292 y-block distributor puzzler

[Stiggy]

Originally Posted by highcking

Stiggy - no. None of the reference manuals say anything about the correct vacuum readings except the reading of 6 degrees BTDC with vacuum advance shut off. But the ATDC reading may be correct anyway. In a regular system, when you check timing at idle, there may still be a small advance contributed by the centrifugal advance weights in the distributor. I’m not sure about that. There’s no obvious way to check timing under true load conditions Where manifold and ported vacuum change as soon as the engine is put under load. I’d like for others to report their vacuum reading at idle with the vacuum lines connected. I still could have a faulty unit though it seems to work ok.

Can you do a vacuum/pressure check on both of the lines going to the
vacuum advance diaphragm? Something is causing it to go to ATDC.
I've checked vacuum at idle and there is none (500 ~ 550 rpm) The timing at idle should not change when the vacuum lines are connected..

[KULTULZ]

If one goes to the SPECIFICATION CHAPTER of the SHOP MANUAL. the vacuum signal levels will be given at the appropriate engine RPM.

Example -

DIST RPM - 200 - DIST Degrees 0
DIST RPM - 500 - DIST Degrees 3 1/2 - 403/4

So there is a modified MANIFOLD VACUUM SIGNAL @ idle. It supplies the signal to advance the breaker plate. This is why the vacuum signal is removed when adjusting base timing.

On the 1956 DUAL DIAPHRAGM SYSTEM (not all 56 engines used it), there are two signals, one modified manifold vacuum (this gives advance) and constant MANIFOLD VACUUM which operates the outer surge diaphragm (which retards breaker movement).

You can see vacuum advance operation/movement with a timing light and revving an engine unloaded. This is one of the checks performed during a tune-up. The same is true on a DUAL ADV DIST, whether using PORTED VACUUM, MANIFOLD VACUUM or a mixture of both.

If the timing shows retard @ idle, it is most likely a bad breaker plate or vacuum diaphragm.

You guys are beating yourselves to death.