Strange timing issue

Strange timing issue - '68 318

C130 Chief said
Per the factory manual, timing should be set with the vacuum advance disconnected, and the vacuum port on the carb plugged (although if everything is reasonably correct there should be no vacuum coming from this port, a good crosscheck).

For those saying I should not set my timing with the vac advance connected. I hear you and I'm not. The only reason I have it connected is because I cannot seem to get the timing mark into view with it disconnected. I wanted to report what it looked like with the vac advance connected as part of the diagnosis, since there is nothing to report with it disconnected.

C130 Chief said
Since you have vacuum coming from the "timed" or "ported" vacuum port on the carb, your throttle plates are slightly open at idle. Also, if you are at 10 BTDC with the vacuum advance connected, and vacuum applied to it, you are probably more like 10-15 ATDC static. This is why your idle drops drastically and the mark moves off scale when you disconnect the vacuum advance. You've confirmed that your vacuum advance works!

Cool, so I will try again to get that mark into view with vac disconnected. Maybe I wasn't brave enough and just needed to keep turning.

C130 Chief said
So what you need to do is set the timing to 10 BTDC with the vacuum disconnected and capped. When you do this, you'll have to set the idle speed back down some, as I bet it ends up about 1500. Then, as you drop the idle back down, the throttle plates should close just enough that vacuum is no longer present at the timed port. This is how a stock 318 should work.

Thank you for this breakdown. I am hoping this is where the problem lies and that now makes mechanical sense to me. My intuition is that the fact that I'm idling at 250 reveals a significant defect somewhere and that my idle setting with the vac connected is overcompensating for it. Regarding my idle setting, I noticed while trying to dial in the carb that there is a sort of breaking point where my idle is either 700-900 or 200-300. With your explanation, I would now guess that this is due to the throttle being open far enough to the point of engaging the advance. I reckon I'll need to get that low idle up without the idle screw.

C130 Chief said
So, there are a couple of other things to check. You want to make sure your mechanical advance is free and working right. To do this twist the rotor arm clockwise and release. It should turn 3/8 to 1/2 inch at the tip and spring back all the way when released. Also, as a quick reality check, the nipple of the vacuum advance should point more or less to the top shock bolt, or slightly behind (or #6 plug). If you have to rotate the distributor (counter-clockwise) more than this the intermediate shaft is either a tooth or two off, or the wires are not in the right spots in the cap. With the timing mark lined up at TDC, the rotor should point to the throttle return spring bracket (left forward most intake bolt). If it does, the intermediate shaft is installed correctly, if not, it isn't (it happens). Either way, the quick fix is to shift all the wires over one socket in the cap. Big picture here is with the engine at TDC, the #1 wire should be in whatever socket allows you to clock the distributor such that you have a reasonable range of motion to adjust the timing. Looking straight down at the distributor, assuming the rotor points in the normal direction, the #1 wire should be at about 5 o'clock, and the vacuum advance nipple should be about 8 o'clock.

I will check this tomorrow. I have a hunch that this is where the problem lies. The distributor has been through the hands of a few mechanics for various reasons and now that I am trying to do this work myself, it seems likely something could have gotten installed slightly off or wired incorrectly. I'll check that the wires are going to the correct plugs and that everything is oriented roughly as you describe.

C130 Chief said
I can't agree that the vacuum advance should be left disconnected. There is no advantage to doing so and many disadvantages. BTW this 318 doesn't have the original timing chain in it does it?!?

My dad and I had the engine rebuilt about 8 years ago, when I was young and inexperienced both the cars and with people. I wouldn't choose the same garage to rebuild the engine again if I had another shot at it. In their favor, they did get it running again after the head gasket blew. Unfortunately, I have no idea whether or not they replaced the chain. I wouldn't be surprised if they didn't.

Thanks again to everyone for all of your help.