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01 Magnum 500 Bad Cam & Low Compression

5804 Views 6 Replies 4 Participants Last post by  Kevcules
Hi All,
I am new to this site and also new to quading. This will be my 1st and I haven't riden one yet. I have been riding dirt and street bikes almost 40 years.

I picked up this 2001 Magnum 500 with about 1300mi, 173hrs, that had low compression. Took the head off and found that the Cam exhaust lob was pretty much gone. I bought a new cam and exhaust rocker and was going to put it in, but the low compression kept bothering me. So the Head is back off and I am trying to find the cause.

Sorry I know I am leaving out some info but don't know what it might be yet.

I did a compression test with the electric starter with a battery pack hooked up speed seemed pretty good. It is a 4-stroke single cylinder with an auto decompressor, so the compression will always test some what low. The book says 50-90 psi and I read 30-35 psi .

So I tried to do a bleed down test. I've never done one before and I dont have the tool. So I hooked the compressor to a controlable air chuck to the spark plug hole and I could control the air flow. Rockers were removed. piston at TDC. I could get it to read about 40psi while I was holding it open but immediately dropped when I released. I thought maybe the chuck is restricting the flow and There was a lot of noise from the chuck so I couldn't hear where the air was leaking out. So I hooked the compressoer directly to the cylinder, but reduced the pressure to about 40 psi.
So it continued to leak-by as fast as it went in.
If I put a hand over the intake and then exhaust ports both of them built up a small amount of pressure that I could hold back with my hand.
The vast majority of air was leaking into the crankcase, from what it sounded like to my inexperienced ears.

So sounds like piston rings blow-by and valves not seating completely.

In all of my reading I have not heard of this before for a circa 2000 magnum/sportsman 500.

So is there anything else I should do before I pull the cylinder?
I am not a machinist by any means, but I do have a vernier caliper.

Any suggestions would be greatly appreciated

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Maybe the piston rings are worn? Try to replace them. If that still doesn't work bore out the cylinder by .010" and get a new piston and ring set. Also make sure that you are using 0w-40 synthetic oil
Maybe the piston rings are worn? Try to replace them. If that still doesn't work bore out the cylinder by .010" and get a new piston and ring set. Also make sure that you are using 0w-40 synthetic oil
Thanks for the suggestions, the rings are worn down to the piston, the piston is badly worn and the piston pin is seized in the rod. I haven't been able to get the pin the move in the rod at all. I am hearing that the rod will almost certainly be bad by the time I get it out. I started looking for a used motor. If you are familiar with what would work that would be helpful.

Thanks, John
I wouldn't get a new engine, not only would it be costly but its also unnecessary. 173 hours is considered low.
Quietseas, I don't think the Magnum 500s got the Sportsman 500 ho engines from 01 up. I would look for a used motor from a 96-00 Sportsman 500. Look on here and CL. There are people parting them out. I bought a bunch of things last spring from a guy in north jersey parting a 2000. He kept reposting the engine for $500. It would be cheaper to get a used engine than to rebuild yours. Good luck.
Just an update- I wasn't able to find a used motor I was happy with so I went to work again on mine. I had to split the piston across the top with a hacksaw, then split it with a couple of screw drivers. I tried again to press out the piston pin and now I could hit it with a torch, but that was still not working so I thought I would try cold. I knew I could control that better, so I ran upstairs and grabbed an aerosol can of air. I tipped it upside down filled the pin with liquid Cold. Then as I was pressing it I hit it with the torch again an pop! It finally moved. I came out ok after that. I dropped the cylinder off to get bored out to 20 over ordered up a wisco piston set.
So now the motor is back together an mounted. I changed the front gear case fluid tonight and hoping to do the tranny tomorrow. Lots still to put back on but I'm getting close. This is about as much work as I have ever done to a motor, so I am excited to fire it up. My only concern is the oil pump. It was just too much work to split the cases to check it out.
I did find that the oneway valve was missing the spring and the rubber plunger was almost all gone. I don't know what would happen with that broken.

I will try to post an update when it is complete.
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Sounds good so far, should work like a new one when you're done!

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