OK, I replaced the oil in the forks of my Suzuki GT380 and they got real stiff. maybe too stiff. I figured that all of the old oil might not have been drained so I decided to take out an ounce. Sure enough, the oil in my right for had old oil mixed in with my new red Honda 10w oil. The left had only a minute trace of the old oil. I drained about an ounce out of the left fork and about 1.5 ounces from the right. My fork actually moved somewhat. A quick ride around the neighborhood indicated that the fork was working. However, although I have good dampening action on compression. I have very poor rebound. What could be causing this?
I am not a motorcycle fork rebuilder or modifier, but I would suspect air.
In reply to Ranger50:
I would think that air would compress, just like in an unbled brake system. SItll I drained soem fluid and compressed forks, no air bubbles. I took a little more fluid out and it was better.
Motorcycle forks generally have the oil level set 1 of 2 ways: either there's specified amount or there is a height with the fork fully collapsed and the springs out. Yours is probably done by oil volume. If there's too much, the fork will hydro lock because the oil won't compress. If the fork won't fully compress, air will be trapped in the damper rods.
The 10wt is way too light for an olde skool damper rod fork, you want 20 or 30 wt. That's what happened to your rebound damping.
In reply to Curmudgeon:
Interestingly, my Clymer book for the GT380 suggets 10w or ATF. Onone of the bike shops around me sell anything heavier than 10W. I tookd some out and I have some rebound now.
If I were guessing (and, er, I guess I am), I'd suspect that the too-small air volume was creating a bonkers-high spring rate, and your rebound damping which would have been okay for stock springs and correct air volume was overwhelmed by the air spring rate.
In reply to Moparman:
No kidding? Learn something new every day. All the stuff I had from that era (including my current XS650) use 20wt or better. We used to use ATF in dirt bike forks after brazing up holes to stiffen the damping, it was supposed to be something like 15 wt.
In reply to Curmudgeon:
I was surprised too. Maybe Clymer is wrong. Took a few mire MLs out and it is just a little stiffer than "normal."
