Hi.Can anyone help me with this... for two years I have always had a strong backfire when I snap the throttle back at 4000 rpms and up. I was looking inside my distributor, and found my vaccuum pod is shot, and it leaks vaccum. I capped it, from the engine side. However, when I had the distributor cap off, I noticed that the plate only moved about 1/4" inch rotation.

My questions:

Isn't the plate it supposed to move much more, for mechanical advance?

What is the slot screw for? I circled it in red, in photo below. I cant undo it, it just wobbles around in an elipse and won't unthread.

How do I get to the bearings below, to check their condition?

Thanks for your help,

-john

I can't enlarge your picture, but I think --

The screw with the elipse head is for adjustment of the points gap

The screw is used to adjust the points gap. It is not a fastener.

As I recall the vacuum advance only moves about 1/4" or so. The linear distance is related to the amount of advance but is not specified or normally measured as an inspection or diagnostic procedure. The test is to measure the change in timing in degrees (using a timing light). The factory service manual has a chart showing the amount of advance degrees versus applied vacuum.