Vacuum solenoid

[dmruschell]

On a side note, i checked my timing and was getting about 12 degrees at idle. I adjusted the distributer to get 13 degrees and it appears to run a little better, but a little lumpiness remains. Im guessing th next step would be valve adjustment?

The stock timing spec is 13 +/- 2 degrees. One degree difference isn't really going to affect the idle quality very much, if at all.

For comparison, I've gotten a huge Buick 455 with a lumpy cam to idle steady at 500rpm with 4 degrees of advance (which is the stock setting for that engine - some Buicks had a stock setting of 0 degrees advanced!). After getting the distributor dialed in, I got the engine to idle steady, but with over 10 degrees of advance. The idle quality should have been vastly different, but when the fuel mixture is dialed in perfectly, the engine will idle just fine. The engine had more midrange power with the higher advance setting though, since it was running more efficiently.

[David T]

The stock timing spec is 13 +/- 2 degrees. One degree difference isn't really going to affect the idle quality very much, if at all.

For comparison, I've gotten a huge Buick 455 with a lumpy cam to idle steady at 500rpm with 4 degrees of advance (which is the stock setting for that engine - some Buicks had a stock setting of 0 degrees advanced!). After getting the distributor dialed in, I got the engine to idle steady, but with over 10 degrees of advance. The idle quality should have been vastly different, but when the fuel mixture is dialed in perfectly, the engine will idle just fine. The engine had more midrange power with the higher advance setting though, since it was running more efficiently.

While the timing will not have a large effect on idle quality, it has a BIG effect on emissions. Back in the day, they only tested for emissions at idle so the object was to get them as low as possible at idle. That meant messing around with the timing. Some of the older cars had more rubber hoses than you could count just to get emissions down at idle and still make the car drive able, especially when cold.

[dmruschell]

While the timing will not have a large effect on idle quality, it has a BIG effect on emissions. Back in the day, they only tested for emissions at idle so the object was to get them as low as possible at idle. That meant messing around with the timing. Some of the older cars had more rubber hoses than you could count just to get emissions down at idle and still make the car drive able, especially when cold.

My 70s cars definitely came from the factory with a few more hoses than my 50s and 60s cars. In researching getting my 79 Corvette closer to stock and restoring (kind of) a 75 Corvette I have, I researched all of the emissions equipment that was used on those cars during the 70s and how it changed over the years. The catalytic converter was first used on the Corvette in 1975, which is also the same year that the Quadrajet was redesigned to use full manifold vacuum for the vacuum advance, like had been done on pre-emissions cars. The only ported vacuum source after 75 was for the EGR valves. I guess the catalytic converter helped emissions enough that a lot of the other emissions "band aids" that were used in the early 70s didn't need to be used anymore. The only emissions equipment left was EGR, a system (like on the DeLorean) that deactivates the vacuum advance until the engine is warmed up, and certain applications used an AIR pump to pump air into the exhaust manifolds. Some of the stuff they used before 1975 for emissions controls still seems odd to me.

[Bitsyncmaster]

What gives our cars crappy idle with advance is the closed loop idle system. The advance brings the RPM up higher (300 to 500 RPM) and that exceeds or brings the idle motor almost fully closed. In most cases you will get excessive idle hunt. Older cars without the idle control don't have that problem. You can test on the D, just unplug the advance solenoid electrical connector on a hot idling engine.

[dmruschell]

What gives our cars crappy idle with advance is the closed loop idle system. The advance brings the RPM up higher (300 to 500 RPM) and that exceeds or brings the idle motor almost fully closed. In most cases you will get excessive idle hunt. Older cars without the idle control don't have that problem. You can test on the D, just unplug the advance solenoid electrical connector on a hot idling engine.

That makes sense. Though, I wonder if adjusting the idle screws so that the butterflies are closed a bit more would resolve that situation.

The advance solenoid electrical connector on my car fell off at some point. I noticed it when looking for reasons my car wasn't running quite right. Reconnecting it seemed to help. It still idled relatively okay, but it did idle better when the connector was in place.

[David T]

The throttle valve plates should be adjusted so that at idle the plates are fully closed. The decel springs should be strong enough to hold the decel valves closed so no air bypasses them. The less air passing through the throttle body the more control the idle motor will have. The throttle stop is a very sensitive adjustment. You want the plates closed when hitting the stop. Otherwise you are bending the throttle shaft and eventually it can break or loosen the plates attached to it. The throttle stop screw adjustment should never need to be adjusted unless it has been tampered with. It was glued so it can't move easily. If you touch that adjustment you may have to readjust the micro switch adjustment too.