Auto advance cam position

[Phil C]

I don't know if this is right. I also don't know what year this booklet is. I think I was given it by the previous owner when I bought my 1965 B40F.  Anyway, read what it says on the "Errata" sticker, and under "Advance and Retard Mechanism."  (Ignore the bit above the errata sticker - that is just regarding the C15S and C15T with the energy transfer system.)

Interesting, eh?

I set mine according to this and it starts okay and seems to run okay (not that I'm an expert by any stretch of the imagination!)

Phil.

[Phil C]

Btw, I believe my bike had a rebore and new piston at some stage in the past. Does that affect ignition timing settings? I'm guessing not.

[AWJDThumper]

The 1965 B40 ignition timing should be set at 33.5 deg fully advanced which corresponds to a piston displacement of 7.14 mm before TDC.

However, using Rupert Ratio as the 'Bible', he only provides fully retarded ignition settings prior to 1965. For 1964, he gives the setting as 5 deg before TDC or 0.18 mm piston displacement. However, for 1962, he gives the fully retarded setting as 15.5 deg corresponding to a piston displacement of 1.6 mm or 1/16" (it may be this value that the above booklet is trying to amend). All this data is the same for both the C15 and the B40.

I believe the advanced setting has always been 33.5 deg - in fact, Rupert Ratio quotes this value for the 1960 models. It is the advanced setting that the engine normally runs at and you only get the fully retarded setting when you start the engine. In principle, with 15 deg auto advance mechanism range, the retarded setting should be 33.5 - 30.0 = 3.5 deg. Where the 5 deg and the 15.5 deg values come from is therefore hard to understand.

The moral of the story is to always set the ignition timing for both the C15 and B30 at 33.5 deg fully advanced!

[hoogerbooger]

So how did you rotate the cam anticlockwise and hold it.??(.whilst checking/adjusting the points were just opening)

& is cam rotation just  3 or 4 degrees(that being all I can get by turning cam by hand)

[JulianS]

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.

[JulianS]

You can find a copy of the 1965  owners manual in the members area of the BSAOC website - look for C15.

[AWJDThumper]

So how did you rotate the cam anticlockwise and hold it.??(.whilst checking/adjusting the points were just opening)

& is cam rotation just  3 or 4 degrees(that being all I can get by turning cam by hand)

It's best to check that you are getting the correct amount of advance which should be stamped on the back of the points plate. Sometimes, you can get this when you pull out the bob weights manually but, because of problem with the mechanism, you might not get this when you rotate the cam. As said, it's best to set the ignition via the advanced setting of 33.5 deg or a piston displacement of 7.1 mm

[courtney]

Many thanks for everyone's replies. They are very helpful and makes things much clearer.

Just to clarify, with the 33.5 degree setting. Do you still need to advance the weights as well as the 33.5 setting? I take it that is the case but just want to make sure.

I'm pleased I'm not the only one with this issue. I'm new to British bikes and it's a bit of a learning curve.

Many thanks.

[hoogerbooger]

thank you as well.

As I have a B25 timing case on my B40 I've used a degree disc to get to 33.5 degrees before TDC. (I've put a mark on the alternator rotor as well to line up with the pointer on the case)

I've removed the cam bolt and can now see how a washer larger than the cam spindle will clamp the cam in place.

BUT turning the cam by hand anticlockwise I still only get 3-4 degrees of turn of the cam. So as 10 degrees is the likely correct fully advanced rotation for my B40 F   It looks like I will need to remove the points plate to check the advance mechanism ? ( having first removed the points and condensor). Won't do this tonight, but happy to have a go.

As you will see in the photo I have marked up the current position of the points plate which is almost at it's clockwise maximum.  The plate seem reluctant to move and there doesn't seem many opportunities for levering against other robust parts. Is there a trick to rotating a reluctant points place ?

( As shown in photo there are two obvious holes in the plate, so two nails in a batten of wood seems an option? being careful to ensure the nails don't project behind the plate ?)

[DAVE BRADY]

Hi,

If the points plate will not rotate easily with the tower bolts loosened then it could be that there is a 'high spot' on the edge of the plate.  Take the plate out and look for rub marks, rub them down a bit until it rotates easily.
I have just done this on an A65.

Dave. 

[Blackthou]

As the cam isn't nipped up, isn't it just a case of rotating to watch it open the points at TDC, then moving the backplate until they do just open? Best way to capture the moment is with a bulb and battery or multimeter.

In addition we're not talking tenths of seconds 0-60 time. The old rod down the plug hole is I'm sure more than adequate. It takes the whole advancing thing out of the picture.

Remove spark plug. Put engine in gear on centre stand. Rotate the back wheel to find top dead centre. Make sure both valves are closed. I usually check by removing the cover above the push rod tube. If they're not shut rotate engine one revolution until they are.

Insert a rod down the plug hole and mark a refrence point that lines up with something noticeable.
Remove rod and mark a second line 7.1 mm above the first.
Insert rod, rotate wheel opposite direction until the new mark goes down into the plug hole and disappears.
Reverse the motion until the line 7.1mm line is just visible. You are now at the correct advance and the points should just be opening. Adjust backplate until they are if not.
Nip everything up.
Remove rod.
Rotate engine again to double check.     

[Phil C]

 So Rick you're saying set it to 7.1mm btdc when fully retarded? Anyway  it appears the 1/16" btdc I set mine to, fully retarded ( as per errata sticker)is probably wrong for my year of bike. But it starts well and seems to run not too bad. Phil

[Blackthou]

Isn't the 1/16 BTDC fully advanced? Perhaps it's because we're talking different models and eras. Sorry for any confusion.

[JulianS]

Below from the 1965 BSA data book showing the fully advanced timing setting in inches, mm and degrees.

[Phil C]

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