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Discussion Starter #1
Just wondering exactly how to do a compression test on a 2 stroke sled? I've heard so many different ways, and I just want to be sure I'm doing it properly. Sled has electric start, but do I pull it over manually? Or turn the ignition? How many pulls? How long to turn the ignition? Engine cold? or engine warm?
Basically, if someone could please explain the steps in a proper compression test, it would be appreaciated. Thanks. Oh yeah.... I do have a compression tester.... so I have the first step down.
 

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Do a compression test both cold and warm then average the results. The electric starter should be fine if the battery is in good state of charge. Crank it over for no longer than 10 seconds with the throttle held wide open and the spark plug wires grounded. After the first tests are completed squirt some oil down the cylinders and retest to see if reading comes up. If it does on a warm motor the rings are poor. If it stays within 5% then you are good to go.
 

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Good tip with the oil test, I have never tried that :thumbsup:
 

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oldslowsledder
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Just to add if your readings are abnormal or low then before tearing anything apart try another tester. They are renown for being inaccurate.
 

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Discussion Starter #5
Just out of curiosity.... why do the plug wires have to be grounded? Is it for safety reasons so you don't damage the computer/chip in the sled? Or does it actually affect compression readings somehow?
 

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Just to protect the electrical system.

It has no affect on the compression test results.

Good question.

Jeff
 

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just turn off the ignition

and give 4 good pulls

as long as they are within 5psi of each other I would not worry

What sled is it?

my Polaris have always been 115-120psi
My Yamaha has ran with 100psi but should have been 115
My Skidoo were 120psi for the fans and 140psi for the LC
My current Cat is around 125-130psi
 

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Discussion Starter #8
Well..... compression WAS 100 and 95. So I put the new rings in it and now compression is 80 each side..... What did I do wrong? Does the sled have to run for a while in order for the rings to seat themselves? I took a bottle brush to the jugs, I re-used the old head gaskets but I don't think there's a problem there, because the needle doesn't drop on the compression tester. What could I have done wrong? I did run the engine for about 10 seconds, but that was it because the neighbors are sleeping. compression was 80 each side, then went up to 90 when I put some 2 stroke oil down the heads.
Also, because the base gasket ripped when taking the jugs off, I built a new gasket out of gasket paper (or whatever it's called). Could there be a low compression reading due to different gasket thicknesses? Also, when I pulled the heads off, there were 2 aluminum gaskets per side. I found that kind of odd, because I don't quite see how the tops of the pistons are supposed to sit tight in the heads with 2 gaskets in there. The engine is a 488. Same engine that's in the older Phazers. In case that helps.
 

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So eh, what was the original reason for replacing the rings? A couple of things to remember, if the "new" base gasket is a lot thicker than the old one, then yes, compression would be lower. You have to be careful when removing the old base gasket, you don't want to gouge any metal in the case or cylinder base, I know you did'nt....right? Also, when you put nice new and perfectly round rings into an old engine, they like to be in perfectly round cylinders, or very close. After a 1,000 or so miles on a 2 stroke engine ( especially an air cooled one) the cylinder gets a little out of round. The original piston and rings kinda set in to this and for the most part can live together ( if left alone) this way for thousands of miles more. I hope you or someone checked the cylinder bores for this, because putting new rings into a oval bore will give you poor compression and probaly engine failure down the road. If you have a low hours engine, then maybe you can get by, but if has a lot of miles and time on it, you may want to worry about this.
 

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Discussion Starter #10
Tomorrow I'm gonna put the pig to the bar for about 20 seconds.... I'll do that a few times, and the rings will either seat themselves properly, or she'll blow. One or the other. Definately don't have time to be playing games like this once there's snow on the ground. Might as well make it or break it now.
 

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485 Phazer engine 1984-2001 should be 115-120psi

BUT try another compression tester as they all read diferently

also did you hone the cylinders?????

do not go full throttle for 20seconds UNLESS you are riding the sled

Feltpro gaskets???? paper gaskets at auto stores

not odd to have gaskets like that as there is a lot of pressure up at the cylinder head
 

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Do a compression test both cold and warm then average the results. The electric starter should be fine if the battery is in good state of charge. Crank it over for no longer than 10 seconds with the throttle held wide open and the spark plug wires grounded. After the first tests are completed squirt some oil down the cylinders and retest to see if reading comes up. If it does on a warm motor the rings are poor. If it stays within 5% then you are good to go.[/b]
What if you don't have electric start?
 

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journeyman sled/motorhead
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did you deglaze the cylinder?
 

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Ok to run with low compression???

I have an 04 Ski-doo MXZ 600 HO. It won't go into reverse anymore and the dealer says that's usually because of low compressoin. I did a compression test and I'm only getting 75-80 psi in both cylinders.

Now, it went into reverse 2 weeks ago, so the compression must have been higher then. I put about 300 miles on it and that's when I started having the problems with reverse.

Here's my question.... Is it bad to run it like this? I don't notice any decrease in power and it runs fine. I'm just afraid that if the problem came about within the last 2 weeks enough that reverse won't work anymore, is there more than just worn rings?
 
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