Newbie question- magneto related Dommie 99

Different timing It does sound possibly the issue, so it's worth checking if they are different.  Whatever others say (and I suspect they will say it), don't overcomplicate your life by changing the plug lead protocol!  As you have found - it runs fine.  It is of course possible that previous owner had the problem and solved it - but didn't polish the pipes.  You might try an infrared thermometer - mine came from the sadly missed Maplin, but they are on the web somewhere.

It is important to check the timing on both cylinders. It can be wildly out (as I discovered many years ago). If so, stoning the cam ring is the answer. Not too difficult, patience required. 

hello the 1960s model 99s were not fitted with magnetos  they where fitted with the D2 distributor   and prs8 lighting and ignition and Rm 15 alternator  with emergency start for there ignition and electrical   system  your model 99 as been fitted with a magneto by the previous owner so therefor a none standard item         Yours  anna j 

My 1960 99 Slimline is in 99.9% original state. The only change is that I have removed the points from the magneto, fitted as standard, and replaced the internals with an electronic ignition gizmo.. Also it never had the PSR8 switch. By the by, the PO's father who originally bought the bike new was a mechanic at a Norton dealers.

I think it all depends on the actual day the bits were assembled and what bits they had in the factory at the time especially if the model and/or company was in a state of 'transition' at the time of manufacture.

John S