Inconsistent Idle

[Joel Talka]

Happy New Year All;

I have another question on my '66 Coronet. The car has a worked 408 small block in it and produces a relatively low vacuum, 10-11". I have a vacuum can in there to generate enough from the carb base to supply the power brake booster. I mention this but I am not sure if this figures into this issue.

The issue is that I can't seem to get the idle to stay put. I'll warm the car up and set the idle to roughly 1000 RPM. I'll take it for a short ride and as I am driving, the idle will not return to that setting but rather adjust up a couple hundred RPM to like 12-1300. I've checked the distributor advance which does not begin to kick in until around 2K. I don't see any cable binding either. I'm wondering if this could be related to the vacuum can......

As always, I appreciate any thoughts or suggestions..

Joel

 

[copper67sat]

I was having a similar problem where idle rpm would increase as the engine warmed up. Traced it to a vacuum leak at the carb to manifold gasket.

 

[AR67GTX]

Soft throttle return spring. Worn/sticking throttle shafts in carb. Worn throttle cable. All a possibility to look at.

Any particular reason for having your advance start at that high of rpm?

 

[Joel Talka]

I was having a similar problem where idle rpm would increase as the engine warmed up. Traced it to a vacuum leak at the carb to manifold gasket.

Good thought.. Where my head was going was the impact of that vacuum canister. Each step on the brake draws vacuum down which has got to be seen at the carb base.

 

[Joel Talka]

Soft throttle return spring. Worn/sticking throttle shafts in carb. Worn throttle cable. All a possibility to look at.

Any particular reason for having your advance start at that high of rpm?

I'll try a stiffer spring.. The carb is new, the cable is original to the car. I have some cable lube which I will also try. Many thanks to you guys, again!

 

[copper67sat]

Good thought.. Where my head was going was the impact of that vacuum canister. Each step on the brake draws vacuum down which has got to be seen at the carb base.

In my case, I had recently had the carb off for a rebuild so I was reasonably sure it was related to that. Turned out that there was a bit of gasket residue on the intake that was hard to see with the poor lighting in my garage. Was easier to see with a drop light after I pulled the carb back off. Cleaned that up and it ran great after that.

 

[copper67sat]

One of my clues was that it changed from engine cold to hot. Can you disconnect the vacuum canister, plug the port and see if the problem persists. That would either point to or eliminate the canister as the problem. Would have to drive carefully as you may not have much power brake assist.