Timing chain & bob weights

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Jan 18, 2006
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Well, all this talk of uneven idling encouraged me to remove the points and AA weights and side cover to check the timing chain.
The bob weights didn't appear to be sticky but definately the slot that the pins run in has a central spot that is shiny and worn.
The timing chain when adjusted to 3/16" play at its loose section is rigid tight at its tightest, this seems to happen at a couple of places around the chain. Is this enough to cause erratic idle (i think so) and where is the best place to buy a chain around Australia.
That's what I found too, worn slots in the AA weights but mine was not serviced, the mechanism was "dry" and the weights did not move smoothly.

Complete AA units are not available new but at least in my case new weights would have been enough (now I got hold of used ones in good condition that I installed).

I have Trispark electronic ignition to install, but first I need a new timing chain as the old one has worn spots making it loose in areas and tight in others. These parts are 30 years old so to be expected.
Another one of my off-the-wall questions.

OK, the chain has places where it is stretched, and places where the links are shorter and it is tighter there. But this chain is moving at a good speed, opening the valves and such. Even at an idle, this chain is turning so fast, how is that bit of unevenness going to translate into some unsteady idling? All this is happening so fast, how are you going to be able to see that the valves are sometimes opening a thousandth of a second earlier, or later than they should? Could you really notice this? The case with the weights and advance... That is a fact...and the reason so many of us have done the Boyer thing. Problem is then gone...period. I would replace the chain, if it is stretched, good idea, but I can't image it causes uneven idle. Rather, I think the general timing of the valves would be changed and you could only feel that at full throttle as a loss of power.

Now it is time for the gurus to step in like they do, and educate me otherwise...perhaps I'm very wrong :wink:
erratic idle

Well the guru's havn't stepped in, yet.
But in my mind's eye ( and this may not be true, I'm no guru) that chain which is worn looser than it ought to be is driving the cam and the points and because its loose and slapping erratically around sometimes tight sometimes gathering up and pushing then the cam and point timing is going to be erratic. At least more so than it ought to be.
I presume that it is loose in the areas that it would do most work ie when lifting the valves so it would be worn when lifting the exhaust valves and when lifting the inlet valves. The points should be opening when all valves are closed in a tight spot but with the loose chain this would mean it would fire a few degees later or earlier or erratically at around 1000 rpm 4 times a second?
Would this make a difference? I don't know.
When i've replaced the chain I will install the electronic ignition so I still won't Know how much the chain has contributed to the problem.

Is it the valve springs pulling and pushing the cam that's causing the slack and tight spots?

I know Camshaft "thrust reversal" from the valve gear has been been suggested elsewhere as a possible cause of erratic ignition timing, as the AA unit could be affected by harsh camshaft acceleration/deceleration as it rotates which could cause the AA unit bob weights to fly open or close momentarily, altering the amount of advance or retard?

It has also been said before that Lucas were not happy about their AA units actually being used mounted on the end of a camshaft in the first place?
I have often wondered ( usually when Im trying to find top dead centre)
how inaccurate is it to turn the engine over by turning the rear wheel?
This is the method suggested in the manual to measure the tension on the timing chain. Turn the back wheel and check tension every so often around the chain.
I guess the rear wheel is turning the engine thru the primary drive - thru the crank which in turn turns the cam chain so it doesn't matter. The effect is the same as pistons driving the crankshaft.
Valve springs affecting chain tension is a whole new situation that happens so quickly its hard to figure its effect on timing and idle.?
Cash, I guess the only way to find out if tight spots are actually caused by valve spring and cam pressure is to remove the chain and measure the wear with the chain off.
Can someone tell me , when you replaced your timing chain for a new one and checked the tension. did you find it felt tight and slack in a couple of spots ? or once the tension was set it was the same whatever the position of the crank/cam sprockets? :?:
This last question...a good one and I also am interested in an answer. My point with my question above, boils down to whether you actually would be able to notice a different and unevenness in the idle speed, just because of a little old difference in the length of a few links, on a chain going great guns even at idle speed. All this speeding and slowing down of the valve opening and closing, happens so fast...if you can actually notice it by listening to your motor...you got better ears than I do. Fur shure, fur shure.

The effect of weights flying about in the ignition...that's really enough to notice, but I bet even the weights, don't move and effect timing on every beat. That would happen every few beats or in a gradual swing from fast to slow advance and a flutter sometimes to boot. This crap and the awful timing caused by weights, made me hate the old Volvo I still have in the shed. I would have traded my first born kid for an electronic ignition for that stinker. Having to adjust the points every two weeks, didn't make me love it, either.

I am interested in the results of the new chain though. Mine has been in since new (about 120,000) and I haven't had the cover off in maybe 7-8 years either , so I have started to wonder if my chain is finished too and if I need to think abut replacing it. If a chain is, as stated, loose and really flopping around, then you will have timing problems. Just not sure how a chain that is a bit stretched "here and there" will have a noticeable effect of the idle.
The effect of the jumpiness caused by timing chain slack is best observed with a strobe. But with out a bench mark and a practiced eye your not going to see the subtle difference. Now with two bikes side by side with a strobe and a dyno you would see the difference very clearly. We have to trust people who have tuned hundreds of Norton's and have done lots of careful observations. And as always use your own powers of observation regarding the system of the motor. As LAB pointed out, Lucas said "your going to put the AA unit on the end of a cam? " Our bikes were built to a price. There are better places for a distributer.
The easy way to tell if its the cam thrust tightening the chain is to turn the cam gently anti-clockwise with a spanner. You'll feel the spring lift and see the cam move a little just before the crank does, hold it and check it then.

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