vacuum advance question

[lnirenberg]

I have been playing with a problem I have been having with my idle speed, whereby it was going up from 700 to around 1000 when the engine warmed up. Yesterday I found and fixed a vacuum leak and I thought that was the end of it. I took the car out for a drive tonight and the idle problem continued. When I got home I moved the vacuum advance hose from what I assume is the port coming off the primaries because it has full time vacuum to a port which I assume comes of the secondaries because it only has vacuum when I rev the engine. I took it for a very short spin and the idle stayed @ 700 and the car ran fine. As it was getting late, it was an around the block spin so I won't be positive until I take it for an extended drive tomorrow. What is the correct port to connect the vaccum advance hose to?

 

[WARVET]

I have been playing with a problem I have been having with my idle speed, whereby it was going up from 700 to around 1000 when the engine warmed up. Yesterday I found and fixed a vacuum leak and I thought that was the end of it. I took the car out for a drive tonight and the idle problem continued. When I got home I moved the vacuum advance hose from what I assume is the port coming off the primaries because it has full time vacuum to a port which I assume comes of the secondaries because it only has vacuum when I rev the engine. I took it for a very short spin and the idle stayed @ 700 and the car ran fine. As it was getting late, it was an around the block spin so I won't be positive until I take it for an extended drive tomorrow. What is the correct port to connect the vaccum advance hose to?

I've been having the same problem. I will look at the vacuum connections on my 75 today and let you know.

 

[Bob Chadwick]

I can't keep the names straight. You want the vacuum advance connected to the port that has vacuum at idle. This advances the vacuum when there is no engine load and drops the advance out when you put your foot into it.

This sounds like a problem with fast idle settings. I haven't played with it on a Q'Jet but older mechanical carbs generally have a cam that controls the idle. It goes to different positions based on the temperature of the car. The one I last played with had a bi-metallic spring. When cold, the cam opens the throttle more based on steps on the cam. As the car warms, the cam changes positions allowing the throttle to close and the idle to slow.

Vetteworks gave the following adjustment procedures.

FAST IDLE ADJUSTMENT

Close the primary throttles and position the cam follower above the high step of the fast-idle cam. Turn the fast-idle screw clockwise until it touches the lever then turn it down three full turns. Readjust the fast idle rpm with the engine cold and running (choke closed).

Automatic should be set at 1,500 rpm cold. I think I saw the manual should be 1,350.

 

[lnirenberg]

I can't keep the names straight. You want the vacuum advance connected to the port that has vacuum at idle. This advances the vacuum when there is no engine load and drops the advance out when you put your foot into it.

This sounds like a problem with fast idle settings. I haven't played with it on a Q'Jet but older mechanical carbs generally have a cam that controls the idle. It goes to different positions based on the temperature of the car. The one I last played with had a bi-metallic spring. When cold, the cam opens the throttle more based on steps on the cam. As the car warms, the cam changes positions allowing the throttle to close and the idle to slow.

Vetteworks gave the following adjustment procedures.



Automatic should be set at 1,500 rpm cold. I think I saw the manual should be 1,350.

When connected to the port with vacuum @ idle it does exactly opposite what you lay out. More info from me is in order--I am running a new Edelbrock #1406 600cfm performer carb w/ electric choke. The mechanical points were replaced by the prior owner with a Mallory electronic conversion kit but the coil remains and I assume it is a stock part. Does this make any difference? I will scope out and try Vetteworks adjustment procedure assuming for the moment that the Edelbrock has the same basic setup for this as a QJet.

 

[Bob Chadwick]

Something's not adding up here. With a timing light on the car, the advance and idle speed drops when I pull the line off the vacuum advance unit. I time the car at 8 degrees BTDC and then put the vacuum line back on. The advance then goes up and I get a little faster, smoother idle. I think it is around 20 degrees but can't recall for sure.

I'm not sure what you are referring to when you say your car does the opposite.

 

[MMM]

Most US cars up to 1968 used manifold (full time) vacuum for the vacuum advance. Starting in 1968 (generally, earlier for cars destined for CA) the manufacturers' started using ported vacuum for vacuum advance. Ported vacuum is tapped from above the primary plates so it is generated only when the plates open.
Reason for the change was emissions control. With a carbureted engine, idle is a range which is hard to control for emissions. Without a lot of detail, ported vacuum caused the combustion temperature to increase, thus an attempt to burn more of the gas entering the cylinders (they also began to run real lean idle systems at the same time).
Going to full time vacuum is better in almost all performance resects, including off idle response and fuel economy. To use manifold vacuum in a car that was set up for ported vacuum you need to reset your carb idle adjustments (speed and mixture) to get the idle speed you desire. I have seen a couple of cases where I had to slightly retard the initial timing setting. In some cases the manufacture actually increased the initial timing to offset some of the negative aspects of ported vacuum. You might also need to change your vacuum advance canister to one designed for use with manifold vacuum (higher vacuum level to reach full advance).
Good Luck,
Mike

 

[lnirenberg]

MMM very helpful and very interesting many thanks. I guess its time to bring out the tool box and mess around. I also downloaded the owners manual--Aughhh head exploding, too much information.

 

[JJS]

MMM very helpful and very interesting many thanks. I guess its time to bring out the tool box and mess around. I also downloaded the owners manual--Aughhh head exploding, too much information.

Where abouts in MA are you? I'd be more than happy to let you take a look at mine. I'm in Worcester.

-John

 

[lnirenberg]

Where abouts in MA are you? I'd be more than happy to let you take a look at mine. I'm in Worcester.

-John

I'm just outside Boston and I regularly reverse commute the Pike past your burgh. Although I often come to CAC during working hours, I am the boss so I guess that counts for something, I'm trying to catch up before vacation. I may take you up on the offer when I get back. Thanks.

 

[lnirenberg]

I'm beginning to think these non-computer cars still have a mind of their own. I took a quick 45 mile joy ride with the advance running off "part time" vacuum and I could feel no negative change in the throttle response through out the rev range regardless of gear. Getting warmed up today was a short proposition as it was 90+ and the idle stayed right @750 any time I had to stop for a light. When I got home, I had an email from edelbrock tech support telling me to run the vacuum advance off the passenger side port which is the part time vacuum port. I am trying to learn to leave well enough alone so for me I will say problem solved--for now.

 

[pgtr]

Most US cars up to 1968 used manifold (full time) vacuum for the vacuum advance. Starting in 1968 (generally, earlier for cars destined for CA) ...

This is absolutely correct and well stated.

To add to this if memory serves, by '71 they had this CEC abomination that did not allow ANY vacuum to the distributor at idle. IMHO these cars will benefit even moreso from a vacuum advance hooked straight to the manifold. In conjunction with that a recurve of the distributor ('71s in particular had LOUSY curves) and to repeat what MMM said, verify the vac can is properly matched to the idle vacuum and cam etc...

 

[JohnZ]

No Corvette after 1968 had manifold vacuum to the distributor; all had "ported" vacuum for retarded idle timing, to increase EGT to make the A.I.R. system more efficient. The only exception was the early 70's cars that also had TCS (Transmission Controlled Spark) - they had "ported" vacuum too, but allowed manifold vacuum to the distributor only when the engine was cold, or overheating, or in high gear.
'66 L-72's and '67-'68 L-68/L-71's also had "ported" vacuum. All will run much better and have better idle/traffic cooling and fuel economy when set up with full manifold vacuum and a vacuum advance can matched to idle vacuum characteristics.

 

[lnirenberg]

No Corvette after 1968 had manifold vacuum to the distributor; all had "ported" vacuum for retarded idle timing, to increase EGT to make the A.I.R. system more efficient. The only exception was the early 70's cars that also had TCS (Transmission Controlled Spark) - they had "ported" vacuum too, but allowed manifold vacuum to the distributor only when the engine was cold, or overheating, or in high gear.
'66 L-72's and '67-'68 L-68/L-71's also had "ported" vacuum. All will run much better and have better idle/traffic cooling and fuel economy when set up with full manifold vacuum and a vacuum advance can matched to idle vacuum characteristics.

Since I moved the vacuum line to the "ported" nipple my car is running better than at any time since I've owned it (next month is our one year anniversary of being together). Having grown up with a father who was a major proponent of the KISS method I hereby declare victory. I appreciate all the advice I got because prior to this all I seemed to encounter were dogmatic responses ie you have to connect to full time vacuum because. Thanks MMM, Pgt & John Z for helping me understand the inner workings of vacuum advance. :cry You guys are the best man. :cry

 

[WARVET]

I checked out my vacuum leaks today. Everything looked ok except for the carb bolts being loose. This could explain the high idle if there was fresh air coming into the carb. I will be driving it tonight to a local show, so I will let you know if that fixed it.

 

[lnirenberg]

I checked out my vacuum leaks today. Everything looked ok except for the carb bolts being loose. This could explain the high idle if there was fresh air coming into the carb. I will be driving it tonight to a local show, so I will let you know if that fixed it.

Where is the show? I'm about to leave my South Windsor office to catch the pike in Springfield. A little side trip sounds good to me.

 

[WARVET]

Where is the show? I'm about to leave my South Windsor office to catch the pike in Springfield. A little side trip sounds good to me.

Inirenberg, The show was in Ludlow. Not bad, but I got there late. I think I might have seen you on the pike. Do you have the t-top holders for your luggage rack? If so you passed me between Chicopee and Ludlow. Sorry I didn't get back in time to let you know about the show so you could stop in. Maybe next time. They have it there every Tuesday night.

 

[lnirenberg]

Inirenberg, The show was in Ludlow. Not bad, but I got there late. I think I might have seen you on the pike. Do you have the t-top holders for your luggage rack? If so you passed me between Chicopee and Ludlow. Sorry I didn't get back in time to let you know about the show so you could stop in. Maybe next time. They have it there every Tuesday night.

I was going to post the same question to you as the blue vette I passed looked a lot like your avatar. I was tempted to follow you on this possibility, but didn't want to seem like a stalker if I was mistaken. Vette show or mixed? I always like the opportunity to see other cars, where in Ludlow?

 

[JJS]

Where is the show? I'm about to leave my South Windsor office to catch the pike in Springfield. A little side trip sounds good to me.

There's a cruise night every Thursday at Hebert's Candy Mansion in Shrewsbury, if you're looking for a diversion from your normal routine. If you get off the Pike in Millbury, you can pick up route 20 in Worcester (about 1 mile from the Millbury exit), head East on route 20 for 3-4 miles and you'll run right into it. Lots of nice vettes, and other cars as well.

 

[lnirenberg]

There's a cruise night every Thursday at Hebert's Candy Mansion in Shrewsbury, if you're looking for a diversion from your normal routine. If you get off the Pike in Millbury, you can pick up route 20 in Worcester (about 1 mile from the Millbury exit), head East on route 20 for 3-4 miles and you'll run right into it. Lots of nice vettes, and other cars as well.

What time, I definately have to catch this before the summer is over?

 

[JJS]

Officially from 5:30 - 8:30, but people start arriving around 5:00. You might want to pick a spot near the road if you don't plan on staying 'till the end. I usually park near the Heberts sign, and that way you don't have to fight the crowd on your way out.
It really has gotten big over the course of the last couple of years, and there are a lot of nice cars each week. You don't see too many "dogs" up there.