I was trying to set timing on my 351m engine in my 54 truck. I plugged vac line and set timing to correct mark but when I plug vac line back in distributor and put light back on it is way off the mark. I tried 5 more times but same results sets fine with vac line off but not when vac is connected back. It is a stock point style dist. What am I doing wrong.

 

Depends on what RPM, and/or what port is connected to whether this is a "problem". Some prefer connecting to a source of manifold vacuum, this allows for lots of advance at idle, as you've discovered. Tends to keep temperatures down.

 

Ah yes, the old ported vs manifold vacuum debate. You're fine, but factory source would be ported vacuum for distributor.

 

Ok guys thanks. So if I understand correctly pull vac hose off dist, plug and set timing. I was just wondering because when I put vac back on dist. and hit with timing light it was no where near the mark. It was right on when hose was pull and plugged but not when I unplugged and put back on.

 

Depends on the carburetor you're using. Basically the only difference between manifold or ported vacuum is whether the vacuum source is above or below throttle plate. I think there is way too much read into this.

Make sure the carburetor is setup correctly, linkage, idle mixture, idle RPM. Normally the throttle plates are completely closed at idle, or nearly so. Just a sliver of light will shine through if you look. The idle mixture has to be adjusted correctly, errors here are often compensated by raising the idle RPM enough to crack the butterflies. Don't do this. The T slot should barely be exposed at idle and this will provide for the idle circuit.

Using a ported vacuum connection, if the idle is set too high then the main metering circuit is in play because the throttle plates are opening up therefore vacuum is being applied to the distributor, and timing is advancing past the 6 or 8 degrees that is "normal". Mostly, if the initial crank, and mechanical (centrifugal) timing is setup it should not
matter one iota where it is hooked up except at idle.

Engine vacuum at cruise should be identical at either port. Idle mixtures though are very lean and like lots of advance, this keeps engine temps down in stop and go driving. Vacuum advance is load dependent (versus RPM) and comes into play at steady cruise speeds or going downhill.

The windup is you have to figure out what you have, and/or want to use and why, because it does take some experimentation, what works on my 64 might not work with power brakes and A/C. But, there's about a 15% fuel mileage penalty if you don't use vacuum advance at all so it's worth using, and makes for better driveability.

 

If your engine is stock, with original carb, cam, distributor, you should use what it came with, either ported or manifold. I would guess a 351M would have come with ported.

An interesting read:

Timing Mech vs Vac advance Good Read - El Camino Central Forum : Chevrolet El Camino Forums

Pertinent excerpt:

"Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more. "

 

That's the GM engineer screed, it gets reposted often on the net, usually without attribution. Great article. Far be it for me to disagree with his conclusions, but..

He is wrong about ported vacuum being an emissions era creation, my 1950 Motors auto repair manual discusses ported as useful for keeping idle steady. No surprise there.

Pg 20 Thirteenth Edition

"Both (Auto-Lite and Delco-Remy) types make use of a spring loaded diaphragm ... The spring loaded side of the diaphragm is air tight and is connected through a vacuum line to the carburetor above the throttle plate so that idling performance will not be affected." (Emphasis in the original)

 

Correct. You successfully set your initial timing.

 

I think someone stated above but I might have missed it. The vac line advances/retards the timing slightly. So plugging the line back in will affect the timing reading.