55 F100 vacuum advance issue

[Rowe]

Recently rebuilt the Ford 94 carburetor on my 55 Y-block; when I hook up the vacuum advance the timing advances about 15 degrees before BTC while idling at 800 RPMs; switched out the spark control valve and nothing changed; thoughts?

 

[dmsfrr]

Is the distributor vacuum line connected to manifold vacuum? or a fitting on the carburetor?

 

[Daves55Sedan]

800 RPM's is way to high for a y-block idle speed setting. If you have a manual transmission, you might be able to go as low as 475rpm, but with Fordomatic trans, you need to go a little higher like around 600. See your shop manual and go from there.

 

[scicala]

It sounds like because of the higher than normal idle speed, the hole in the throttle bore of the carb that's for vacuum to the distributor is being partially exposed too early, and causing too much vacuum to the distributor advance.
If you really want the idle speed that high, I don't think it's hurting anything.

Sal

 

[Southtowns27]

What carb? It needs to be hooked to ported vacuum, not direct to manifold vacuum.

 

[KULTULZ]

Recently rebuilt the Ford 94 carburetor on my 55 Y-block; when I hook up the vacuum advance the timing advances about 15 degrees before BTC while idling at 800 RPMs; switched out the spark control valve and nothing changed; thoughts?

IF the CARB and DIST are both original LOAD-O-MATIC, you have an incorrect vacuum signal (or possible vacuum leak w high idle).

- http://m571.com/yblock/loadomatic.htm -​

All manifold vacuum to the distributor passes through the spark control valve. Under normal road load conditions, the spark valve is held open against the pressure of a pre-set factory calibrated spring by a combination of atmospheric pressure outside of the diaphragm and manifold vacuum from within. When accelerating, manifold vacuum momentarily drops below a predetermined point and the calibrated spring closes the spark valve, shutting off the largest of two passages through the spark valve. The passage left open is a caliwww.jalopyjournal.netbrated by-pass which permits a small amount of manifold vacuum to reach the distributor vacuum line. Closing the valve prevents excessive spark advance, while the manifold vacuum passing through the spark valve by-pass prevents full spark retard. As engine speed approaches the throttle setting, manifold vacuum increases sufficiently to open the spark valve and allow a higher vacuum to operate the distributor. This carburetor also employs a distributor passage ball in the manifold vacuum passage to prevent weakening of the vacuum to the distributor whenever venturi vacuum bewww.jalopyjournal.netcomes stronger than manifold vacuum. By use of the spark valve, more efficient spark advance upon acceleration is obtained without the excessive "ping" of the normal road load part throttle spark advance or the sluggishness of wide-open throttle spark retard.

The spark valve functions in a similar manner to provide an intermediate spark retard whenever the load on the engine is increased to a degree where the normal road load spark advance would be too great and the wide-open throttle spark retard would reduce the efficiency of the engine.

SOURCE - https://www.carburetor-parts.com/spark-control-valve-70-5b

The 1955 WORK SHOP MANUAL will have a full description and diagnostic procedure.

 

[Rowe]

carburetor. the vacuum line is connected correctly

 

[KULTULZ]

It sounds like because of the higher than normal idle speed, the hole in the throttle bore of the carb that's for vacuum to the distributor is being partially exposed too early, and causing too much vacuum to the distributor advance.

Sal

See the ILL Below -

​

If CURB IDLE is not set correctly, this passage (shown) will become exposed and introduce manifold vacuum to the signal possibly causing the HI-RPM and timing advance.

Your DIST VAC SOURCE is connected to the throttle body just above the SPARK CONTROL VALVE?

Both CARB and DIST are original LOM and neither has been changed over and/or modified to DUAL ADVANCE DIST?