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97 Magnum 425 - Motor locks up with plug in but fine when out??

n4cer26

By n4cer26
in Polaris ATV Forum

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n4cer26 Apprentice

n4cer26

Posted
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Just bought a non-running 425 Magnum 6x6. Guy said previous owner had motor rebuilt (3 years ago or so.) It reportedly ran great but recently the electric start began failing and he put a new recoil on it which he broke immediately.

 

I get it home and put a new recoil on it and the pull is incredibly hard....so much so that I think I'm going to break the new recoil assembly. I also installed a new starter which won't roll it over either.

 

I'm thinking maybe a hydro-locked situation so I pull the plug and the motor rolls over fine with both pull start and electric start. The cylinder was dry however.

 

Checked the valve lash and it was right on at .0006".

 

Checked the compression relief spring on the end of the cam and the spring was strong and keeps the metal piece against the dowel like it should. The cam lobes look good from what I can see.

 

I have a small camera that I dropped into the cylinder. Could t get a pic of the entire chamber because of the angled engine but there are marks on the center piston and a couple of grooves once side of the piston top. I will include the photo.

 

Anyway....I'm at a total loss at this point. Not sure how to proceed and diagnose. Would sure appreciate anyone's help who might know what the heck is going on! I was able to get it to turn over about 5 times with electric start with compression tester installed which read 145 lbs. I don't know what else to do!

 

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Mech Grand Master

Mech

Posted
Posted

In some of the manuals for those things they have a maximum compression reading listed. Apparently, that's the maximum it should have for an easy start if the de-compressor is working. It shouldn't be higher than that figure if the decomp is working as it should.

Have you read what it should be for that bike ?

Perhaps the decomp isn't working.. perhaps there is wear.. 

 

n4cer26 Apprentice

n4cer26

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The engine was rebuilt a couple years ago and I can tell that is true by having gone through as much as I have so far. Looking at the loabes of cam through the rockers....they are pristine. Going to watch the ball bearing today to see if there is any movement in the decompression ball bearing when I that metal piece away from the dowel.
I am not sure what the max compression should be.......can't find it anywhere.

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Mech Grand Master

Mech

Posted
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I've looked in a couple of books and they seem to expect no more than 90lbs if the decomp is working right..

You still haven't confirmed that it will turn over if you leave the plug slightly loose and turn it slow enough for the compression to escape..  That will prove it's only compression and not anythng mechanical.

Gwbarm Grand Master

Gwbarm

Posted
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At 145 your compression valve is not working, mech is right about the 85-90 # , maybe it didnt get back together right after the rebuild, I would still think the starter should turn it over, if you put oil down in the cylinder does that help it turn over at all,

n4cer26 Apprentice

n4cer26

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I've looked in a couple of books and they seem to expect no more than 90lbs if the decomp is working right..
You still haven't confirmed that it will turn over if you leave the plug slightly loose and turn it slow enough for the compression to escape..  That will prove it's only compression and not anythng mechanical.
I'm sorry, completely forgot to respond to that question. Yes, it turned over a bit easier with the plug loose.

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n4cer26 Apprentice

n4cer26

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Ok.......figured out that the spring for the decompression was on so that the opposite side of that metal piece was against the dowel. It shouldn't have made any difference when starting, o ly when running. I corrected it. Also discovered that the fuel pump is not working so it wasn't getting any gas at all. I bypassed it direct to carb. I hit the starter and it fired right up and idled. Took it for a test drive and it runs great. Go figure??! I do have an oil leak somewhere though. Will have to sort that out.

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n4cer26 Apprentice

n4cer26

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Sorry, I forgot to answer that question Mech. Yes, it turned a little easier with the spark plug a little loose.

I also realized that during the rebuild, someone put the decompression spri g on wrong and it was holding the wrong side of the metal piece against the dowel. I corrected that. The motor still turned hard.

Also discovered that the fuel pump is garbage. I bypassed it and ran directly from tank. I put it back together and hit the starter......it fired right up and idled. Go figure!

Took it for a test drive and it has gobs of power. 6x6 works so I'm almost there.

I do have an oil leak to chase down.

Thanks for hanging with through this......certainly was different!19f0d1aab0fc2826d34e89218eb13747.jpg

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Mech Grand Master

Mech

Posted
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Hi Steve.  It's possible, but then other than too much compression, which should get released by a working decomp, it shouldn't cause a problem that only shows up when the plug is in. If the cam timing was out and two valves hit, then yeah it could cause a lockup, or if one valve set hit the piston, yeah it could lock up.. but then it wouldn't matter whether the plug was in or out.. The valves would hit or they wouldn't.. 

 

What we need here is someone that has seen a worn or faulty decompressor that can explain what wears or goes wrong with them..

 

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