I decided to pull the whole top end off the motor rather than try and perform spring replacement surgery with just the top cover of the motor off.
Of course this led to inspection of the moving parts in the top end. The cams were scored, flat spots and worn out chrome on the rocker arm faces. The plate on the cam sprocket was brazed in two spots. I'm replacing all that stuff with a pretty fresh looking set I found on ebay.
The tops of the pistons and cylinder head have hard carbon deposits as Loud Mouse referred to above.
And the springs sure were loose. There was probably 2-3mm of slop. You can see it where the loop of the spring pokes through the hole in the weight and in the play between where the weight is supposed to remain in contact with the center of the sprocket.
I tried the new springs but the slop was still there. I pulled them off and compared them to the old ones worn ones, and both sets were about the same length. So, I bent a little slack out of the old ones, replaced them, and now I have a snappy tight spark advance unit.
I hope this cures the issues I had with starting the bike, sluggish take off, and inconsistent timing between the two cylinders. My only remaining concern is how to best remove the carbon deposits from inside the cylinders. Zep morado? Wire wheel? oil and carb cleaner?
nander
advance unit springs
I'm reassembling the top end and double checking everything. It all seems good, I just wonder if I may have created too much tension on the advance springs.
Ideally, should the initial position of the advance weights be in contact the center boss on the sprocket? In all of the four units I have removed, there seems to be a couple of mm free play before the spring action begins. nander...
I pulled my springs out the other day (my top end is still together) and like yours, the old ones are the same length as the new ones. As you have noted, a simple swap won't cure the slop so I will end up doing some bending on the springs as well. I'm not convinced that the weights (arms) should be tight to the center boss in the resting position. I'm guessing that the limits of travel for the arms is determined by the diameter of the two holes in the sprocket and the diameter of the two posts at the ends of the arms. These posts appear to have a neoprene? outer surface, and in my case, this has been been pretty much worn away. That reduces the diameter of the posts, thus increasing the travel of the arms to the point where they now stop against the center boss. The free play is not because the springs are too long. It's because the arms are coming in too close when at rest. (It could also happen if the rivets are loose, but in my case they are not.) I wanted to see how much my advancer travels from resting against the center boss to full advance, so I put a protractor on the end of the cam and measured very close to 23 degrees of cam advance (I haven't put the new springs in yet.) This equates to 45-46 degrees of crank advance. Bill Silver says it should be 40-43 degrees, so I'm thinking the worn neoprene has led to 2 or 3 degrees more advance. He warns about not exceeding the maximum, so when I get it all back together, I think I will retard the spark by 5 degrees at idle to compensate. I haven't had my cam sprocket out of the head and I haven't seen any good photos of one, so I'm guessing about all this. Now, to bend my springs... Wilf
wilf,
i thought the missing neoprene might account for some additional travel. please let me know how you intend to measure the 5 degrees you want to retard the spark at idle? are you just going to time the bike dynamically so that it doesn't exceed the full advance marks and disregard what is indicated for timing at idle? if so, won't this result in a very retarded initial ignition timing and slow acceleration at low engine speeds? nander nander,
The 5 degrees is the difference between the F and the T marks on the rotor--instead of static timing to the F, I'll time it to the T, retarding the spark by 5 degrees. That will indeed result in retarded idle timing. I don't know what it will do as far as acceleration. From what I've read, it's more important to not exceed the max advance and the possible damage it could cause. Am I correct in assuming your neoprene bits are worn or missing? Wilf
yes, nothing that really resembles neoprene on the three advance units i've inspected.
what concerns me is that the reason i got into this whole mess was the timing -- my being forced to set the timing of one cylinder more retarded than the other. i had trouble starting the bike, it lacked power at low engine speeds and had sluggish throttle response. i attributed this to the excessive retardation of one cylinder. i don't have the book in front of me right now, but bill silver mentions a "performance modification", welding up the holes in the advance unit to limit the travel of the advance weights in the holes. i guess this increases initial advance but limits the amount of advance possible overall). if that modification indeed enhances "performance" (he claims better acceleration from stop/low speeds) by limiting the the amount of advance, i would imagine the (opposite) condition we are encountering, with potentially more degrees of timing available, will lead to poor performance. what do you think? is it time to weld up a couple of stops on the sprocket? Difficulties in timing led me into this mess as well. Using a timing light at idle, the timing marks were moving all over the place and I had difficulty getting a consistent idle--the engine wanted to rev too high or die, no in between.
I put a protractor on the cam (this was before I removed the springs) and found there was a 9 degree difference between when the weights were at rest against the center boss and when they just started to contact the springs. 9 degrees of slop on the cam equates to 18 degrees of advance slop at the crank, so it's no wonder the timing marks floated around so much at idle. That's when I ordered the new springs. I now believe that new springs are not the cure, and bending the hooks on the springs is not the cure. The problem is the worn neoprene resulting in too much advance arm travel. And I can't see an easy cure for that: somehow make the sprocket holes smaller; somehow increase the diameter of the posts on the arms? I don't know how to weld, so I'm left with two options--bending the hooks on the springs or trying to find a new or 'newish' sprocket/advancer. Bending the spring hooks should make idle timing/advance more stable but will not reduce the total advance. That's why I'm considering retarding the spark at idle. I'm not sure how much retarding will be required. I'm thinking that because the springs will have to stretch more for increased travel, it may become more difficult for them to do so, and that may help to limit the total advance at higher rpm's. I've attached a photo attempting to illustrate some aspects of our discussion...
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