There is a noticeable shimmy when I am idling or at a light, but not when I accelerte. Smooths out to a smooth purr when the gas is pressed. Is this merely a carb adjustment issue or a problem with the timing?

 

An engine vibration can be caused by a lot of things.
Ignition timing (that is, degrees off of TDC) is probably at the bottom, as it would affect all cylinders equally.
It is possibly a carb adjustment, but it would be for the idle mixture screws, not the external adjustments.
If it idled well before, then leave it alone. Carbs rarely go out of adjustment by themselves, but require human intervention.

I would first suspect a miss in one or more cylinders.
Most likely caused by problems in your ignition system. Likely culprits are old or shorted plug wires, cap and rotor - any of the ignition components.
Secondly by vacuum leaks - around the base of the carburetor and the intake. Vacuum leaks will also cause similar symptoms. especially if minor.
Thirdly by engine wear - low compression in the cylinders with a miss. Could be rings, a collapsed lifter, wiped camshaft or anything doing with compression.

I would check and replace ignition components first.

Failing that as a fix, I would check for vacuum leaks. Using an unlit propane torch, I play the gas around the bottom of the carburetor, around the intake where it bolts to the heads, around the distributor vacuum advance canister, and anywhere there is the potential for a leak. If the idle speed goes up at a spot where the gas is sprayed, then you have found a vacuum leak.

Checking compression requires a compression tester, and any actionable information from a compression check will involve engine work.

 

Assuming I have a lack of anything other than basic mechanical skills, is this something I can do alone or do I need to visit a mechanic? And does this explain why it drives smoothly, but only idles roughly?

 

Can you do the work I described? Absolutely, as it is at the bottom of the skill levels needed to maintain these trucks.
I would recommend you get a Haynes or a Chilton's manual for '65 - '79 trucks and study it. Get both in fact, they are well worth the money.
I consider myself to be about the middle of mechanical skills, so this kind of work is very easy for me.
Call around to get a quote on a tune up for your truck - and you may very quickly decide to tackle it yourself. Tune-ups are gravy for mechanics, and a tune up may cost you about a bill and a half- but still not find or fix the problem.

Please accept my apologies if my comments seem condescending, I certainly do not mean to appear that way. It is difficult for me to evaluate what your "basic mechanical skills" would be compared to mine, so I assume you are starting your work on vehicles.
I am very glad to help any way I can.

Yes, it can. That's why I made these suggestions in an ascending order, to help track down the problem.
Any one of these areas can be causing identical symptoms.
And it could be something else entirely that would reveal itself only after more thorough testing.
And it could be something simple, too.
You need three components for an engine to run:
1. Spark, fired in time (ignition system)
2. Fuel/Air, properly metered by the carburetor
3. Compression, achieved and maintained at the proper time by the rotating assembly (pistons, valves etc).

If any of these fails or is inadequate, either in the engine as a whole or in a single cylinder, the engine will misfire, or not run at all.

For example, you ask if these can make it run smoothly when driving but it runs rough at idle.
The answer is yes, because the engine needs enough compression to run right.
If one or more cylinders have low compression, they can make it run rough because the low engine speed (say, 500 - 600 RPMs) allows the compression charge in the engine to escape before the plug fires.
However, when running at 1000 RPMs, the engine is running twice as fast, which gives less time for the charge to escape before firing.
Things that lower compression and allow the charge to leak out include worn rings and block, worn valves and worn valve rotating parts (including a wiped cam and wiped lifters).
The answer is also yes, because of wear or changes in the idle circuits or the power circuits in the carburetor.

But you cannot tell for sure until you eliminate the other possibilities such as ignition or compression.

Hope this helps.

 

That helps tremendously. I'm hoping for a cheap fix and not something wrong with the engine.

 

If there's a vibration, it may not be caused by the engine...as vibrations can radiate thru the entire power train, frame and body.

Could be caused by broken rubber engine/trans mounts, bent wheel, bad u-joints, broken shackle that has caused the rear axle to shift over to one side.

 

Appears suggestions pretty much covered; add my $.02; if the pulley, damper, etc, have been replaced may not be match set; and/or, belt alignment or tension needs adjusting??

 

C3AZ-6312-E .. Harmonic Balancer-Contains 3 holes to mount accessory outer pulleys to add P/S & A/C / Reproduced

1960/65 Ford Passenger Car: 352/390 & 1966/67 410/428 / 1965/67 F100/350 352.

A rubber gasket is placed between the inner damper and the outer single sheave pulley, then the 3 parts are pressed together forming a one piece assembly.

Over time, the gasket age cracks and/or shrinks, causing the outer pulley to begin to "walk away" from the inner damper.

If not noticed in time, the pulley will fly off, might end up in the radiator core. Not pleasant!