B33 ignition setting

[t20racerman]

Hi all
Just picked up a badly running (but very nice!) 1950 B33 and the timing is way out.
I looked up what the setting should be, and sources vary. It's either 7/16" or 3/8" BTDC depending what source you look at.

I was hoping that someone here who has a B33 could share their experience here - what works best?

I've owned and worked on old Brit bikes for over 40 years, so am perfectly OK with how to set it correctly, would just like to hear from someone who runs one of these bikes.

[idie]

I have not noticed a lot of difference whilst using either setting on my 1955 B33.

[Jim S]

I have a 1949 B33 and I set the fully advanced setting at 7/16in. With the manual advance, you can always slightly reduce the advance while riding if it doesn't like the full 7/16 advance setting.

Jim

[t20racerman]

Thanks guys. I'll go for 7/16"

[idie]

I think the 3/8th s÷thing was for the old pool petrol available after the war.

[limeyrob]

We seem to be going full circle: pool petrol burned fast and needed less advance, then we got ethyl lead and 5* and could run a high comp and big advance, then we got unleaded and had to retard again because it burns faster (there is a theory that the Buncefield overpressure was much larger that predicted because it was unleaded) then a few weeks ago I read that petrol now has suppression additives to keep it stable in high pressure injection systems that also delay combustion.  I don't know if this last claim is true but it makes sense.
I wonder where we are now?

[t20racerman]

Update:
The bike was flat - definitely down on oomph. Redid the timing at 7/16" and it's running much better now. 
Thanks for the comments guys.

[iansoady]

With a manual advance magneto I generally set it so that full advance is a little more than recommended. Then use the manual lever to find the sweet spot and then retime so that that is the full advance setting. As said, petrol has changed dramatically over the years so the book values can be taken only as a guide.

[limeyrob]

I'm thinking the same. I'm running my A10 RR at 5/16" as this was the recommended setting for good power back in the day, but the BSA book figure is 3/8".
The plugs always show slightly rich when I know its not but black plugs can mean retarded ignition so I'm thinking I'll try 3/8" as its manual A/R.