I have seen a lot of posts where people advise using a vacuum gauge to set the timing. Can someone elaborate on this or point me to a thread on doing this. I haven't done a tune up in 30 yrs and now have a "66 with a 240 and a '64 with a 390 (not original) to play with.
Thanks. john

 

Engine timing isn't just the initial or "crank" timing. It's just part of it, there is mechanical or centrifugal weights inside the distributor. And vacuum advance. Just Sayin'.

Usually a V8 will run best with around 34 to 38 degrees of "total timing" which, despite the name is not counting the vacuum advance, this usually adds another 10 to 15 degrees when engine load is light, and vacuum is high.

Now, we can divide that 36 degrees say, however we want between crank timing, and mechanical timing, but we always want about 34 to 38 total. Too much initial usually causes starter kickback, especially when warm, though. Engine manufacturers usually have a pretty low initial crank timing - 4 to 8 degrees say - and then a whole bunch of too much mechanical advance - and, adding insult to injury, comes in very late, or at a high RPM, close to 4000 RPM or even more. If you're routinely hauling gravel and using the truck as a truck, that's probably what you want. If not, not.

What I'm getting at with all this though, is be careful. Advancing the crank timing, without decreasing the mechanical advance, usually doesn't work so well, there isn't "room" so to speak, especially with a high compression motor. Best results by curving the distributor. May need to use premium fuel for maximum performance. By installing lighter spring(s) in the distributor, and limiting the amount of mechanical advance versus crank advance, we bring in a little more initial timing, a little less mechanical, but earlier in RPM say 2800 RPM. This makes for a lot better performance and acceleration.

Here's how to set the basic ignition timing with a gauge if you want to experiment with that. Disconnect and plug vacuum advance. Connect gauge to a source of manifold (constant) vacuum. At book idle spec, advance distributor timing clockwise. The idle RPM will increase along with the indicated vacuum. A normal, healthy stock V8 will pull a maximum of around 19" or 20" at sea level. This may be too much advance, though.

As you continue advancing clockwise the idle will at some point stop increasing, and start to hunt or stumble and wander. The engine vacuum will start to drop off. Retard the timing back, about 1" from the maximum smooth idle point that is (before it starts to break up), usually around 18" something like that. In Tucson it may end up maybe a little lower, because of the altitude. Take for a short test drive. Check for pinging on full throttle acceleration. Back off a little if pinging or knock is evident. Reconnect vacuum advance. Check for engine knock or pinging during part throttle acceleration at cruise and, at steady highway cruise. A very light occasional knock on part throttle acceleration is OK.

When you get a chance, verify the timing with a light. Very likely you will find this vacuum timed engine "sweet spot" is around 12 degrees or so. Again - severe pinging may result if the distributor is not adjusted to allow for this. Excessive advance may break a piston ring or a hole in the piston itself. But, a vacuum gauge will allow one to adjust their engine and carb to a high state of tune. They are a great diagnostic tool as well. Let us know what you find.

 

I'm tooting my own horn but I know tuning, and use vacuum a lot. Tedster9's post is pretty good, and he put a lot of effort into a reply to a short incomplete question.

I'll help, but we need more information. Do you have a good timing light and marks? How far into this do you want to get? Do you just want to loosen and turn the distributor or more? Is this a stock engine, what are you goals, etc.

I need to modify one thing about what Tedster9 said.
At this point several things need to happen before moving on. Very likely timing will be way too advanced and idle RPMs too high. That's too be expected and a good sign at this point if it is.

The first thing that needs to be done here is to put a timing light on it and take note of the timing. The timing & idle will also likely be unstable to get a good measurement. Adjust the carb to slow idle and readjust timing to max vacuum. Then take note of the timing, I call this number 'ideal idle timing'

Next shut it off and try and start it again. It's likely that the timing will be too advanced to start, that it will kick back. Retard the timing and try again until it starts easily. Re-adjust carb and note timing, I call this 'Ideal starting timing'

There's more but at this point it's safe to ......

 

You're right, I forgot to mention to bring the idle back to spec.

It's a good plan to readjust idle mixture on the carb too, anytime the timing is changed. Using a gauge is a good training exercise, if people can just learn to put the timing light away for a while.

Find where the engine runs best and don't worry too much about what it ends up at. The Y block in my truck seems to run best somewhere around 12 to 14 degrees. No starter kickback, idles smooth, runs good. The distributor has limited mechanical advance as described though, total is still around 34 degrees. This is where a timing light is really required - seeing how much total timing there is. and when the advance comes in RPM wise.

Some timing tape helps to see what's what for this.

 

I completely agree, often even when it comes to total timing. It all depends but often I tend to rely more on how the engine reacts to timing then the actual numbers. I use the numbers as a reference point not really for the number itself. A vacuum gauge, and my ear will tell me a lot more then a timing light. That said I have a knowledgeable ear, very few can use that method. No one set of numbers are ideal for any two engines.

Biggest factors are if I have any reason to trust the accuracy of the marks and if I can hear a ping over the exhaust.

However number wise A LOT can be done with a piston stop, timing tape, and a plain back to zero timing light.

 

Thanks for the responses. My 240 runs well but I want to give everything a check. I just rebuilt the carb on the 390. The truck had been sitting for at least a year and a half. It runs and I can drive it but I can tell things need adjustment. I kept reading in threads about slipped marks on the harmonic balancer that may cause problems when doing a tune up. The '64 with the 390 will mainly be for camping so I don't need optimum performance like if I were hauling big loads. The 390 has a 2 barrel carb but the ID tag # isn't on the ref chart in my MOTOR manual.

 

It's probably a good plan to check for this. The damper absorbs torsional vibration of the crankshaft. Once the elastomeric (rubber) bonding breaks down it isn't doing any of that

 

You have bigger problems then any kinda of advanced tuning I could talk for hours about. Nor did you answer if you have a timing light. You CAN NOT replace a timing light with a vacuum gauge. Get one, FWIW I prefer the old school ray gun looking back to zero lights. These, mine looks exactly like this

This one is Craftsman but they were sold under many names with the only difference being the sticker on the side. Back to zero versions have a dial on the back that delays the flash a # of degrees. Well made tough simple suckers. I'm 99% there's no new ones you have to find them used.

You need to find out if your balancer has slipped. For this you'll need a piston stop and a tailors measuring tape. Piston stop tool will look like this

It goes in the spark plug hole to stop the piston. You install it, turn the engine by hand in one direction till it stops, mark the balancer, turn the other direction till is stops, mark the balancer. Then use the tailors tape to find the point exactly in between the marks, that will be TDC.


I personally dislike Autolite 2100/4100 carbs, I'd replace it with a 4bbl manifold and a Holley based carb. I'm also guessing you still have points. If you really want to tune just get at the very least a DSII setup and you'll need an adjustable vacuum advance. IMO MSD distributors are very worth the money, consider it.

 

Sorry about not providing more info. I have a timing light, dwell meter, vacuum gauge, vacuum pump, pressure gauge (fuel pump pressure), compression test gauge, remote starter, and feeler gauges. The engine still has points. At some point I would like to put a different distributor on it along with a diff manifold and a 4 barrel carb but other things have priority.
I really appriciate every ones input. It's been 30 plus years since I've done an old fashioned tune up and I'm reeducating myself on the subject.

 

Well then .......

 

Update. Finally managed to do some work on the 390. Between adjusting the carb and setting the timing to about 6* (was at TDC). The engine really smoothed out. Took it for a short drive up the mountain and had good power and response through out the gears and rpm ranges.

 

So... used a vacuum gauge?

 

Fords 6* was on the conservative side. Take my 302-4V Cougar and the recommended 6*. When I got around to opening my distributor guess what I found? I had a cam with a 10L and 15L with the 10L selected. The engine had been running a total of 26* for years from the factory. Even when I changed a cam long ago I wasn't knowledgeable to look into a distributor at the time. After moving the initial up to 15* the car was a whole different animal.

My 410 and 390 are both at 10*.

 

Not related to problem, however this information is needed for this thread.
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Thanks! you always have those little tidbits that are handy!