front fork travel ?

on a 77 LTD 1000, what is the distance the front forks should travel before they bottom out?
I have looked in the FSM and dont really understand the weight to distance chart.

The distance I am seeing on the bike, seems to be about (looking at the wipe area) 1 5/8 inches, I am wondering if this is too much thus causing the bottom out feeling when I hit a bump

If the forks are bottoming out easily, there is too much sag, the springs are sprung, the fluid level is too low, the fluid is worn out, or most likely, a combination of these.
How long has it been since the fluid was changed and the forks carefully cleaned out? If you can't remember, that's too long. Doit.
Later model LTDs have air-adjustable forks. If yours are too, try a little more air pressure. Be careful, it is very easy to over inflate them. In a perfect world, the forks should compress (sag) about 30mm from completely unloaded to bike and rider weight on them.
Here is a link to an article that explains it in quite a bit of detail. It is for scoots with much more adjustability than KZs have, but the ideas are the same.
www.sportrider.com/motorcycle_suspension/index.html

with the front wheel off the ground measure the distance from the axle to the bottom clamp. then with a second person's help take a measurement while your sitting on the bike. that should turn out to be about an inch to an inch and a quarter. things i can think of are. the springs are very slacked and bottom too easily. there isn't enough oil or the oil is too light in wt. the springs are way to stiff making the ride feel harsh. the forks are overfilled with oil or the oil is too heavy a wt. most big road bike's fork travel was 5~6 inches give or take back then.

Tex makes a good point about old fork oil. i have seen old fork oil that was turned into something close to a black margarine. that prevents the forks moving and makes for a harsh ride.

I replaced the seals last year, and cleaned and changed the oil, it had a name on the shocks of someone who had rebuilt them, but I dont remember the name.
It does not have air adapters on them, I have seen them on other bikes.
I am getting the feeling that they are bottoming out when it hits some bunps, and that is not a good feeling.

When you replaced the seals, what fork oil did you use (brand and weight) and how did you measure the amount that you added?

dont remember the bran, but it was purchased from z1, I do remember converting the amount from metric to english and measured it out that way, both were done to the same amount

The reason I asked is you may want to consider changing the fork oil again and going with a little heavier weight. I use Bel-Ray 15w in my KZ650C1 and have never had the forks bottom out even when riding 2-up (I weight in at about 205 and my wife is 115ish). If 15w doesn't make the front stiff enough you may consider Bel-Ray 20w. They make a 30w, but I suspect that would be overkill as you don't want to lose front fork movement or sensitivity. Regarding the amount of fork oil, if you have a factory service manual for you year/model bike it will tell you the correct distance to measure from the top of the fork tube down to the top of the fork oil level when the front end is up off the ground. This measurement on the KZ650 varies from model to model / year to year, so it's important to use the correct distance for your model. This is the only really accurate way to ensure the fork oil level is correct. If you don't have the manual, I'm sure someone on this site can provide the correct spec. Ed

There are also specifications in the service manual for the free length of the fork springs, and a chart that shows the spring rate. If you want to check the rate, you can get a bathroom scale, set the spring on it on its end, and compress it by hand to a certain length (say 15" for instance) as measured on a tape measure, read what the scale reads, and compare what you've found with the chart and see if your spring has softened. I use a similar process with a drill press as a valve spring rate checker. You might have to convert your measurements into metric before comparing to the chart, I don't remember if it has english values.

Oh also, I'm not sure if your forks are the same as the kz900 which I have, but the shock absorber cylinder in the bottom of the fork has about 6" of travel itself. I'm not sure if the spring binds (coils completely closed) before that point or not, but the max travel you will have is that ~6".

Bobby:
My FSM calls for 245cc of 10W fork oil for an '81 KZ650D & F, and 291cc of 10W for a KZ650H (the CSR).

Before replacing the seals in my forks, I drained the old metallic filled oil from the forks and temporarily replaced it with 20W-50 motor oil. Could hardly detect any difference in front suspension stiffness.

When rebuilding the forks, my springs showed some wear where they touch the inside of the fork tubes. The spring stiffness is surely reduced by this wear. With no air pressure, my motorcycle would "bottom out" when crossing railroad tracks, etc.

With 15W fork oil, and no air pressure, the travel from "bike upright with rider mounted" to full compression (bottomed out) was only about 1 3/4". So I gradually added air until it rode firmer and quit bottoming out. Resulted in carrying about 15 psi with rider on the bike. With that pressure the distance increased from 1 3/4" to almost 3". I realize that I am compensating for weak springs by using air pressure.

My guess is that your springs are getting quite weak (as evidenced by the 1 5/8" travel). Best, although costly, solution would be to install progressive rate springs. I believe the springs are the problem, and that oil viscosity and cleanliness although good maintenance practices, have little affect on suspension firmness. Just my $$$ 000,000.02 worth.

The fork oil viscosity most certainly DOES affect fork firmness; it is the single greatest influence on firmness. Fork oil is forced through small orifices as the front forks compress and rebound. Higher viscosity oil flows more slowly through these orifices than does low viscosity oil. Therefore higher viscosity oil results in slower compression or rebound of the front forks. This is what you feel as “firmness.” The fork springs also affect firmness, but more from the standpoint of how much WEIGHT does it take to compress the front end rather than how FAST does it compress or rebound. The springs do control the amount of sag, that is to say the height of the front forks. If you were only relying on the springs for firmness the front end would bounce up and down like a pogo stick or like the body of a car with worn out shocks. The factory service manual does give the spring length service limit that you may want to check. Also, use FORK oil not motor oil, baby oil, olive oil, whale oil, or canola oil. You wouldn't run fork oil in your motor. Why? Because you know it is not the same as motor oil. So why would you think motor oil is the same as fork oil and put it in your forks? Ed

650ed:
Your post is very informative. The first time I drained my fork oil, it was loaded with metallic particles. I ordered new 15W fork oil from Z1, but until it arrived, I used the motor oil as an experiment, and to keep from pouring the nasty fork oil back in. (I knew that I would be draining and internally cleaning the forks anyway). I just could not detect an increase in firmness.

After cleaning out the forks and adding the proper amount of proper fork oil, the bike would still bottom out on railroad tracks, IF there was no air pressure added. My springs are probably weak, so I have been using air pressure to compensate. Progressive springs some day.

My main point is, that there is a total travel length of about 6" from fully extended to fully compressed, and when the "average ride height" is low, with only 25% from being fully compressed, there simply is not enough distance to keep from bottoming out occasionally. What I have found, is that when my ride height is from about 40% to 50% of fully compressed, that I do not experience bottoming out. I am open for ideas.