Author Topic: 1956 C12 Ignition Automatic Advance  (Read 1389 times)

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Offline 1956C12

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1956 C12 Ignition Automatic Advance
« on: May 25, 2015, 10:09:04 am »
Hello again, as mentioned in my last clutch related thread I have a problem with the engine of my C12.
The engine started after a bit of a bother when I purchased the bike. 1st thing I did was fit an iridium plug. This had the instant effect of the engine at least starting off one kick. (I was exhausted cawing on it previously.)
I bough the bike a long time ago but had the opperchancity to road test the beast until the past few weeks when time permitted.
As it went, it was more a shake-down than a fun blast. I had to push the B%*tard home first outing from 2 miles away. Damaged the 'new' £80 visor of my £560 Shuberth when it slipped off the stupid handle bars in the process, which irked. (I have since calmed down.)
The bike failed due to the rear spindle nuts being slack which let the chain fall off. As mentioned before a lot of the fasteners were slack. But I have since went over everything with thread lock and a spanner.
On that first run I noticed the engine performance was very poor. I pulled the cover off the points and checked the static timing. I couldn't get it to work as the book says. This was due to the points being fitted 180º out. The capacitor at the top, points on the bottom. I also had to pull the auto advance unit off the camshaft and sort out the damage to the auto advance hub spindle, That had suffered Hammer Rash from a previous keeper.
I also found the drive pin in the auto advance taper had been 'compromised' when previously fitted out of line but who-ever undeterred battered it in with a hammer anyway. That had the effect of chipping the pin keyway and mashing the drive pin. I dug out the pin from the taper and made and fitted a new one.
Clashed it all together and proceeded to set up the timing.
I set the piston at TDC (Induction, Compression then back to lower the piston 1/4" back down the bore). I did this several times to be confident I was exactly 1/4" before TDC.
Then I gripped the cam of the auto advance with pliers and advanced the points opening as the points back plate was tightened.
(In a post on here someone suggests turning the points advance using the bolt on the end of the camshaft adapter. That was confusing as it will turn the camshaft and engine so upset the piston position. Pliers worked fine on the cam of the auto advance.)

The engine certainly runs tons better than previously but I am not certain the auto advance is doing it's job properly as it feels like the brakes are holding then releasing when the engine reaches full load (or there-abouts). It runs evenly with the throttle just off the stop though.

I could cheat and convert to 12v and fit electronic ignition to resolve the worn timing parts etc. but I wanted to know if there are any simply cures or better ways to set timing.
I also find that a lot of the the OEM information for the C12 is a hotch-potch of info for the C10 and C11. This only adds to the confusion misinformation. Yes a lot of the stuff is similar in principle and I'm sure with experience it is all clear but that it no use to one versed in specifics. :)

Offline timsdad

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Re: 1956 C12 Ignition Automatic Advance
« Reply #1 on: May 25, 2015, 10:56:14 am »
Anything with similar Lucas a/r and points can be set the same. I plonk the advance/retard and cam mechanism in about the right place in the taper, easy if there's a key or pip to locate it, then fit the back-plate and points. Set the points at .015" and rig up a test light through the low-tension coil wire so it goes out when the points break.

A bigger washer, about a 3/8" perhaps, now needs fitting under the long bolt head that holds the ignition cam assembly on, so the a/r cam can be twisted to full advance and the bolt tightened to lock up the cam on full advance. You then time the ignition however takes your fancy - piston travel before top dead centre or degrees advance on the alternator rotor, by rotating the back plate in its slotted holes.

When it's spot-on, with the test light very dim with the crank at the right place, you remove the big washer under the bolt head, replace the bolt, twiddle the advance to and fro to make sure it's not stuck and you're away.

Degrees of retard is not so important so it's always best to time it on full advance where it's working hardest.

On a twin cylinder that doesn't have the later, split points back-plate I time both cylinders separately and get the timing on each side correct by compromising on the points gap. My Thunderbird is timed correctly with about .010" gap on the left hand points and about .018" on the right. Opening up the points by one thou will advance the ignition timing about one degree. Therefore your points always need adjusting before you do the timing and not after!

Works for me!

Just a motorcyclist.

Offline 1956C12

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Re: 1956 C12 Ignition Automatic Advance
« Reply #2 on: May 26, 2015, 01:46:51 am »
Thanks Ray, I had used a similar method but I was dead posh and used a avometer continuity with the buzzer function. I set it several times to be certain it was just as the points opened.

I'll whip the cover off and double check my setting like your guide suggests.

The A/R was in a state of distress when I had it off a couple of weeks ago. But I thought it had cleaned up a bit though not perfect.
It may still be interfering with the drive shaft or something.
I fitted the washer behind the contact breaker bolt. I could probably remove that now and try it again.