This is the sticker on my valve cover. It's a 77 F250 with a 351m. No A/C. Hope this helps

 

Well...I hemmed and hawed and went out and put base to 12, total ends up near ~45. Figured - it's in the middle....more conventional numbers than I had earlier today, or yesterday for that matter. I figured ford would say all 351m were ___BTDC....say, all 1/2 ton rigs were 8, or 12, or whatever. Did they tune every one? Or set them differently if they had AC, or some other hotel load?

Good stuff. Grateful for the info you fellas offer. I'd have zero idea otherwise, and not many options for fellas near me who have finesse for rigs with no ODBII port.

 

What I'm getting at, is 34 or 36 BTDC is the important number.

The initial timing # really isn't. It is important that the distributor mechanical timing works properly and smoothly advances and retards to spec up and down without sticking or scatter. The amount of internal advance is a problem with remanufactured distributors, no telling the application it came from before it was rebuilt. Can kind of sniff it out by watching the total timing.

Ford distributors limit the amount of timing through "slots" underneath the reluctor. 10L, 15L, 18L, etc are typical. This is 1/2 the crankshaft degrees. So a 10L slot = 20° internal centrifugal timing advance. Add 14° for initial that is your total timing - 32° BTDC.

Can play around with different slots, different springs, different amounts of initial timing, but it always adds up to around 34° BTDC total timing for practically every OHV ever made, not just Ford. Vacuum advance does not factor into this, a very common misconception.

If the ignition timing is retarded the engine will tend to run excessively hot, poor fuel economy and sluggish performance. Most stock engines run fine with the distributor connected to ported vacuum. This is another widely misunderstood subject to say the least. You could have a defective distributor, open it up and see what you have.

 

I understand - I'll target 34~36deg total timing and be right where it ought to be; base timing will fall out where ever it needs to (20deg in my case). I am getting confused with what the vacuum does afterward. My distributor is brand new. Nothin ' fancy. Oreilly spectra brand. But works much better than its predecessor (which had sheared with an oil pump failure; and the vac advance was broke). And I hear you about how the vac advance should come into play under load, not RPM - something I've yet to understand how, but that'll come. So, if I go set total timing to 35, and vac pushes that to 55deg, should I try adjusting the vac port until...? or perhaps, if it's running fine, just let it go?

Aside - had been under the impression that the distributor should be hooked to ported instead of manifold. Could be wrong there too.

The adjustments haven't made a noticeable difference in they way this rig runs, from what I can tell, but it does change my comfort level knowing it's running like it's supposed to.

Thanks again!

 

As a general rule, a stock or OEM type engine should probably be connected to ported. If you don't feel you have a good handle on how it works yet this is a good plan for now. Performance or street driven hot rods might want (or have to) use a manifold connection due to the valve overlap of the camshaft.

Connect your vacuum gauge with a long enough hose so you can mount it in the cab or be able to glance at it on the road. Then you can see how the engine vacuum works when accelerating, cruising, decelerating or going up a slight incline etc. It's all over the place and constantly moving most of the time.

The vacuum advance canister is a little more tough to calibrate without special tools because there isn't any load whatsoever on the engine just checking it in neutral with a timing light. Basically you can overshoot it a little bit. The engine in my slick pulls 21" vacuum in neutral revved up, and maybe 16" or 17" vacuum cruising. There isn't very much load on the highway, but there is some.

So while it might show 52° or 54° ignition advance on the balancer with a timing light in the garage, it would be somewhat less than that when actually on the road. I can measure vacuum, but I can't stick a timing light on the balancer when out on the road. The spec in the manual for my application is up to 47° BTDC or somewhere right in there, but they used vacuum pumps and distributor machines to set it.

The exact figure you want to use is based on weather, altitude, fuel quality etc. Basically though as a practical matter crank it in till it pings or rattles at part throttle and back it off slightly, till it doesn't. A little part throttle rattle every now and then on deceleration is acceptable. Don't confuse this with the engine knock under heavy load conditions. Give the engine what it wants, not an arbitrary number. Remember, just the vacuum advance is adjusted at this point. The mechanical or centrifugal curve is already done, so don't mess with that. Don't retard the mechanical or centrifugal advance timing to try and compensate for too much vacuum advance.

What people have a hard time understanding is the two systems work opposite or complement each other most of the time. When one system is advancing the timing, the other is usually pulling some out. The only time they really are additive in a big way anyway is steady cruising, on level ground, at part throttle.

In that specific situation the RPM is relatively low, manifold vacuum is high, and the mechanical weights and springs aren't quite at their maximum advance, and even if they were it wouldn't be near enough ignition advance for cool, efficient combustion. This is why vacuum advance was invented. It's a little tricky to setup sometimes but makes for better all around driveability even in stop and go driving and typically will lose 3-4 mpg hwy without it so it's worth taking the time to set it up right.