell. If the rings were upside down would this cause this much pressure loss?
 

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Rick
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make sure to put them in right also. there are lil tabs on the pistons that they line up with. and they are offset from each other a lil also.
 

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Discussion Starter #14
I am at a loss. The rings are good, the pistons are good. It must be the cylinders. I can pour gas on top of the cylinder and it disappears in seconds. Maybe they weren't honed or bored properly.
 

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Rick
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take jugs and pistons in and have then checked..
 

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Rick
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it is right then the arrows need to point at the flywheel or front if the arrows face that way. crank seals won't make it loose that amount of psi. they will suck in air and blow a motor or leak out oil but not do what yours is doing. i think you have the wrong size pistons for your bore. do you have a book for it? if so did you check the ring gap also?
 

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measure your ring end gap with a feeler blade and tell us what you get. Sounds to me like the pistons and rings are not sized to the cylinders properly
 
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