The Stovebolt Site Forums The Shop Area Engine Shop Vacume Advance / Timing

Having an issue with the vacuum advance being fully on. It does not change even when I turn the distributor either direction. Engine starts ok, sometimes with a slight hesitation when turning over. Idles ok without stalling at 450 RPM's, Hydra-Matic RPM.

I have properly set the timing making sure #1 is on the compression stroke and set on the BB on the flywheel. Have even tried setting the timing it a little before the BB.
Distributor is set in the engine so that it aligns with the oil pump slot.
Made sure the distributor wires are in the correct firing order and #1 in the correct location on the cap.
Carburetor is installed correctly with the insulator in the correct position.
Have tried setting the idle mixture screw at 1 1/2, 2, 2 1/2 turns on the carburetor.
Have new manifold gaskets and are properly tighten.

Fresh rebuilt 1960 235 truck engine.
Carburetor Rochester B as in the Truck Repair Manual page 6-52. Although mine is stamped C Rochester GM it looks the same.

Try a little more detailed description of the problem. Do you mean the vacuum advance is in the "full-advance" position all the time? If so, you're either connected to the wrong vacuum port or there's an internal problem in the carburetor. Generally, the vacuum port in the carb only has vacuum on it in the "off-idle" position. If the idle speed screw is turned too far clockwise it can result in the throttle plate uncovering the port for the distributor line and resulting in vacuum being on the line all the time, but that will usually result in a too-fast idle speed. If the initial timing is WAAAAAYYYY too late it might be necessary to open the throttle that far to keep the engine running, but other performance problems would also be present. Is it possible you've got the distributor vacuum line connected to the intake manifold instead of the carburetor?
Jerry

To eliminate a little low hanging fruit, confirm the diaphragm in the advance is not ruptured. You will have to apply some vacuum to the advance to see if it will actually hold vacuum or just sucks air. I have a little stub steel brake line with a rubber hose attached. Screw the stub in and blow into the hose. If it blows through, it’s ruptured and will need to be replaced.

Jerry, the couplet was the idle screw. Lowered the idle speed and vacuum advance problem solved.

I have the timing set a the "C" on the flywheel. is that a better setting ? I am getting 32 degrees of dwell @ a little over 400 RPM's.

"Dwell" is measured in degrees of distributor rotation. Are we comparing apples and oranges? If you're measuring crankshaft degrees of timing advance with a delay-flash timing light, you might be a little low. I usually disconnect the vacuum advance and set the engine speed to 1800-2000 or so, and set the initial + centrifugal advance to around 35-36 degrees. When the vacuum is reconnected, expect the total advance to go to about 45 or a little more during light-throttle cruise. If the engine doesn't get a detonation rattle on a long uphill pull in high gear, you should be good to go.

32 degrees of dwell, measured at the side terminal of the distributor with a dwell meter, is about right, depending on the part number of the distributor. You'll still need to check the total timing at cruise RPM, however.
Jerry