Red Delta

So, the overly thick manual doesn't want to let me in on the secret if I should have the vacuum line connected to the distributor while setting idle speed.

It wants 500 rpm in drive. I have it set at this, with the vacuum line attatched, but it just doesn't feel right. Think I should disconnect and readjust the setting?

Everything else is set nice. Dwell and timing is bang on.

Nilsson

Yes with vacuum advance connected for final idle adjustmet.

rocketraider

If the distributor vacuum advance is connected while making idle speed adjustments, it's "possible" the vac advance will do its job and advance the timing, which will increase idle speed. You do not want vacuum advance at idle. That's why it's connected to ported vacuum instead of manifold vacuum.

Disconnect the vacuum line at the advance chamber and plug it to avoid a vacuum leak. Then set idle speed to specs.

Set the initial timing to spec at the specified idle speed. You may notice an increase or decrease in idle speed. If it does, set idle speed and timing again until everything is stable. Once everything is stabilized, then set the idle mixture for highest and smoothest idle. Turn the idle mixture screws in 1/8 turn or until RPM drops about 30 RPM, then set final idle speed. If carb and ignition are operating correctly this will be your best idle speed. You can verify it with a vacuum gage, since best idle/timing will produce a high stable manifold vacuum reading.

Then, connect the vacuum advance to its vacuum source.


Now, as to the why of unhooking the vacuum line when setting idle speed- if car has a DTVS (distributor thermal vacuum switch), it will switch vacuum advance between ported and manifold depending on coolant temperature. It does this to lessen overheating at idle speed; in other words, it will switch to manifold vacuum at high coolant temp and advance the timing, which helps cool the engine.

If you leave the vac advance connected and coolant temp goes high enough to cause the valve to switch over to manifold vacuum, the you'll have vacuum advance at idle and idle speed will increase.

Even if car does not have DTVS, messing with idle speed can possibly open the throttle plates enough to create a vacuum signal at the carburetor vacuum port, and actuate the distributor vacum advance.

Clear as mud right?

Texas Jim

Rocketraider, What year did this DTVS come out on the 455? And would you not have to go quite high w/ the idle to cause advance? And are we talking about idle mixture adjustment or idle speed adjustment? Your idle speed adjustment, I would think, should be done w/ the vac. advance connected, as you want that correct idle speed when you go to enguage the tranny. You set the idle speed w/o the vac. adv. connected, and then when you connect the vac. adv., you have a different idle speed. What good is setting the idle speed to be used under normal driving w/o everything hooked up just like you were driving the car? Quite possibly I'm not understanding your sequence of adjustments.