jumping tach???

[onthecheap]

i have a 77' 305 rebuilt and all that good stuff,my queistion is when it idles you can hear and feel a very slight miss, not enough to be worried about but i look down at the tach it jumps about 200-300 rpm back and forth? but when i give it gas to about 2500-3000 rpm it goes away? anyone had this problem??? :confused:

[bonecrushins10]

check the timing with a light ,

[onthecheap]

timing is at 10 deg.

[Dimemaster]

Vacuum leak.