Timing for 1983 Carb Mercedes

[Rollin]

I'm going through the routine checks and maintenance on my W123.
It is the 2l M102 with Stromberg Carb.

I'm doing the ignition timing and I'm never sure whether I'm doing it correctly.

The car is meant to run on unleaded and I use super unleaded.

The ignition timing at idle, according to the manual, should be 13 +/- 3 so I always set to 15.

Car runs OK, but has a slightly lumpy idle.

If I turn distributor to advance the timing above 15, the revs increase. It's probably revving highest at the stop where I can't turn anymore. I can't discern any pinking at that point either although difficult to be sure it's not happening due to other noises.

I've read timing should be advanced when using super unleaded but how much is too much?
I also realise that setting could vary car to car, but would a setting of 21 be unusual if the manual recommends 13?

[E-bmw]

I may be totally wrong but I was told on an old-school set up, which you have to adjust for highest speed, back off a little until it is still smooth & then check for pinking, no pinking means it should be fine.

Obviously, this is in the absence of a defined setting, or means of checking, but the theory should still work unless I was told b0llocks.

[Rollin]

Thanks.
Yup, that's what I heard too. Problem is, by doing that, the advance is way beyond what the manual suggests...even though there's no pinking that I can hear.

[richhead]

back in the old days when unleaded was becoming the thing, we used to retard the timing a few deg.