Rebuilt shock has no compression dampening?

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Mar 26, 2001
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#1
I just rebuilt the rear shock on my 83 YZ250. I have never rebuilt a rear shock before but I figured it was worth a try before I spend $100 having somone do it for me. I dissasembled the shock, but did not dissasemble the valve head. I also replaced all of the seals and o-rings.I used ATF fluid like many on this board have suggested. Once I had the shock assembled and charged I put in on the bike to test it out. The shock has good rebound dampening, but almost no compression dampening. It almost feels like the shock has no oil in it at oil. Here are a few things that I think might be causing the problem, would some of you suspension wizards please tell me what you think.

1. Some air might be trapped in the shock.
2. Incorrect pressure on the nitrogen reservoir.

Are you supposed to fill the reservoir with as much oil as it will hold before you fill it with nitrogen? I just pushed the plunger in the reservoir about half way in before I filled it with oil. That means that half of the reservoir is filled with nitrogen. Could that much nitrogen be compressing and letting the oil flow in and out of the reservoir under compression? Any other ideas? Would someone please go through the steps of filling the shock with oil. Thanks.


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marcusgunby

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#2
Setting the piston height for the N2 is critical to proper working, i think it is 90% of the volume of the N2 chamber.Im not sure it is the only problem though.It sounds like you also have air trapped or maybe something else.

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MACE

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#3
First my assumptions of the situation:

1. 1983 YZ has a remote reservoir with a floating piston (not a bladder).
2. You replaced the shaft seal by removing the lower clevis - since you say you didn't disassemble the valve head (piston with all the shims).
3. As indicated above, I assume a 1983 YZ has a piston with shims similar to later model bikes.
4. This isn't the YZ with the B.A.S.S. valving.

Since you have rebound damping, I assume your piston ring is sealing adequately.

I bet you have air in the top of the shock and/or in the remote res line. That air bubble flows through the compression needle orifice resulting in little damping for small to normal hits. You may be getting some damping after the bubble has been forced into the res and fluid is working in the needle valve instead of air.

The rebound doesn't see the bubble because the shock piston and rebound circuits are down below the bubble.

If you have a reservoir piston (not bladder) set the level as follows: Guys, correct me if I get this wrong!

1. Orient shock body and reservoir with the open ends up. Raise the remote reservoir until the res line is always climbing (i.e. no low or high points in line). Open end of res must be lower than open end of shock body. Clamp both bodies with vise and or clamps & lumber so you won't run out of hands as you bleed the system.
2. Open compression valve full open.
3. Fill shock body until fluid fills reservoir completely.
4. Rap all parts with a screwdriver handle to dislodge any trapped air bubbles.
5. Wet the oil side of the reservoir piston with fluid, make sure the res is full to the top and piston ring is in place - insert the piston into the res and push all the way down until it hits bottom. This will force fluid back into the shock body. Hopefully no bubbles will still be coming out by the time the res piston is bottomed.
6. Shut the compression adjuster all the way (lightly seated).
7. Wet the shock piston and insert into the shock body. Work the shaft up and down to dislodge air from the valve.
8. Open the compression valve and
9. If the res piston moved, push it back down.
10. Fill the shock body to the top and with the piston valve near the top, install the inside seal head retaining clip, the seal head and the outer clip. Fluid will be everywhere on your work bench by now. You'll learn to like it.
11. When you installed the seal head you should have seen the res piston move due to the fluid the seal head displaced. Now push the shaft in and you should see the res piston move some more - probably about 1/3 of the res stroke. If you don't have enough free volume in the res you can overpressure the shock on full compression. If you are really overfull you can hydraulic lock the shock.
12. Reinstall the res caps and you are ready to charge 'er up.

Keep in mind I'm not an expert on this shock. I just thought I'd take a swing at your problem and figure if I'm too far off the real experts in this zone will correct me.


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MACE

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#4
Marcus,

That sounds like a reasonable procedure. If the line from the shock body could be removed, the last bit of air might be removed after letting it sit overnight. Getting the air out is most important.

imq707s, Would you do your own dental work if it saved you money? ;) (Just kidding)

Filling the reservoir half full with oil sounds like a DANGEROUS thing to do. What if it hydraulic locks or busts apart from too high of a pressure? You need to be sure there is adequate volume for the nitrogen to compress as the rod displaces oil into the reservoir. There should be a measurement as marcusgunby said ~90%. Please be careful for your own safety.

James

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MX265

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#5
This is why I leave this stuff to the experts. I hardly have a clue what half of this converstation means. All I know is that shocks have oil, nitrogen and a very hevy spring. I'd be lucky to get through replacing the spring without getting by head taken off, or worse, without having the spring fling through the windsheild on my wifes Jeep Grand Cherokee. I'd loose something worse than my head. :eek:

My theory, when in doubt, let the experts do it. It's worth the $100.00. It's safer and easier. I did learn a heck of lot though.

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