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I have a 1940 GMC one ton with a 1955 248 engine. I want to fine tune the timing. First time timing a vehicle. I have read past posts stating using the bb or U|C with the "|" as the mark for TDC. The truck sounds good running but just want to make sure everything is up to snuff. I unhooked and plugged the vaccum line. Started the truck and set it at TDC per the "l" between the U and C. If I use the bb it idles too fast. The octane selector is set about 2 or 3 degrees of advance at this stage. I have my idle adjustment screw 1 and 1/2 turns out. I adjusted the distributor with a timing light so I would be right on the "|" between the U and C. Sound good? Do I need to check the vacuum timing or am I good?
Thanks in advance,
Bill

The octane selector is a throwback to the 30s and 40s when gas quality and octane were all over the place, and driving at high or low altitudes required an adjustment. The plate allowed some easy adjustments and a hard stop at 10 degrees advanced or retarded to compensate for the bad fuel or altitude. With today's fuels, you don't usually make those kinds of adjustments.

Now, if you look at the octane plate and the clamp you will see that the octane plate is held down by a bolt and the distributor is secured by a clamp and a screw. The screw is what you use to set the timing, not the plate.

So ideally, you want to set the octane selector plate at 0 and set the timing using the screw clamp arrangement. You also need to know what the factory recommendation is for the timing on the motor you are using. Usually, when you see UIC on the flywheel, the bb is set for some amount in advance which is also marked on the flywheel. So if the flywheel is marked 5 degrees and has a bb, and your motor is supposed to be set at 5 degrees advanced, you set it on the bb, not TDC at the UIC mark. If your motor is specified for TDC, then UIC is fine. I have actually never seen a flywheel that is marked with UIC and no bb. All of the TDC flywheels I have in my stash are just marked with a bb for TDC. So, Disconnect the vacuum advance and plug it. Set your timing, then reset your idle speed if its lower or higher then it should be. Adjust your idle mixture screw, readjust idle speed if necessary, recheck the timing.

I respectfully disagree with using the factory timing spec. Fuel octane was MUCH lower when those specs were written. I haven't used a timing light in years. Start your truck, let it warm up then adjust to the fastest/smoothest idle, lock it down then readjust your idle. If needed you can fine tune using the octane selector but that likely won't be necessary. If your truck doesn't ping under load or drag the starter from being advanced too far you're good to go.

Tiny,

That is how I have done it too, besides bb. on my 503 is not visible because of firewall, advance it 'till it pings slightly, then back down a hair.

Ed

Those methods work too, and I have used them on occasion myself. But nearly all of my 235 and 261 motors motors are modified so I prefer to work from a baseline because I tend to keep pretty good records for each of the motors I build to see, as precisely as I can, what each motor likes (and they all seem to like something different in timing and other adjustments). So I start with zeroing out the octane selector plate (and I don't move it from zero), getting the timing to the factory specification using the distributor clamp, and then tinker with what the motor likes from there, TDC or something more advanced. Hey couple of ways to skin a cat, lol.

Indeed there are.

Dragsix, Tiny, and Ed,

Thank you for the insight/advice. It is much appreciated!
Bill

To give you all a feeling for the octane in gas back then. Note that for today's octane ratings you'll need to go a minimum of 10 deg advanced from stock spec.