Ok, thanks. Now for a dumb question. When holding the advance in your hand how much play is normal? The weights are held back by the springs, but the center section that slides onto the cam has play when holding the unit in your hand and inspecting it. In fact the hold thing seems rather fragile. Also, on the rear of the unit where the weights are connected there are small blazed metal plates near each end. I think someone mentioned those could be custom weights? I don't really want to buy another of this one is ok. It does advance to "F" when the engine is reved up as indicated by a timing light.
They do seem flimsy but they aren't doing all that much. They wear in a few places over time. The springs get weak, most of us will replace springs when trying to solve timing issues. The little springs are not cheap either and they sell each not by the pair. 12 11 1 357 627 $10.39 according to Max but there are two different weights of spring and I didn't find another part number so I don't know if this is the one called "heavy duty". I think you want the HD ones. If you decide to get these. The "D" shape that indexes the advance unit is worn and the advance can be rotated in advance or retard direction before the small nut is tightened . This actually helps sometimes to get the bike timed. It's just something you have to remember when the plate doesn't seem to move round far enough. There should be some small fiber material washers and very small E clips that hold it together. A lot of riders will polish the pieces so they can slide easily against each other. You seem to have a restricted amount of movement to the advance. This could be something done on purpose by a former owner or it might be worn springs. It could also be bent posts, the posts the springs attach too. The small posts I mention should be straight up. If they are then they aren't bent. Another way that Hot Rodders try to customize the advance unit is to weld or braze the hole that one pair of posts is moving in. By making the hole smaller the amount of advance is restricted. If this has been done you should be able to see either weld material or the more common brazing bronze material on the top piece of the advance with the holes in it. No evidence of modification? Then you probably need new springs. You have the latest edition advance unit, that is the latest that they made before they stopped making them. It is the one you want to have. There is another more rare problem that does pop up sometimes. You should check for a double timing image. You will notice that because of the physical arrangement of things the two lobes of the advance unit each fire for one side of the motorcycle. Don't get this confused with the lost spark or extra firing. Both plugs fire at the same time so they fire at every TDC. One cylinder on compression is a working spark and the other cylinder on exhaust is a lost spark. This is checked with a timing light, inductive pick up non advance timing light is about the best, easiest to use. You have to watch in the timing window carefully for a double image of the timing light firing through a wide range of RPM. If the advance unit lobes are uneven or the tip of the cam is bent the firing of the plugs will be uneven. If the light is showing the F mark or the S mark and then it shows the next firing a blank mark it will be difficult to notice because you think you are seeing the mark you want. It sometimes looks like there is a ghost mark. In addition keep inmind that there is sometimes a little irregularity to the firings and the normal mark is not dead steady. Good luck with this. I have tried to make it as plain as I can. Maybe I succeed, it's plain as mud to me. You are doing well. The bike is or should be rideable as long as the full advance is being observed.
I'm holding a brand new advance unit in my hand and yes, they are a bit sloppy feeling. remember, the top plate with the D shaped hole is bolted to the end of the cam, and the other part is centered by the shaft itself. That will eliminate 99% of all "sloppiness". There are two thin plates welded on the bottoms of the weights.
Thanks to you both, and others here for the info. The bike runs like a bat right now so I shouldn't really complain, but I'd like to get it perfect at some point. The timing issue naws at me. So, when I have time I dig back into it.... I think I'll order the new springs. Not all that much in cost, and if I don't need them at least I won't be out much cash, if I do need them it's a cheap fix... Can't lose with that logic.
sorry to resurrect this, but did you ever figure it out? i've always had the same problem and have never been able to figure it out, as seen here. http://advrider.com/forums/showthread.php?t=840445
I'll add another thing to test. When I first got my 90/6 and started doing the research I read about the 'double image'. Now I knew that the timing chain was old and slack - I could hear it thrashing around in there - but only had a single image when I shone a strobe through the timing hole. Then one day after adjusting the points using the static timing method the bike ran really badly; popping and backfiring, not running at all well. I borrowed a strobe and pointed it at the advance unit. A double image for sure, but out by a lot ... I went back to the static timing and rotated the engine to check both sides a few times. Found that they were way out of sync. I made a paper indexing disk, glued it to the alternator and measured the difference to be around 30 degrees. Clearly, the cam nose was bent, but not by much - it doesn't take much of a bend to make a big difference. You can see by the first pic that the original threaded end of the cam had been broken off and replaced with a bolt. Somewhere along the way somebody had also tinkered with the shape of the points cam. Since I couldn't afford a new advance unit I resorted to reshaping the points cam again, and got it pretty good. O, and I also replaced the timing chain. I now have a comfortably close double image through the timing hole when I use a strobe. I'm throwing my unusual experience into the mix, just in case it is useful to others out there. If you have a 40 year old bike with stock points and you can't see that second image, look further. It may be well outside the window. Good luck.
