Just wondering how much movement there should be inside a SVDA distributor when you suck on the vacuum pipe? I have four spare distributors and I’ve tested the vacuum advance on each by sucking on the pipe and one moves considerably more than the others. How much should the plate move?
It should move by 6 degrees rotation. Check on the distributor that has more rotation (probably more like 11 degrees) that the rotor arm angle is locked to the drive dog. If so it's a very old SVSA vacuum only distributor and should be sold to somebody interested in period correct engines. In these all the advance is vacuum. The 11 degrees becomes 22 degrees at the crank shaft ( the distributor turns at half the crank speed so all the angle changes in the distributor are doubled at the crank). Which gives e.g. 8 degrees at idle, 8 plus 22 = 30 degrees at full vacuum for an old Beetle. On newer SVDA distributors the rotor arm also moves relative to the drive dog, one way will be spring loaded and move 10 or 11 degrees, the other way, nothing. That's the centrifugal weights moving out to their end stops. This is added to the vacuum advance taking total advance to something like 40 degrees plus/minus a bit being timing advance for a lightly loaded bus engine. If you floor it, vacuum vanishes, centrifugal advance limits to 28 degrees to keep the engine cooler and avoid melting bits off the pistons..
