Per common tune-up instructions, I have adjusted my timing and gone through the idle mixture adjustment.
An odd thing then occurs: as RPM advances slightly off-idle, I would expect the vacuum reading to go DOWN from 15.5 - but it goes UP to about 17.5 !

This occurs around 800 RPM, give or take a bit. My best guess is that the first part of the advance curve starts kicking in, but I thought that happens significantly past 1000 RPM.

Any ideas?

Minor points: I have the vacuum to the choke warmer circuit blocked with a screw. I did remember to adjust for highest vacuum, then backed the idle mixture screws out a half turn. Yes, the gauge is plumbed to intake manifold, and the distributor vacuum advance goes to ported vacuum. This is after complete warmup. Although I have not indexed the TDC mark, I believe the Initial Timing is 15 degrees on the original dampner.

Kevin

 

Sounds about right. Too far advanced makes it hard to start. Too much advance could make it ping (valve clatter). Carry your timing wrench and screw driver with you for a while in-case you need to make some fine tune adjustments.

 

Yep, that's normal. All is well. Keep fine tuning

 

Now that I've verified TDC. Initial is correct.
Now, back to my question... Why?

Kevin

 

Why what? Why does vacuum increase? Because engine speed is increasing. It's sucking harder. Now if you had the engine loaded, such as accelerating in gear, the engine speed would not increase as much, and the throttle would open more relative to speed. THEN vacuum would be going down, until engine speed caught up, then vacuum would start rising again until the desired speed is reached, then it would hold steady.

 

Oh the lost nut down the intake. Well where did you find it?
As we did not hear back of the recovery mission ?

Also did you have your vacuum advanced removed from carb when timing it?

Orich

 

^^^This^^^

When using a vacuum gauge it's important that the idle RPM is set at factory spec. Say, 500 - 600, maybe 650, for V8. For some reason everybody wants to spool the engine WAY up when using a vacuum gauge. Get that needle in the green, I guess. Don't do this.

You may as well use the vacuum gauge to set base ignition timing. Warm engine up, disconnect and plug vacuum advance port on carburetor. Put the timing light away for now. Advance distributor body clockwise for highest steady vacuum reading typically around 19" or so at Sea Level elevation, stock motor, stock cam.

Then, back off (retard) the distributor 1" of vacuum indicated from the highest obtained, in the example here, 18". Your engine may be different, but you get the idea. Lock down the distributor hold down bolt, and re-connect the vacuum advance. Check for ping on heavy acceleration. This should be very, very close to optimum.

Engine has to be warmed thoroughly to set idle mixture, first ensure float level is correct and finalized before proceeding. Usually 1 or 1 and 1/2 turns out from seated is the start point for mixture screws. This is an arbitrary ballpark setting, just to get the engine to idle without stalling is what we want, they can't really be left at this setting for a good result.

It's very important that the throttle butterflies are completely closed or nearly so, at idle, and ONLY the idle circuit is providing fuel at idle. See pic. Adjust the mixture screws before messing with idle RPM too much when possible and make that carb work for a living. It will start to hiss when you're headed in the right direction. As you turn them in, the vacuum indication on the gauge and idle RPM will go UP, so turn the RPM screw back DOWN to spec.

Adjust mixture screws evenly 1/4 turn at a time for highest vacuum obtained with smooth idle. Use your ears more than the gauge though at this point, giving a little time for the engine to get used to each new setting before continuing on, a quick blip of throttle to clear it out helps. One potato-two-potato, it should be purring like a kitten now. I wouldn't then "back the screws out a half turn" after doing all this, you may as well throw the vacuum gauge away! Some tuners even recommend a 1/8th turn in leaner after finding the highest vacuum. The Ford manual specifies always favoring a slightly rich setting, but a half turn out from highest vacuum is a lot.