These bikes greatly benefit from a gearing change, but just a small one. When it comes time to replace your sprockets, keep the front stock (17T) but go down one or two teeth on the rear. Stock rear is a 34T. Get a 32T or 33T. On a 16" rear wheel, I like a 32T, on an 18" wheel I prefer the 33T.
And I never did comment on fork oil so here goes ..... I stick with the recommended 10wt. oil. One of the issues with these forks is they're not real responsive to the little bumps, jolting you through the bars instead of soaking them up. I feel that using heavier oil would only make this issue worse. However, I do feel more than the stock amount is needed and works better. As mentioned earlier here, the stock amount is about 5.8 ounces per leg. I use 6.5 to 7 ounces. I'll go with 7 if the forks were torn down, all cleaned out, and totally dry inside. I'll use 6.5 for just a drain and fill. I have a homemade leveling tool I use to equalize the amounts from side to side. It's basically the same thing as the store-bought ones, a length of brass tubing with an adjustable stop, and a little syringe on top .....
I did some fork "tuning" on my '83 recently after the rebuild the forks got. Since they were stripped and cleaned, I started with 7 ounces of oil but that proved a bit too harsh. So, I sucked some out of each leg. I ended up going with the often mentioned setting of 6" from the tops of the tubes, springs removed and forks fully compressed. I measured the total amount I sucked out and it was about an ounce, so that means I ended up with about 6.5 ounces per leg. That may be the golden number there for these forks. Adding the extra oil greatly improves the fork function. They won't dive nearly as much under braking. I also noticed when I filled the forks on my '78 with the stock amount of oil, they were very "squishy" when pumping them up and down. They actually made squishing noises. Adding more oil fixed that too.
Going back to the unresponsiveness to little bumps, many of us modify our damper rods by doing something called the "Minton Mods" to them. Basically, this is just enlarging the holes in the damper rod slightly, and adding one more small one at the top .....
This allows them to flow oil easier and better, and that improves the fork response. It's a simple, free mod you can do and it really helps.
For fork springs, I put the MikesXS progressive springs in my '78 and they work nice, but I can't recommend them today. I bought them years ago when they were cheap but they have raised their price so much now, you can get real Progressive Suspension springs for about the same. On my '83, I thought I'd try to save the cost of new springs and just add some preload spacers to the originals .....
I always liked the stock springs, they are a nice dual rate type. They were always too soft though, even with the preload cap adjusters cranked all the way up. The caps have two steps, each adding 10mm of preload, so you can add 20mm total. I figured spacers a bit longer than that should do the trick, so I made them 1" (about 25mm). On paper, this looked good, about a half step stiffer than the stiffest cap adjuster setting, but it was actually more. What I didn't take into account was the washer I had to add under the spacer, that added a few more MM to the preload. So, I was actually preloading them 27 or 28mm, close to a full adjustment step more, and that proved to be too much. The forks were too harsh and stiff. So, I shortened my spacers about 3/16" and now, combined with the washer, they're giving me 23 or 24mm more preload. And this seems to be just right.
So, to recap, 6.5 ounces of 10wt. oil, the Minton Mods, and 13/16" preload spacers added to the stock springs, and the forks are good.
there are loads of differing opinions. The FACTS are that yes rotella diesel oil is a ONE of the recommended oils for older 4 stroke wet clutch bikes. I have run it with no problems as have many others.
