Hello gentlemen and ladies.
My 1976 Ford F250 with 390FE 4 barrel has been running OK but not great and the carb and advance have only ever been adjust "by feel" so i've recently purchased a vacuum gauge hoping to tune in the engine with that. I've been trying to look around at a proper order to do these steps in, but maybe it doesnt make a big difference? This is what i'm thinking of doing and the order in which i plan on doing it. IF you guys would do anything different please let me know!

1. Replace existing capped vacuum lines with vacuum caps (unused ports were previously plugged with whatever hoses we had laying around with screws in the end, some have cracks now, not great i know but that was 13 years ago)
2. Check other vacuum hoses for cracks/leaks in hose or connections
3. Plug vacuum gauge into an empty port on back of manifold tree behind carb
4. Adjust carb (float level, idle mixture screws, idle speed)
5. set timing based on max steady vacuum reading

Does this seem correct? I've not done this before and any tips would be very much appreciated. Thanks

 

The problem with that is it only sets your initial timing which seems nice but it's actually the least important aspect of ignition timing. under load and operating RPM is where the work is done as well as the damage. initial is meaningless here.

You need to set your timing with a dial back light . get your mechanical all in at 2000-2500 , it should be 30-35 normally. then hook your vacuum advance up and altogether it should be 50-60.

Now you go for a drive and get rolling good and nail it, under load do you hear any detonation ( pinging ) ? if so you either need more octane or less advance. if you don't either leave it or advance it a bit and see if it pings. you may have to limit your vac advance if it is too much. once you're happy with the way it performs then see how it starts. if it kicks the starter you need to change the curve in your distributor. if it starts good you're done.

Now you can use your vac gauge to set your idle mixture.

 

A timing light is necessary because it's very important to know what the distributor mechanical advance is doing at different RPM, from idle to wide open throttle. As mentioned the initial # itself isn't as important as the rest of it. Disconnect and plug the vacuum advance port. Then rev up the motor to 1000, 2000, 3000, etc. Make sure it moves smoothly up and down the RPM range without "scattering", and advances to spec, usually around 34° BTDC or so, and returns to the same point without sticking as you look at the marks. Be careful, avoid moving parts. Pay attention.

A vacuum gauge is useful for finding that optimum initial ignition timing setting for any engine, but this likely will end up to be too much advance on the far end unless changes are made internal to the distributor, custom tailoring the advance or "curving" a distributor. Most people don't want to do that and it's a pain in the *** on most Ford distributors because the springs and weights are buried deep.

After reading the forums for a while and seeing the problems with older engines I recommend that the TDC "0" mark on the vibration absorber (damper) be verified with a tool called a piston stop. Defective dampers due to age are pretty common.

If you want to try it anyway (I knew you would) advance the distributor housing in the (clockwise) direction slowly when engine is idling at around 600 RPM Idle and look for the maximum # steady vacuum gauge needle. At Sea level, and a stock healthy engine it will be right around 18" hg of vacuum. If distributor timing is advanced too far engine idle will start to wander and "hunt" and the vacuum will drop off quickly. Play around with it a bit and get familiar. Back off juuust slightly from the highest steady reading achieved, maybe 1/2" of vacuum from that exact point, and lock the hold down bolt tight. In the example provided this would be 17.5", every engine is going to be slightly different. This setting should be close to optimum, for pump gas. Before re-connecting the vacuum advance take a test drive and perform some full throttle accelerations and check for engine knock or "ping".

 

Maybe i will just leave the timing as is for now depending on how the hose check and carb tune goes.

 

It's adviseable to check it through the full RPM range prior to carburetor tuning.

 

I've always tuned carbs at idle. Set timing at factory spec, maybe advanced a couple degrees, set base carb settings - float level, mixture, idle. Then adjust mixture for best vacuum. Reset idle speed, if necessary, then go back and tweak mixture. Usually go back and forth a few times before I get both the idle and mixture when I want it. Other than float level, everything else adjustable is just for the idle circuit, unless you want to go into the carb to replace or modify hard parts, there's no adjustments for on throttle fueling. After the carb is set you can play with timing advance curves.

 

Right, but any significant change to the initial ignition timing also changes the manifold vacuum. That's why it's advised to make a second pass through on the idle mixture adjustment if timing has changed. There is little to no air flow through a carburetor at idle, so manifold vacuum is incorporated through the idle circuit for proper fuel atomization, this depends on a strong crisp manifold vacuum signal.

By the time he gets into the jetting and power valve stuff it's important that the ignition timing curve has been finalized.

 

I'm pretty sure the OP isn't looking to recurve their distributor, but IDK his engine specs, I'm assuming it's close to stock. If he set the timing at the factory spec, plus 1 or 2 degrees, he'll be fine. Then like I said go back and forth with mixture and idle speed to get the right curb idle speed with the richest possible mixture and best vacuum. For any stock to mild performance engine this will get you a really good base tune. If he wants to wring the last 10% out of it he can go and recurve the dizzy or start swapping internal carb parts.

 

Well he never really stated as such, but I get the impression he doesn't have a timing light.

Mechanic's vacuum gauge is an essential tool, though so is a timing light. It's very important to verify proper operation of the mechanical advance throughout the RPM range and a timing light is the only way to do that.