1. 25mm 'timing' means the injector pump lift is set to 1. 25mm
This translates into ? degrees advance. The fuel is injected into the cylinder 'in advance' of reaching top dead center
"TDC"
Many 1st Gen. trucks were set (and factory called for) 1. 25mm lift setting for emissions reasons i think.
The timing being used by many on this site seems to range between 1. 35mm and 1. 70mm.
I personally am set at about 1. 37/38mm. The increase in timing will usually improve performance and decrease smoke when you 'up fuel' the Cummins. It can also have the effect of raising your operating temps I THINK! There are some really great posts on timing,injectors,AFC housing adjustments etc. Just run a search and don't overlook the "Archives".
Regards, Bush'