Chumley

Hi all, all the Chevy truck sites are unclear on the concept of how to allow new members to register, so here I am. I know most GM engines of this vintage (early 70's) use the same procedure for setting initial ignition timing. I run a Rochester 4MV on my trucks 402, and in the past always set the timing with all hoses connected. After learning that GM says to disable the distributor vacuum advance when setting the timing, I did that. Then after reconnecting the vacuum line and checking, the timing mark was not even on the scale. Runs fine, but I can't think of why there is a difference from the way I set timing before and the timing mark read to where I set it. The reason I'm confused is that, after a lot of research on the subject, a Rochester (or most carbs for that matter) don't start pulling vacuum until just off idle and above. This is with the vacuum line connected to the ported, or timed vacuum port, where it should be for a common street engine. My plan is to hook a vacuum gauge to this port and see what it is doing at idle. Bottom line question: Why disconnect vacuum line from distributor if there is no vacuum anyway at idle form the timed port? I also used to adjust the timing on my 70 SX with all hoses attached (usually at around 12BTDC) and the engine ran great. Thanks in advance for input or insight

oldcutlass

You need to figure out why you have vacuum going to the advance if you're hooked to ported. The reason you block the vacuum advance is to ensure that there is no vacuum to the advance and your settings are accurate. Are you still running points?

Sugar Bear

If there is vacuum at idle where it shouldn't have vacuum, check for a throttle plate that is sticking partially open or too high of an idle speed screw adjustment.

Fun71

The reason for disconnecting the vacuum advance while setting the timing is, at least on all of my vehicles, the timing is set at an engine speed above idle, as in 1100 RPM. At this point there may be some vacuum present on the ported source, and that would impact the timing setting. Disconnecting the vacuum to the advance canister ensures there is no vacuum advance at all when setting the initial.