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Discussion Starter #1
Hi guys.

I have a 2001 Polaris SCRAMBLER 500 2X4 (A01BA50AA) motor.

I got the machine running for about 10 minutes. Ran great!

Then, it died and haven't been able to get it to start since. I have good spark, I have good fuel. I rebuilt the carb, have new oil, new plug, etc...

When I started this project it had 75PSI compression. I just ran another compression test and for some reason, now its dropped to 40. Anyone have any idea's of what went wrong and what to do now?
 

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How does it sound when it cranks over? Two options one is a hole in the cylinder or other is stick valve not building compression. Pull the plug and shine a light down the hole or take a pencil with piston at top dead center poke around. Good piston pull valve cover and turn over by hand make sure everything is moving freely
 

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Also the 500 have a compression release built into the camshaft so it will never be an accurate reading. A small amout of starting fluid and put the airbox cover back on to see if it would even pop over might be another option
 

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Discussion Starter #4
The engine actually SOUNDS fine. I don't hear anything unusual as it tries to crank over. Spinning the clutch by hand also feels good. It still does the poofing sounds like it did before (like compression release sounds). I tried starter spray directly into the carb last night, with no effect at all.

There's no evidence of oil or radiator coolant leaking anywhere on the motor. Although one thing is weird - the oil reservoir, or oil cooling tank seems to be overfull of oil. The level is just below the dipstick hole. There is a possibility that I have the oil return lines switched the wrong way, but I never saw how they were supposed to be hooked up. Can someone tell me - the upper oil line of the reservoir itself, - does that line go to the upper or lower input on the motor?

Also, even if it was 'overfull' at the reservoir, that can't hurt the engine, right?

I'll take a look at the cylinder today.
 

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That is just like one of our 2001 Magnum 325 engines even though it is very different engine.

We pulled the top cover and the compression release looks to be working well and the cam lobes are in spec per the Polaris shop manual. Got the high end bore scope from HF Thursday on a double sale price for $160 but was waiting for better weather which should be this week to look at it and pull the head and cylinder since we will be just a few bolts away.

At this point I am thinking leaking head gasket or broken rings. There is no way an engine that will start, idle and run with power will loose compression from wear in the matter of a few minutes.

http://www.polarisatvforums.com/forums/atv-repair-maintenance/49019-40-psi-reading-compression-test.html
 

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Discussion Starter #6
I read your thread. It appears that if you opened your carb while doing the compression test, your readings went from 40 to 95PSI? I tried this, and the readings only went from 40 to 50. Why would that be?
 

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I read your thread. It appears that if you opened your carb while doing the compression test, your readings went from 40 to 95PSI? I tried this, and the readings only went from 40 to 50. Why would that be?
Sorry for the confusion but we have two 325 Magnums. The one that was running and died does not make any more compression PSI with the throttle open than closed nor can the air compressor pump the cylinder PSI above 25.

The 'project' 325 Magnum with the blown low end engine (blew oil cooling line per the seller) will only make about 40 PSI with the throttle closed but will hit 95 PSI with the throttle OPEN. It will set and idle just fine for long time but if put into gear and given gas it will just knock and die. This indicates a good carb and top end to me. We have not pulled the rod cap off of it yet so we really do not know the extent of the damage and where the damage is. Often the rod inserts will rotate a bit and lock up on the crank.
 

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Hay mason, it lil'rascal. You did the same as me trying to find info on your machine. These guy are really good and I'm sure you'll get your problem figured out.
 

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Discussion Starter #9
Hey Rascal!! We just took the top of the motor off and found the main head gasket is melted on the water side. Thinking that could be the cause of low compression. Heading to Polaris dealer now to get new gasket. Everything else looks good.
 

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Discussion Starter #11
Well, ok, not everything looked good.....Took the head cylinder off last night and got a look at the cylinder. Looks like we have scoring down one side of the cylinder wall. We ended up using boring stones and got a lot of it out, but not all.

Turns out, we did have the oil lines switched the wrong way - no way to know this, as every diagram we looked at didn't specify which line went where. So, with the lines switched and not knowing we had to prime the oil to the motor, she ran for about 8 minutes with very little oil. Tonight I plan to replace every ounce of oil in the motor and the reservoir and new filter, and we'll try to start it again. Doubt it will start though. If it doesn't, it will require Polaris dealer to bore and put in a new piston and rings. OUCH. That's not gonna be cheap. But, at least it'll be an almost new motor when I am done.

FYI - PRIME YOUR OIL PUMP - hard lesson to learn.
 
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