Having a tough time wrapping my head around this. I need someone to explain it in simple terms. I have a timing gun with the dial back on it and dont quite get it. I though i had it but the motor just isnt running right.

Total timing = initial timing + vacuum timing + mechanical timing, Short answer initial timing is at idle with vacuum advance to dist. disconnected if present. Total timing is at about 2500-3000 rpm with everything hooked up.

Depending on your engine, your initial timing at idle should be anywhere between 12 and 20 degrees BTDC. As your rpms increase, your timing advances (due to the internals of your distributor). At some point (this should probably be between 1500 and 2500 rpm) this advance stops and at this point you have your "total timing" which should be around 36 to 40 degrees.
I hope this helps.
Good luck.

When you use your timing light, use the dial to get the timing mark at 0, when the engine is idling. look at the knob on the light, that is your intial timing. Useful when there is more than a few degrees of timing at idle. Set your timing light knob to 0 and rev the engine and use the timing light set at 0. That's your total timing.

Initial timing is set at idle. Total timing is set at whatever rpm the distributor is setup to stop advancing at. An stock distributor may only stop advancing at 4000rpm where an aftermarket or recurved stocker may hit total advance at 2500, 3000, or whatever rpm the guy doing it set it up for. All timing is set with the vacuum advance can on the distributor disconnected. Vacuum advance comes in on top of total timing.

At idle the distributor's shaft is turning so slow, not enough force is present to start operating the mechanical advance mechanisim inside the distributor(which is weights and springs slung outward by centrifugal force), or at least not operating it very much. This is where you set your initial timing by rotating the distributor. A stocker might like 10* initial timing where something with a big cam might like 20* initial.

Total timing = initial + mechanical advance. Mechanical advance is however much timing is built into the distributor. This can be tailed to suit your engine. Some stock distributors have 30* of advance built into them where other recurved distributors can have 20, 10, or however many is wanted. So if you have an engine with a big cam, you might hypothetically have 18* initial + 18* mechanical advance = 36* total timing all in by 2800rpm.

If your distributor is curved wrong for your engine, you will never have it right. Too many guys think you can just plop a distributor on out of the box and go. You can not. They need to be tuned for your application.

I've never understood why the advance should be all in by 2500-3000 RPM. Shouldn't the timing continue to advance as engine speed increases?

I thought the point was so that you get the maximum amount of fuel burning at TDC so the piston is forced down with as much force as possible. If the advance stops at 3000 RPM and you spin the engine to 6500, arn't you leaving a lot on the table? I've never understood that.

Quick points:

> Mech timing must not begin until at least 100-200 RPM above idle speed, or idle will be unstable.

> Vac advance must be connected to ported spark fitting on carb - if there's vacuum at idle, carb needs rework (throttle blades open at idle)

> Tuned right, 55 deg total at high vacuum cruise really keeps engine clean and doubles-triples ring life. Most factory cals were in this range.

Rick