BSA Sunbeam scooter timing

[Dean Southall]

I have fitted boyer igntion to my sunbeam 250 twin engine. The manual gives timing for full retard (5deg) but the boyer required setting at full advance.
Can anyone advise (TTJohn?) what the full advance setting would be in degrees btdc?
cheers
Dean

[Dean Southall]

bump

[TTJOHN]

Dean. I just timed mine the old fashioned way, TDC then bring it back about 5/16" if I remember correctly, if you are fitting a Boyer you can adjust that easily once you've started it, to get the best results.

[Dean Southall]

Thanks for your reply John
I have tried setting by calculating the advance by measuring the degrees of travel on an old  Sunbeam mechanical advance unit and doubling this. Fortunately the external flywheel makes it easy to mark up TDC, advance etc.
I've tried tweeking it but with no joy; looses power abive 1/4 throttle.
I have spoken with someone with experienec of these machines and he says they are very fussy with regard to ignition timing.
I'll just have to try by trial and error. (the neighbours are pretty understanding thankfully!)

[TTJOHN]

Dean, are you sure it's the timing, seems to me like carburation problems, I know they are difficult little blighters to get correct and a lot of the people that supply carbs are not quite as good as they seem and know bugger all, I bought a cheap version of a Dellorto carb off of e bay £27-50 but I had to change the jets and settings before I got it correct.

[Dean Southall]

I did not have an orignal carb but found that Allens Perfomance do a Mikuni kit for the Sunbeam.
Have tried a range of jets and needle settings, and the bike dyno'd and it showed carburation good throught the rev range on the original set up.
Hence looking at timing (again!)

[BILL NELSON]

We used to have one as a field bike when I was at school. I didn't know anything about them, so set the timing at about 40 degrees full advance as that was the best I could do with a school protractor - we still laugh about it, as we were given it as a non-runner. I can't remember the details as I've never worked on one since & everything was new learning at the time.
Over the years with unknown engines, I've usually started with 38 degrees full advance & worked from there.

[Dean Southall]

Hi Bill, thanks for the advice. 34-38 is what I set the 650s at. Would it be less for a smaller engine? It seems not is you set it at around 40 and it ran.
I had to work backwards to get an estimate. I checked the rotation of the mechanical advance and found this to be 12degrees. Double that as irs cam driven so half engine speed gives 24 degrees and add the 5degrees full retard that was the setting given in the manual. So I set it at about 29btdc.

When my mojo returns, and all the running bikes stop having mares, I'll try advancing to around 38^o