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Discussion Starter #1 (Edited)
Hi All,
1st off thanks to all of you posters that I have already learned so much from.
I am new to this site, and new to quading. Actually I've never really riden one yet.

I picked up this 2001 Magnum 500 with about 1300mi, 174hrs 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

Thanks,
John
 

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If it is blowing into the crankcase then yes it is the rings themselves that are shot. As far as the valves I do not know if they are hydraulic on the machine or if there is anyway to shim them. If they are hydraulic it may just need new seats and to lap the valves. Check the edges of the valves to see if there is any wear or chips in them as this is usually signs of wear caused by the motor sucking in dirt particals which will also wear out the piston rings.

Best bet is to pull the motor down fix the valves, find an oversized piston and rings (like maybe 10 thousanths over bore) and have the cylinder professionally bored and honed so you are starting new and fresh for a healthy motor

Bigtime
 

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Discussion Starter #4
I was afraid of that!

I was really hoping to get it running on the cheap, but that would probably be a big mistake.
You dont think that the material that wore off the cam could have caused extra wear in th cylinder and rings?
Should I stay with the oem pIston and rings?
Thanks,
John
 

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It is very possible that is what happened. When you took it apart was there dirt in the airbox tubing to the carbuerator or in the carb itself? I would suggest not doing it on the cheap especially since it always costs more in the long run. Look at my post entitled Not Again. Thank god I dont have to pay for it.

As for the piston I would try to stick with oem if you can. It all depends on if there is cylinder wall damage and whether you can get overbore pistons that are oem. If the cylinder doesnt have scratches at least measure the cylinder to make sure it is square or uniform dimensions all the way through. Again look at my thread Not Again. If you decide you dont need to bore it at least have the cylinder honed to put crosshatching in it to help the rings to seat better and faster.

Bigtime
 

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Nothing cheap about this fix. I would first get a clymer repair manual, so you have the correct proceedures and specifications. To bad it just wasn't the valve seats, and you could do a valve job pretty easy, and also described in the clymer. I would start first with the valves and you really can't mess that up, its just very tedious and time consuming when done right and carefully. I would mike all the parts before you do any repairs tho
 

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Discussion Starter #7
I don't know how to put up pictures or I would. I ook the cylinder off today and the piston sides and skirts are scraped up pretty bad. I am having trouble removing the piston pin. I tried to press it out with a c-clamp, tried adding a little heat to the top of the piston, but still not coming out. So I need to Get the piston off then I plan on bringing it to the machine shop so I can figure out what piston and rings to order. I still need to evaluate the valves too.
 

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I usually heated up the piston with a propane torch, ALOT. All around the circlip area. The aluminum will expand much quicker and more than the forged wrist pin. It most likely is oil seized in there from burnt oil varnish. Fuel oil or kerosene sometimes breaks it down. Just dont do any pounding, dont wanna bend the crank journal. I too would stick with OEM, I've never had good luck with aftermarket, other than our race machines that were rebuilt and replaced often. Check out speedwerx/hot seat if you do go the big bore route. I had lots of quality work from them.
 

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Wiseco makes an oversize for it, if you removed c-clips from piston you can usually use a 3/8 drrive extension to drive pin out
 

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you should also be careful when preforming a leakdown test if the air goes into the crankcase it could blow the crank seals out then you would have another problem on your hands so don't use too much air psi
 

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Discussion Starter #11
Update

Wayne I read about the sels in the manual, but I assumed that wouldn't be a problem if the valve cover was off. There would be plenty of roo for the air to escape out the cam chain area. Did I assume wrong? How would I check to see if the seal was blown out?

The pistonis worn pretty badly on the front and back. The piston was not moving freely on the pin. The machine shop estimated .020 over would do. It was a very quick estimate though.

The Piston pin is stuck in the rod, not in the piston. Inhave tried heating up the rod and the top of the piston, but it wont budge.

I figured I would take the motor out and bring it to a shop, but what are the chances of pressing that rod out and not damaging the Rod? I was trying to figure out how to get the Drive clutch off and came across a post listing the bolt needed to pull it of as a 3/4" fine, so I started looking around. Turns out a trailer hitch ball comes in 2 size threads and the smaller on is 3/4" Fine. I had on lying around and it was long enough to thread i past all of the threads in the clutch. So I cut a spacer about 4" out of an old motorcycle rear axel, ground it smooth and tried it out. the clutch popped off with no problem.

I am thinking at this point I may be better off picking up a used motor. Would a Sportsman 400 fit and would it have enough power?
 
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