Manifold and ported vacuum

[75gmck25]

Your description, and the use of the terms fast idle and normal idle, are confusing.

Now that I think of it, let's simplify your setup and answer some questions.

Do you like how your base and mechanical timing setup performs?
- Disconnect and plug the vacuum advance line to the carburetor
- Set base timing wherever you like (12 degrees BTDC?) at 650-700 rpm and verify the mechanical advance works as you rev the engine. Total timing may go as high as about 32 because it would be 12 degrees base + 20 degrees from the mechanical advance. It will always advance with higher rpm.
Drive it around for a while and make sure you like how it performs. Keep playing with settings until you get it to perform at its best. Changing the mechanical advance curve (at what rpm it advances, and how quickly) will require different advance springs in the distributor.
- Now reconnect the vacuum advance line to ported vacuum. It should have no change in idle because with ported vacuum would be zero.

Is the vacuum advance, combined with your base timing and mechanical, causing the engine to ping?
- Drive it around for a while again and make sure you like the performance. Make some light throttle acceleration runs at about 60 mph and make sure you don't get any pinging. If you get pinging you will have to set base timing back a few degrees or get a stop to install in the distributor to limit total vacuum advance.

Do you like manifold or ported vacuum at low speeds?
- Now connect the vacuum advance to manifold timing, which will make the idle go up. Just set the idle back down to about 700 rpm, or wherever you like it (I set mine a little higher in Park so I can run the A/C and it idles smoothly in Drive). Drive it around for a while and see if you like it better with manifold vacuum. It may not be much different than using ported vacuum.

---------------------- More info ---------------------------
If you disconnect and plug the vacuum advance line, and set the base timing to 12 degrees BTDC at about 650-700 rpm, this is your base timing. This base timing will not change, regardless of manifold vs. ported vacuum.

Still disconnected vacuum line - slowly run the rpm up and note the increase in timing due to mechanical advance. It will usually top out at about (20 degrees + base of 12 degrees) 32 total at about 3000 rpm It may start increasing slowly as low as 2000 rpm. You usually want this mechanical timing to come in at a lower rpm because it helps off-the-line performance.

Manifold vs. ported - If you first use ported vacuum and the base timing is 12 degrees BTDC at about 650-700 rpm, when you switch it to manifold vacuum it will increase the timing to about 32 (12+20) and the idle will go up because of the timing increase of 20 degrees from the vacuum advance. If you don't like that idle speed you will have to adjust the curb idle down lower.

The change in idle timing due to vacuum advance (ported or manifold) will have no effect on your base timing or your mechanical advance. You could even run with vacuum advance disconnected, since it's mostly there to provide better economy at cruise rpm. Racing engines often run a distributor with no vacuum advance.

 

[Ricko1966]

I hooked the truck up to manifold vacuum today. The fast idle goes way up, to about 1800-1900 rpm, from about 1200. The normal idle is about 100 rpm above what it is when using ported.
The timing is now advanced when in idle, this I can see with the timing light. When comparing to the tab, its about 6-8 degrees advanced. So about 18-20 total in idle. When revving the engine, the timing should fall back to 12 (which is the base timing) if what you are saying is correct. But, it does not. When revving the engine, the timing increases with the RPM and I’m reviving up to 2000 rpm. The higher the revving, the more the timing increases

This does not seem in line with what you guys are saying.

The centrifugal advance is doing its job like it's supposed to. Did you read ignition 101 and the link I put to it,you need to understand all the parts of the distributor and how they interact. Watching short you tube videos isn't going to do it. Post 24 go to the link open it go to timing 101 open it.