Maybe they rebuilt the engine, but reinstalled the old, clapped out distributor? Take the vacuum can back and start shopping for a new distributor. As already discussed it sure sounds like the mechanical advance is sticky or erratic. It will never run right if that's the case.

With all that said keep in mind 30° at idle is not too much advance provided the vacuum advance is what's used to get there, by connecting to manifold source of vacuum. In the situation you describe though, with it disconnected, something else is going on.

 

Do the balancers tend to slip with these trucks giving inaccurate TDCs? When I did my timing almost a week ago it was 18° up to 38° and it's at 28° up to what looks like would be ≈45° but the balancer doesn't read that high. Could I use the screwdriver method that I mentioned in my second post? No store near me has a piston stop.

 

If the balancer HAS slipped, it's a boatanchor and needs to be replaced. Or rebuilt, there's a couple places that do that. Should have been part of the engine rebuild imo.

 

Turns out the arm of the vacuum can had slipped off the little post it rests on. The snap ring broke when I tried to take it out with a hook so I was hoping it's stay there like someone had said above. That explains why the vacuum advance still wasn't doing anything so I'm off to find a tiny snap ring to keep it on. Why else would my timing have jumped up from 18° to 28°? It's not a boat anchor, I drove it an hour going 70 on the way home and drive it daily without a problem. If it was a bad distributor wouldn't it not really run right at all?

 

With the arm of the vacuum can sitting on the post you can see how the mounting screws don't line up. I can push it forward to line up the screws but wouldn't the diaphragm then always be partly open? It's identical in dimensions to my old one.

 

Normal. Just some pressure to hold the pick up plate firmly against the stop when there's no vacuum. It's got a return spring to return it to it's resting position.

IMO These mechanical trucks are great platforms for learning how to make an engine go vroom - vroom. You can't just plug them in to find out that's wrong with them and hang a new part or sensor. You really have to think about how the systems operate, what they do, when they do it, and how it effects the operation of the vehicle.

In this case I encourage you to think about how the vac advance actually operates, what it does, when it does it, and most importantly why. Then you go from hanging parts to understanding what an engine needs and how it operates.

Edit: When you asked what pinging sounded like I reckoned you're a young buck who's only known modern computer controlled vehicles. And then I realized it's been so long since I've heard it myself I've about forgotten what it sounds like too! LOL. As a suggestion, once you've got your timing dialed in, carefully mark it so you know how to get back there, and then experiment by advancing the timing until it pings. You should notice it first as you gently ease into the throttle going up a hill in high gear at about 50 MPH. If you give it more gas the pinging might worsen, and then it might go away as... the manifold vacuum drops and vacuum advance drops out of the equation.

Be gentle with it as you're testing the outer boundary of your ignition timing and back it off once your done playing. Left unchecked, pinging will damage an engine.

Anyway, I'm glad you're here and interested in learning about your truck!

 

You need to learn to read for content. Lots of people on the forum will provide advice that they've learned over the years but they aren't going to repeat themselves or argue.

If the balancer has slipped, it needs to be replaced, that's what's meant by that. There's a couple places that rebuild dampers - "Damper Dudes" and "Damper Doctors". Distributors are the precision mechanical "brains" of the ignition system when they get worn one thing they can get is erratic. Not saying this is for sure your problem here but it's not unusual. The advance plate loses the free movement it needs to work properly. It sticks, one day it works so-so, the next worse. If it's high mileage it should probably be replaced, especially a rebuilt engine, they deserve it.

 

It's been fixed. The mechanical advance was sticking just a little bit and the extra advance after putting the new can on was caused by the arm popping off and keeping it held open a little. Thanks for keeping it simple since I'd said I hadn't touched timing before, but don't assume that means someone's completely new to working on cars. It comes across as condescending when you assume someone knows nothing. I've actually touched almost every system on a car, just new to diagnosing timing problems.

 

I don't think anybody means to be condescending, there are hundreds of questions here every day, impossible to keep track of everybody and their skill level and experience, plus add to that we're trying to troubleshoot remotely. Sorry if you took it that way.

 

If you compare the new advance unit to the old one, I bet the arms are different lengths. That will sit the plate inside at a different position, and that messes your timing up. Afterall that is how it works, the vacuum advance moves the plate which varies the timing.

I believe on the fuel injected distributors with no advance, they have a bar in there instead, they have a name for it. And they have different bars for different applications.

 

No offense intended, sorry if there was any taken.

However, if you want to dig into mechanical distributors the "advance the base timing until it pings" exercise is still a good one. I hope this isn't taken wrong but... back in the day that was a valid tuning technique. Advance base timing until it pings and then back it off until it doesn't. Done. From there you might consider the point (conditions) under which it begins to ping and make adjustments to the advance curves. Maybe stiffer springs to delay the mechanical advance, or adjust the vac advance (if you have an adjustable one). Or try ported versus manifold vacuum.

Tedsters advice is also spot on. If the mechanical or vacuum advance mechanisms are sticky, or the bushings in the distributor allow the shaft to wobble, that's trouble.

Anyway, glad you got it sorted and we could help. We're here to help.