Hi - I would be grateful if anyone can explain the set up on my 1946 B31 advance / retard mechanism. I am fairly new to these manual advance / retard levers and so I expect I must just be missing something obvious.... I am having trouble with the bike kicking back...
Having had the mag dyno rebuilt, I have reassembled the bike and retimed it it, although I now cannot get it to stop spitting back or kicking back when attempting to start it. I am sure that the timing is set up correctly - 7/16" before tdc on the compression stroke. That all looks OK, with the points just about to open in the 10:00 oclock position. The manual says to time the bike with the advance/retard in the slack position.
So I can understand that in normal running set up, the engine is presumably firing just about at TDC - is that correct? But on start up, I believe that you are supposed to pull the advance/retard lever in, which is supposed to retard the firing slightly? But it looks to me that when the lever is pulled in, the points are then opened wide, ascompared to the standard advanced position, when they are only just starting to open.
So that suggests to me that the plug will spark earlier with the lever pulled in, because the points are opening earlier? Isn't that therefore advancing the timing? It would certainly explain why it is kicking back at me!
I know my logic must be wrong but I cannot understand where. Can anyone explain please?
Thanks
Bob
Hi Bob,
First question relates to the advance retard mechanism - looking at the end cap on the mag does the cable enter down to the right or down to the left?
This is kind of important as the earlier mags entered to the right which was 'pull to advance'. Later mags were reversed and thus 'pull to retard'. I think a '49 would be the earlier but you never know so could you let us know which you have please?
Cheers,
Bernie
Hi Bernie
Excellent - I think you might have resolved my query!
My bike is a 1946, and the cable goes into the right side of the mag as you look at the end cap.
If therefore, as you say, this means that the control lever is 'pull to advance', then firstly, I have incorrectly timed the bike with the advance/retard in the retarded position, and secondly, I have been pulling the lever in to start it, thus advancing the timing even further. No wonder it is kicking back at me!
Is the logic above right?
Thanks
Bob
Spot on!
If you want to visually check then remove the end cap and watch the direction of rotation of the base plate when you pull the cable.
I think you will see it move against the rotation of the points thus advancing as you pull.
Therefore you need to time in the fully tight position.
Just as a footnote - I believe the reason the design changed to the LH side was the idea that if the cable broke then the spring in the cap would pull it to the 'slack' position and it would be a better 'get you home' option if the ignition was then fully advanced rather than fully retarded.
Cheers,
Bernie
Hi Bernie
Excellent advice - it is now timed correctly, and the advance/retard also seems to be correct.
The bike runs well in the advanced position, and the engine slows down as you retard the lever slightly, so all well and good. However, the bike only seems to want t start in the fully advanced position - it is reluctant to start with the lever even only very slightly retarded. Is that anything to worry about? I'm not sure if I can do anything anyway, as I am sure it is all timed correctly now?
Many thanks
Bob
The internal timing of the mag is such that the position of maximum flux, and most intense spark, occurs when the mag is at full advance. When retarded the spark is less intense.
You mention that the mag was rebuilt - did that include a rewound armature?
I would recheck the timing and if that is OK I would speak to the mag builder.
Hi Bob,
Might also be worth checking the contact breaker gap is spot on and that you don't have a 'nib and trough' on the points that can affect the setting when using a feeler gauge. This is when arcing transfers metal from one side of the points to the other resulting in a pit on one side and a sort of mountain on the other - when this happens the points need to be replaced or dressed back until smooth.
The info that Julian has supplied is important as magnetos are rather different from coil ignition. With a coil, you can break the LT voltage and generate a spark in the HT regardless of any outside factor (assuming you have 6/12V on the coil) and it will spark well.
A magneto is different because instead of using battery voltage it generates the electrical energy as it rotates and will only be able to generate a healthy spark if the points open at or around that point of 'maximum flux' which is the self-induced energy build up.
This means that the points gapping has a direct effect on timing and therefore the ability of the mag to generate a good spark - and also that at extreme positions of advance/retard the magneto may not be able to generate sufficient electrical oomph for a good spark.
I hope that that helps rather than making things more confusing but if in doubt, remove the points, dress back smooth and reset to 12 thou. Then check the ignition timing again. Make sure you rotate the engine forwards to the 7/16" BTDC to eliminate backlash and you should be good - assuming the magneto is working well.
Hope that helps,
Bernie
Thanks both
I have checked the points as suggested, they were clean, flat and were exactly 12thou. Having reassembled them, I then redid the timing which I now believe is spot on again.
But the advance/ retard lever still needs to be in the fully advanced position in order to start the engine, it will not start if the lever is retarded even the slightest bit, (but does that actually matter?).
When the engine is running, it sounds good as the revs increase or stay constant, but there is a bit of popping on deceleration. This occurs whilst the bike is on its stand, so the engine is not under any load. Actually, this popping has always been there but its not too bad, so I wonder if anything else could be causing that?
The mag was rebuilt by Priory Magnetos a few months ago, and he did put a new armature and capacitor in. So as Julian suggests, perhaps I should speak to Andrew to see what he thinks....
Thanks again for all your advice. The most important lesson I have learn't so far is that my advance/ retard lever works the other way around to that expected!
Bob
However, what is more worrying, is that the bike will not restart so easily after it has run for a few minutes.
Once the bike has started will it still idle as you retard - or cut out? I normally start with just a tad of advance to avoid a kick-back and the engine will just tick over very slowly at full retard.
Have you checked that you have a spark when retarded by kicking over with the plug removed? This won't be a definitive test as if the spark is weakening in the retarded position it may spark OK against the head but not under compression - but if you have no spark at all it would indicate a problem with the internal mag timing.
My M21 will pop if I close the throttle at high revs without also retarding and my guess is that is the result of it becoming over-advanced as the revs drop off.
Cheers,
Bernie
Thanks Bernie
I like your last paragraph! I had a feeling that the popping might be due to something like that, so I won't worry about that any more!
Once started, if I retard the lever, the bike definitely cuts out, it will not run or start when retarded. I haven't checked for a spark.
Also, I messed up the last post a little bit, in that I only half stated that the bike does not restart easily after it has been running, even for just a minute. Given that I am confident that the timing and points are set correctly, (although of course that does not gaurantee I have got it right!), it does look like a remaining problem with the magneto?
Regards
Bob
Hmm, might well be time to talk to Andrew and see if he has any ideas.
The only other thing that occurs to me is if the cam plate is not in the correct position rotationally.
I'm away from my bikes at the moment so can't check to see if that's possible - from memory there is more than one slot in the cam base plate that the control plunger can slot into but isn't there also a cutaway that limits the rotation advance/retard extremities - and this sits over a peg? If so then there's probably no option for getting the plunger engaged in the wrong slot - but just wondering....
Cheers,
Bernie
I think your suggestion could be viable, but given that the mag has been reconditioned, I would have expected that sort of thing to have been checked?
I'll have a chat with Andrew to see what he says.
Thanks
Bob