Auto advance cam position

[Blackthou]

7.1mm out ie 33.5 degrees. Are you sure you were fully advanced at the time?

[AWJDThumper]

If it is set to 7.1mm (0.280") fully advanced, what would that be fully retarded?

I have checked all the available data from both the user manuals for 1961 and 1965 and the data provided by Rupert Ratio and the only reliable data appears to be the fully advanced setting of 33.5 deg/ 7.1 mm. As said, it is the fully advanced setting that the engine uses most of the time it is running and therefore needs to be set accurately.

The problem is that the corresponding retarded setting depends on what the auto-advance mechanism provides. This was 15 deg prior to 1963 but was then changed to 10 deg. However, using these valves, I have found it impossible to verify any of the retarded settings given. Basically, the expected retarded settings should be either 3.5 deg (33.5 - 2 x 15) or 13.5 deg (33.5 - 2 x 10). What is provided is either 5 deg or 15.5 deg. Even more confusing, the 15.5 deg / 1/16" setting given for 1961/62 should have been 3.5 deg!

It would be nice to sort out the retarded settings but, even with the benefit of being an applied mathematician, I can't make the numbers add up. Hence, the best advice appears to be to stick with the advanced setting for the ignition.

[hoogerbooger]

RR does say the BSA advice on timing changed with the prevailing weather condition at Small Heath on the day. So maybe that explains why the maths doesn't work...

(It's raining here today, but not sure how that changes the settings I should use)

[JulianS]

Remember that the settings given were based on the fuels in use at the time. Modern fuels may mean your engine runs better on a slightly different setting and you will only know that by experimentation.

I agree with the good advice given to time the engine on the fully advanced figure.

The retarded figure is too unreliable - and depends on you bike being fitted with parts to original specification.

Also 1 degree at the advance unit equals 2 degrees at the crankshaft.

[AWJDThumper]

The other thing to bear in mind is that the C15 and B40 are in such a mild state of tune that most people are unlikely to notice a 5 deg error in the ignition timing - they might not even notice a 10 deg error. If you are using a gauge down the plug hole to set the piston position, the accuracy you will achieve is probably going to be between 5-10 degs. It's certainly better to think in terms of removing the chaincase cover and positioning things more accurately using a timing disk attached to the alternator rotor or a dial gauge if the head is off.

In terms of using modern fuels, the advice I've seen is to back off the timing 2-3 degrees (lower octane rating) but, again, for the C15/B40 this refinement would probably make little difference.

[DAVE BRADY]

Hi,

This is not necessarily going to solve the problem but I can remember getting C15s and B40s with distributers running and then riding along turning the 'dizzy' by hand to get the best position for performance.  Not so easy with side points.

Dave.

[hoogerbooger]

4CA Points plate off. Only circa 5 degrees of turn of the cam when I rotate manually anti-clockwise.

The  bob weights have cut-out guides between their pivot and the end of the bob that spins out. The pins through the two guides only travel to just over half way along these guides when I turn the cam anticlockwise.

So is this right ? or faulty ?    Is 5 degrees of turn on the can 10 degrees of advance ? and correct ?


Also. Points plate is a very tight fit and will not turn. The plate is somewhat dished.  No obvious single sticking point. so will sand down gradually until it can slide. Is it worth applying a tad of copper slip on the plate edge when I have sanded to just slide ? or leave dry.

( I don't think the picture adds anything ? looks like it should in the guides)

[Phil C]

Forgive me if this is absolutely irrelevant, but I think that to get more adjustment I removed the cam from the taper and repositioned it slightly when putting it back on. If it's any use, the discussion started on this forum on 3 Feb 2018 on this forum, called something like "Ignition timing adjustment." Phil

[JulianS]

The back of the advance unit will be stamped with its advance range, I think 10 degrees for B40 but they can also be found with 5 and 12 and maybe other ranges.

[hoogerbooger]

OK will take auto advance out....to see what it says on the back. Then I will know.

Can I check I am correct to think that when I re-seat it in the taper it doesn't matter whether the spindle has rotated from the current position.

[Phil C]

Isn't that the same as repositioning it, as per my post above?  Phil

[Phil C]

Actually, no, I guess it's not. Maybe I marked the taper and the cam, to help me put the cam in a different position on the tape when I refitted it? Am I talking rubbish?   Phil

[courtney]

The distributor was changed for one with a smaller advance range during late 1962/early 1963.

To lock the AR unit find a washer with centre hole diameter just greater that the cam spindle then when you tighten the AR bolt with unit in fully advanced the washer will clamp the cam and  it will be locked in advanced. This information comes from the 1965 C15/B40 owners manual.

Set the piston position, then set the points plate with fixing screw about central in the slots, position the AR unit best you can and tighten fixing bolt. You can then adjust the timing by moving the points plate.

This is the most helpful and least confusing explanation of how to do this. I must have read and downloaded everything but this bleedin' owners manual! Many thanks.

[hoogerbooger]

auto advance out. It's a 10 degrees unit & whilst it doesn't look like 10 degrees of rotation to me, the bob weights do turn out to the maximum the unit will allow ( which I can now see from looking at the unit from the back.

BUT the weights do not spring all the way back. One spring in particular looks stretched and the tab it attaches to from the base plate is bent. See picci.

So I should get replacement springs (if I can find the right ones) 

Before I try bending the tab back to perpendicular to the base plate. Any advice ?    (seems just one spot weld attaching the tab to the base plate & if that breaks, substantial extra expense/time/electronic ignition seems an outcome.

idea so far is with narrow nosed mole pliers might be able to clamp the welded part of the tab to the base plate to reduce risk to the weld`. Sounds like, from RR, the cam is a push fit over the spindle and I shouldn't try taking the auto advance apart to make the tab easier to clamp ? 


BUT a better idea might be do nothing and just put it back in with less ability to self retard ?

[courtney]

Mine is exactly the same as yours with the bend. It worked fine before I took it apart, so it's staying as it is. Replacement springs are available though.