R100GS Timing

1989 R100GS
Running smoothly.
Decided to install a Red Centre ignition system.
Current bean can is on the limit of adjustment to get S mark aligned at idle - approx. 950 rpm.
Fitted the Red Centre, can get it to run OK but at the limit of adjustment and can only get the Z mark showing in the window and at 1150 rpm.

I am wondering if the cam chain is off by 1 tooth. Would it even run if so. Records show the chain was replaced approx. 10,000kms ago but not by who.
There is record of a persistent 'surging under load at high revs' issue in the last 10,000 kms (after the chain replacement) which may be unrelated. Records show the bean can was replaced to solve this issue but no mention of whether replacement was a new or s/h unit. Notes say surging problem returned.

I can chuck the old bean can back on and ride around happily for now but I reckon something is amiss.

Hoping to get some input before I start disassembly to have a squiz at the cam chain in case I am totally off track.

I wonder would the cam chain being a tooth out be apparant if you turned engine over by hand and watched piston rise to see when it reaches TDC and then see if the movement of pushrod/valve was in line with that. i.e. the valve clearance pushrod gap hasn't either begun to open up again before reaching TDC or isn't closing just after TDC?

I don't know this ignition but the Boyer has to be timed at full advance i.e. 3000rpm plus as it messes with the timing at tickover to generate a smooth idle. Is this possible with yours?

Stock timing mark can be + - 3 degrees, that is some bikes like 29 degrees, some 35, even on a bog stock bike.
If you dont have a rev counter or timing light what works quite well is to adjust the bead can to give the fastest tickover at the point where the charge light goes out, then retard it to just off the point where the revs start to drop.
Works just as well if you if you do have a rev counter and timing light ----------.
Back in the days when rolling road dynos were rare we would then take it to a long steep hill where the bike was pulling hard and do a few test runs to get the best performance, now most folks let the dyno guy do his stuff.