Vacuum advance, timing prob

Im currently trying to set up my ingnition timing on my 1983 got gti 1.8 dx engine code. I have set the timing with a strobe light on the 6 degree mark with the vacuum pipe off and plugged at 900 rpm but when i replace the vacuum pipe the revs raise and the timing is out at about 1100 rpm. the vacuum seams to be altering the timing even at tic over. Is this correct? It doesn't seam right to me. i thought it only altered the timing during increased revs/load.

No thats right ,
well its how i do mine anyway