The Stovebolt Site Forums The Shop Area Engine Shop Setting timing using a vacuum gauge

Which vacuum source should I use, ported or manifold? I've never tried it using a vacuum gauge before, alsways used a timing light. But after reading some of the posts about setting timing, I would like to try it. I'm just unsure about where to hook up the gauge.
Just for the record, my dizzy is an aftermarket one that does not use a vacuum advance, so there is no line from the carb for one. I also have dual carbs, if that is an issue.

Does you intake manifold have a vacuum port for a vacuum wiper?

Yes, it's plugged at this time, as I have converted to electric on the wipers. My carbs also have the port for the advance, they're just plugged.

I like to get the engine running at close to cruise RPM, maybe 2,000 or so, and use full manifold vacuum to the gauge. Carefully start advancing the timing until the vacuum maxes out, then go a bit further until it drops slightly. Back up to the best vacuum reading and lock the dizzy down at that point.

If the engine is able to start without kicking back from too-early initial timing after making the cruise-RPM adjustment, you're good to go. Otherwise, you'll have to re-curve the centrifugal advance system to get the best compromise of starting and cruise-speed advance.

One little precaution- - - -on several dyno runs, I've observed that one degree of over-advanced timing drops as much horsepower as five degrees of retarded timing from the optimim HP point. Always err on the side of a little less timing, rather than having too much!
Jerry

Thanks Jerry, that was just the onformation I was looking for. Figure it's better to ask someone that's done it before than go in blind and make things worse!

Jerry,

Is the procedure the same for a stock 235 distributor with the vacuum advance connected.

Yes, it works the same way as long as there's no load on the engine. As soon as the engine goes from light-throttle cruise to pulling or accelerating, the vacuum advance drops out of the picture and the spark is retarded somewhat, because of a drop in manifold and/or venturi vacuum. That prevents detonation from over-advanced ignition timing when the engine is in a loaded situation. It's usually a difference of 5 to 8 crankshaft degrees, sometimes as much as 10 degrees on some distributors. For instance, a small block V-8 develops its best power under load at about 36-37 crankshaft degrees, and the best fuel economy at light throttle cruise happens at around 42 to 46 degrees of advance.
Jerry

I printed this topic to try it out.

edit:
Do you leave the vacuum line hooked to the distributor or unhook and plug it. Is that why you take it to 2000 rpms to get full mechanical advance.

When I'm setting the max mechanical advance, I want enough RPM to overcome the return springs on the mechanical advance flyweights. That usually happens by 2,000-2,500 RPM. The vacuum advance needs to be disconnected at this time. Taking the RPM back up to cruise speed with the vacuum line connected allows me to measure the difference between maximum centrifugal advance and the additional advance added by the vacuum chamber. Since his vacuum chamber has an adjustment, he can tinker with his total advance number to get the best fuel econony without over-advancing and getting into the detonation range.

Some highly-modified engines need to have the centrifugal advance system restricted to prevent over-advancing when extra initial timing is dialed in to improve throttle response when a very radical camshaft grind is used, but that's a whole different can of worms!
Jerry

(already STFA)
Sorry if I'm just not understanding something. I feel like I'm getting bits and pieces and need the whole procedure. Here's what I gather is the process, from accumulated research. (specifics relate to stock 250 with non-computer controlled HEI distributor)

So I disconnect the vacuum line to the dist, rev to 2000-2500, adjust timing to max vacuum, back off 1" of vacuum. Reconnect line to dist,and then what? Seems that would then add in another 10deg or so of timing. Or am I supposed to leave it connected and time for max vacuum at idle then back off 1"?

I also gather I should use manifold vacuum (researched this exhaustively and this makes the most sense), that the full vacuum advance should be all in around 10" of vacuum or so, "all in" should be about 12-15deg of additional advance, mechanical advance should start coming on at 1000 and be all in by 3000rpm, and it should be roughly 20deg of additional timing.

All of these seem to end up, on my vehicle anyway, at about 14-15deg initial advance.

So at light throttle 15 initial plus 15 vacuum is around 30 total, or at WOT and high (for mine, 3000) rpm,15 initial plus 20 mechanical/centrifugal is around 35 total.

With all of this, discovered my vacuum can Is doing diddly, will replace with adjustable and my mechanical only seems to advance 5-8 degrees, so a recurve kit appears to be in order.

Comments anyone?

If you're only getting 5-8 crankshaft degrees with your centrifugal advance, something is restricting the movement of your distributor cam and/or the flyweights. There should be about 15-20 degrees of centrifugal advance, plus another 10 degrees or so from the vacuum chamber. Most engines, 6 or V8, run best at around 36 or 37 degrees of initial plus centrifugal advance. The extra advance from the vacuum unit happens at light throttle cruise and is only for increased fuel economy. It goes away as soon as passing or other acceleration demands are made by opening the throttle,and the advance drops back to whatever is being provided by engine speed.

If you're going to use a vacuum gauge to estimate ignition timing, do so with the vacuum advance disconnected, and adjust the timing for the highest vacuum reading at no-load cruise RPM. Then reconnect the vacuum, and do a test drive to see if the engine detonates under slight acceleration at cruise RPM. In case of detonation, retard the timing a degree or two at a time until the detonation disappears.
Jerry

Good feeback, I think I will leave it be. I have another problem now.