Also make sure there is full manifold vacuum to the mod and no restrictions in the vac line. There must be good flow thru the vac line.
If you have a rubber line from the manifold all the way down to the mod, the rubber line can collapse and "trap" vacuum in the line. That is not good.
There should be a hard line for the vacuum to the mod with short lengths of rubber hose on each end.
Restrictions in the vac line are more common than you might think. The line can get restricted from carbon buildup.