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Low compression

Arctic Cat General Discussion

 
 
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  #1  
Old 02-27-2011
westman995
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Default Low compression

Ok so i put together my friends motor for him that he blew up. The new parts that were put on it were two new SPI pistons, one cylinder, crank seals, and all new gaskets.

Well, it has low compression. It only has like 90-100 psi at the very most. It runs, it has power SOMETIMES.. I feel as if it shouldnt even run as decent as it does with only around 90 psi per cylinder.

Now im wondering, why in the hell would it have low compression on brand new pistons and rings? One of the jugs is the same one that was on it when it blew up and it had over 120 psi or something like that before.

Is it possible that the rings were put on upside down There are two rings per piston. And i checked over the instructions, nothing about which way the rings went on. They rings were squared all around, none of the edges were rounded or anything.. The SPI pistons and rings i put on my 700 polaris had to be put on a certain way and were rounded off of whatever on one edge of the ring. I didnt see any way i could have put these rings on the wrong way since they are square all the way around?? Hmm..
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  #2  
Old 02-27-2011
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The ends of the rings would have notches for the indexing pins and could not be put in wrong side up.
They have to seat in and the compression will go up.
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Old 02-27-2011
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How long has it been sense the rebuild? It will take some time for the rings to seat and the compression will come up once it has done that. Don't go over half throttle or so for the first tank of gas or so.
Where the cyclinder bore's nycil (chrome) or cast iron bore? Did you mic the bore for size and out of round? Did you Mic the piston for clearance? SPI rings are rounded on both sides and I put them in with the lettering facing up. Compression will also be higher on a warmed up engine than a cold one.
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  #4  
Old 02-27-2011
westman995
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Quote:
Originally Posted by OCR View Post
The ends of the rings would have notches for the indexing pins and could not be put in wrong side up.
They have to seat in and the compression will go up.
Well, i tried putting them in the other way and while i did get it in, it wasnt easy and the rings dont really move around very good when i put them on this way. So i must have had them on right.


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Originally Posted by DoubleClutch View Post
How long has it been sense the rebuild? It will take some time for the rings to seat and the compression will come up once it has done that. Don't go over half throttle or so for the first tank of gas or so.
Where the cyclinder bore's nycil (chrome) or cast iron bore? Did you mic the bore for size and out of round? Did you Mic the piston for clearance? SPI rings are rounded on both sides and I put them in with the lettering facing up. Compression will also be higher on a warmed up engine than a cold one.
Its been ridden a couple times since the rebuild. Its maybe had two tanks of gas ran through it. We checked it before running it at 90 psi then after it was ridden a while and it was the same i believe.

They are nicisil bore cylinders. I did not mic it or anything of that sort. These SPI rings were not rounded off on any sides, or atleast one side was not more rounded off. I also did not see ANY lettering or markings when i took them out right now.

And the reason its taken apart again is cuz he sunk it in a pond haha. So the motor was filled with water so we had to pull the cylinders and clean it out and what not.. Now it needs new base gaskets as one of them ripped when we took the cylinders off.
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  #5  
Old 02-28-2011
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check with another guage and see what it says. i have a tester that most people would call like new. but it reads 15 psi low.

did you check the bores and pistons for clearance specs?

the rings you got right, upside down on spis and the rings stick in the pistons.

if you sink the sled you dont have to pull the engine apart, remove carbs and drain them, remove pipe and drain it, remove seat/tank and drain it, drain the oil resivoir or after it sat over night atleast bleed the water out of the system,

for the engine itself simply remove crank case plugs to drain water or pull spark plugs and pull engine over and water will clean right out.

mix gas in next tank for safety.
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  #6  
Old 02-28-2011
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^^^^^^^^^^^^^^^^^^^^^

It's a Cat, don't have case drain plugs!
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  #7  
Old 02-28-2011
westman995
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We did try another gauage, read about the same.

I didnt check the clearance or anything i guess.

Ya the rings did stick in the pistons when i put them in the other way ha..

And he is right, there is no drain plugs on the case, which really sucks.
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