MGA or MGB 123 Tune Ignition Timing Curve
I had trouble finding an ignition curve for the 123-tune distributor. Fuels have changed since 1959 and my engine has also changed to a 1950cc stage 2 built by Oselli. Standard timing with 63 years of wear didn't work at all. My vacuum connection is from the carburettor. Static timing is top dead center.
Here's my map, and I'll put notes below about how it was set up.
The centrifugal advance curve deals with full throttle. Set the vacuum advance to 0 across the board. Apparently 32 degrees is about as much as this engine can handle.
I started off with a curve I found on the internet and tried full throttle from low rpm on a flat road. At rpm where I heard pinking I backed off the timing by 5 degrees and tried again until there was no pinking. It's sensible to be a good few degrees away from pinking. It took quite a few runs to get a good timing curve for full throttle.
My curve ended up being a straight line.
Driveability will be poor with just the centrifugal advance curve. The engine needs a lot more advance on part throttle.
My vacuum map ended up being a little odd. It's only the negative part of the scale that is used with the left side being idle and the right side being full throttle. Again I started off with something from the internet but I had a lot of trouble with pinking on shallow hills and backing off the vacuum advance quite early on fixed that.
My idle was poor with only 10 degrees advance but adding another 12 degrees using the vacuum has made it lovely. For the first time in 32 years I have a nice stable idle at around 750rpm. My vacuum take off is from the carburettor not the inlet manifold.