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Misc: Everything Else Ignition Timing Related Posts


Cyborg

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VOC Member
It has been forever since I owned one of those, but 39 degrees BTDC if my memory is correct. DBD34... and it seemed like every time it backfired when starting, you may as well just give up and go get the degree wheel. Probably should have lapped the pinion onto the mag.
 

vibrac

Well Known and Active Website User
VOC Member
Ah one of the joys of the electronic BTH a rod to lock the mag while you do up the nut
Now why couldn't the prince of darkness think of that? Because nobody at Lucas had actually timed a magneto on a motorcycle since 1925 That's just about the last time anyone from Miller tried to ride a motorcycle down a winding country road in the dark. No wonder the car products were largely better.
 

bmetcalf

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VOC Member
The D Jags raced through the night at Le Mans then, I assume they were Lucas-equipped..
 

oexing

Well Known and Active Website User
VOC Member
. . . . but I guess they did not have the crappy rotating coil mags . . .

Vic
 

Magnetoman

Well Known and Active Website User
VOC Member
DBD34 ... and it seemed like every time it backfired when starting, you may as well just give up and go get the degree wheel. Probably should have lapped the pinion onto the mag.
The pinion is lapped onto the mag. I won't name names, but someone obviously screwed up when he tightened it...

The DBD34 in question is a U.S.-only 'Competition' model, supplied by BSA with 10:1 pistons. The same someone screwed up years ago when he had the chance to install a more reasonable 9:1. And yet I continue to allow him to work on my motorcycles...

Since I was in the middle of changing jetting and had just cut the #3 slide to make it a #3.33 the carburetor was the initial suspect in the bad running. Even if the DocZ hadn't been given to me for free, it would have paid for itself several times over in the pain and suffering I didn't endure while getting to the point where it was clear the problem was with the ignition, not the carburetor. Although it ran badly when started, at least it did start, and it's a lot easier to troubleshoot a motorcycle that runs, even if badly, than it is one that won't even start. There's no way I could have started it "manually" (leg-ally) once the timing had slipped.

If nothing else, getting the bike to the present point is a reminder that correlation does not imply causation. The bike started out running OK-ish, I changed the carburetor's internals, and it ran badly.
 

Cyborg

Well Known and Active Website User
VOC Member
Meant I should have lapped mine. I wouldn't dare suggest someone with a handle like Magnetoman would omit such a thing! Maybe I did lap it at some point, but don't recall as it was back in 1970 or so. I do recall being told by someone to slide the pinion on and before putting the nut on, use a socket and a hammer to set the pinion onto the taper so it wouldn't slip when the nut was tightened. Didn't really like that advice. Can't imagine the bearings appreciate that.. or the magnets for that matter. I have a sprocket to install on a different project (that BTH TT above) and wondering about making an installer that threads onto the armature and then a threaded collar with a radial roller is snugged down to encourage the sprocket to stick. Maybe overthinking it...
 

Magnetoman

Well Known and Active Website User
VOC Member
A well lapped taper doesn’t need much of a tap.
Personally, I always follow the double tap rule.


However, the details get complicated. I use the inductance of the primary to determine when the points open, since that changes from 0.00 (on the mH scale) to ~5 so there's no mistaking it. However, when you have a resolution of ~0.1-deg. the inductance doesn't just abruptly jump between those two limits. It sort of oozes between them. Further, if I stop advancing the engine just before the full value is reached (or anywhere up to that point) and wait, it slowly oozes lower over a period of many seconds. I think part of that oozing may be due to backlash of the gears relaxing, but some of it may be due to the points rubbing block relaxing under the pressure of being held partially open by the spring. If I had ~10x lower resolution, so I could tell the difference between 38-deg. and 39-deg., but no better than that, I wouldn't see these effects.

If I'm right about the relaxation issues, both would be different in operation. The engine would keep pressure on the gear train so there wouldn't be an opportunity for the backlash to relax, and the rubbing block would take on some steady-state amount of relaxation. The only way to know how much an effect this would have would be to accurately set the static timing, then check the timing with the engine running.

The other "detail" is the value of advance to use. Fifty years ago my Gold Star was tested by the factory at "39 degrees." I can set it to 39.0+/-0.1 degrees but with today's fuels, would it be better to use something other than 39 degrees? Unfortunately, I haven't found any definitive information on this. Yes, there are plenty of unsubstantiated allegations that more/less advance is needed because modern fuels burn slower/faster, but I've yet to find any actual data showing one or the other.

27018
 

vibrac

Well Known and Active Website User
VOC Member
Reading here the number of times I have read of deviations of shorter full advances eg 34 not 38 on fairly standard machines would indicate this is dictated by modern so called petrol
I ended up with a shorter advance than book on the Dyno over the last decade
 

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