SOHC.co.uk Forums > CB750
Cb750k timing marks
Martin6:
The ignition is at full advance at quite low rpm, at about 2700rpm, IIRC. So when we're riding, it is fully advanced nearly all the time.
There was a comment on the US site that it's best to think of the advancer as almost an on off switch, to retard the ignition for a stable idle. I doubt if it goes to full advance at 2300rpm say, it will make much difference out on the road. But if your advance springs are worn out, it may be advancing even lower, down at idle revs. That will lean your idle and could result in uneven idling if the advancer is moving around. I've seen mention of nipping one end of the springs off, just where the hook is formed at the end of the spring and reforming that hook. Not needed to try that, but if you have an unstable idle, that could sort it.
For my 750, having checked my advance unit is free and smooth, I don't use a strobe to time it. I static time it using one of those cheap screwdrivers with a bulb in the handle and a wire with a crocodile clip at its end. I adjust the points plate to trigger on the 'F' for 1&2. Rotate 360 degrees to check, then move on to 2&3. Adjust the points first though, as this affects the timing. 👍
K2-K6:
Martin6, yes agree the full advance position is more important in reality.
Essentially controlling fuel burn at maximum duress and load is much more important than idle control.
Retarded position is just to facilitate easy starting and tickover speed control by retracted timing for convenience really.
Can't really see a problem with early advanced curve though, assuming no pinking, as it will improve engine torque generally if you can get it advanced as soon as possible.
I dont know why there's advice floating around to delay the advance curve by shorter springs, seems undesirable to me. Competent control of retard to ensure even tickover, yes. But other than that there's no reason to restrain in reality.
Ideally, whichever method of initial setup, then maximum advance should be checked and verified as correct as good general practice.
Bryanj:
Shortening springs is not to delay curve but to stop erratic advance at tickover, or at least thats the way i see it
K2-K6:
--- Quote from: Bryanj on July 11, 2024, 12:55:20 PM ---Shortening springs is not to delay curve but to stop erratic advance at tickover, or at least thats the way i see it
--- End quote ---
Yes, I can see why they've tried to correct if it's obviously slack or showing advance down below about 1500 rpm but not considering it'll increase the spring rate, which will have more influence on delaying the onset of advance.
cbxman:
--- Quote from: K2-K6 on July 11, 2024, 12:18:40 PM ---Martin6, yes agree the full advance position is more important in reality.
Essentially controlling fuel burn at maximum duress and load is much more important than idle control.
Retarded position is just to facilitate easy starting and tickover speed control by retracted timing for convenience really.
Can't really see a problem with early advanced curve though, assuming no pinking, as it will improve engine torque generally if you can get it advanced as soon as possible.
I dont know why there's advice floating around to delay the advance curve by shorter springs, seems undesirable to me. Competent control of retard to ensure even tickover, yes. But other than that there's no reason to restrain in reality.
Ideally, whichever method of initial setup, then maximum advance should be checked and verified as correct as good general practice.
--- End quote ---
I did the cut and shut trick on my 750 advancer. It was a little sloppy, so I put it in my pillar drill and put the tachometer on it. I marked the bob weights with white paint and ran it up to find that it advanced too early. I cut the spring loops as described in 'the other place' and ran the test again. Full advance 2500rpm.
It was simple to do, but when you have only one shot at it, or mess it up and have to find another advancer.....not so easy for most of us.
So it does work, but there is likely some variability between old advancer springs.. I think it is possible to do one spring, but that might give you a two stage advance curve. Probably wouldn't be noticeable I suspect.
Have fun!
Cheers,
Jerry
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