Got Time? (Part 3)


Done correctly, there will be no backfires from the ignition source.
The engine will light right off and provided the centrifugal and vacuum advances are working ok it will run fine.
If you are doing the “breaking in the cam bit” this will get usually get you through the 20 minute break-in period just fine.
Some like to shoot the timing right after start-up and all that’s required there is to have the vacuum advance disconnected.

In fact, if there’s one golden rule about dynamic engine timing and the checking of same it is to have the vacuum advance disconnected.
Pull the hose at the distributor and plug it so you don’t have a self induced vacuum leak.
Golf tees work well as small vacuum line plugs.

So, now that the cam is broken in or if you’re just dealing with a tune-up on an already broken in engine, here’s the drill on timing.

The book figures for the engine we’re dealing with here - the 210 HP 327 - shows the timing figures as follows:
Initial = 2 degrees BTDC.
Centrifugal = 32 degrees BTDC at 1975 rpm.
All-in = 34 degrees BTDC. (All-in equals Initial timing plus full Centrifugal timing.)
Vacuum = 16 degrees BTDC at 15" of vacuum.

All-in timing refers to centrifugal timing only and as mentioned is derived from the rpm level of the engine.
Centrifugal timing is zero (don’t be confused by the 2 degrees initial here, we’re talking only about how much timing the centrifugal advance mechanism puts in) until 450 rpm.
The 450 rpm level is where centrifugal advance starts advancing the timing.
The timing is all-in (hence the term) at 1975 rpm for this particular engine.
The book shows the idle rpm for the engine to be 500 rpm so idle speed will have to be backed down to 450 rpm to double check the initial setting.
If you can’t get there, but you do get it to idle at 500-600 rpm or so and assuming it’s a smooth idle - even with a hot cam, although the engine specs we’re quoting here are for a stock cammed engine - you can do a little extrapolation.
Extrapolation being a big word that in effect means to just think about it a bit and make an estimate as to where you should be with the information you do have.

The other way around this is to check the all-in figure at say 2000-2200 rpm.
Provided the dampener or timing tag is marked for that point.
You can mark the dampener for the all-in timing figure of 34 degrees and match it up to the zero point on the timing tag.
This necessary if the timing tag is marked off in degrees and the dampener just has a scribed line at the zero point.

With the vacuum advance disconnected as mentioned above, fire the engine - and making absolutely sure to stay out of the plane of the fan blades - run the engine up to 2000-2200 rpm and note the reading.
At these rpm levels it’s much safer to shut the engine down, change the distributors orientation if desired - only small adjustments in distributor orientation is required - fire it up and check again.

If the timing light shows no change in the centrifugal timing when engine rpm is changed, then you have a problem in the centrifugal advance mechanism.
Centrifugal advance problems are fairly rare.
At least the ones where it doesn’t advance at all.
The usual problem is when you have a distributor that’s been tinkered with a bit.
Most times by novice hot rodders who change weights and springs without realizing what they are trying to do.

As a small side note, when fan blades come off they can do considerable damage.
Two that I’ve seen, one in person and the other after the fact; a 53 Chevy, stock six cylinder lost a blade. It went through the hood and partway through the garage roof.
The other, a guy I knew had a fan come unglued, the blade hit his left forearm and the resulting scar tissue made it look like a hand grenade went off in his pocket. A more than serious injury and he was lucky to regain full use of his arm.

Now that the centrifugal timing is where you want it to be and operating correctly, all that remains to be done is to check the vacuum advance timing.

Couple of things to know beforehand is where the vacuum source comes from.
Not always as easy as you may think although the general rule is: ported vacuum comes from above the throttle blades and full time manifold vacuum comes from below the throttle blades.

Note that some Holley carbs have a full time vacuum bib above the throttle blades and others use a bib in the same general area for ported vacuum.
Carter carbs and their twin, the Edelbrock - they both come off the same production line - many times have two bibs below the throttle blades.
One reason for the confusion in selecting a vacuum source as far as Carters go and perhaps for the Edelbrocks as well is that the instructions point to the passenger side bib as the one to use for vacuum advance.
This is incorrect for GM engines and in fact the passenger side bib is ported vacuum.
The drivers side bib is what you want and it is full time manifold vacuum.

You can source full time manifold vacuum from several other places as well.
Any fitting that taps into the manifold proper.
Even those in just one intake runner.
The large and sometimes medium sized bibs at the front and rear of the Carter carb base.
As well as the small hex shaped vacuum manifolds usually found at the rear of GM engines.
These are hex shaped as mentioned, have several bibs of varying sizes and you can either T into one or if you have an unused bib, use that.
Be careful you don’t tap vacuum from a thermostatically controlled vacuum switch because you may end up with no vacuum at all due to the engine temperature not being sufficiently high.
T’ing into the power brake vacuum line also works well.

Now that you’ve sourced your vacuum advance properly, checking it for proper operation is easy.
With the distributor vacuum can connected or re-connected as the case may be, fire the engine, let it idle and take note of the amount of advance.
It will be quite a ways above the initial setting of 2 degrees BTDC at idle.
Assuming you have 17-19" of vacuum you should show the full vacuum setting.
IE: 16 degrees BTDC plus the 2 degrees initial making a total of 18 degrees BTDC.
You’ll probably show a bit more than the 18 degrees BTDC because the advanced timing causes the engine to speed up and once the rpm raises the centrifugal adds it’s little bit to the mix.
Which means you may have to re-adjust the idle rpm once the vacuum advance is connected.

Here’s how you check the vacuum advance for proper operation.
It’s not enough to show full vacuum advance, you want to make sure the vacuum advance is operating freely and retarding when it should.
Retardation coming in when the throttle is depressed putting the engine under load which makes vacuum levels drop.

Paying attention and once again staying out of the plane of the fan blades, wing the throttle lightly and watch to see if the vacuum advance is backing off.
Don’t be fooled by the centrifugal advance coming up as rpms increase.
Winging the throttle in this case simply means cracking it open quickly, which drops the vacuum levels and then letting the throttle return to idle without much gaining much rpm at all.
You should see the timing back off when you do this.

If the engine stumbles when you wing it, run it up to 1000-1200 rpm, take note of where the advance settles out and then wing the throttle.
You’ll have a little more advance on with the rpm spun up a bit and added to the full vacuum advance, but the vacuum advance will retard just as it should when winging the throttle.

If you run the engine up past the 1975 rpm mark the total timing will be 50 degrees BTDC.
That due to the centrifugal advance has reached it’s maximum due to the rpm level and the vacuum advance has reached it’s maximum as well due to the high 19" or so vacuum level as well as the 2 degrees initial timing.
It’s not a problem.
There is no detonation because the load on the engine is light and just like at idle the fuel/air mixture is lean and takes while to burn.

Aside from just paying attention to where your vacuum source is, you can double check it with a vacuum gauge, timing light or tachometer.

A vacuum gauge will show 17-19" vacuum at idle with a stock cam indicating full time vacuum.
Spinning the engine up with light throttle settings under no-load conditions will have the gauge indicating the same 17-19" of vacuum regardless of rpm.