Here's a breakdown of the process, but I strongly recommend seeking professional help from a mechanic experienced with classic Cadillacs. Improper adjustment could cause serious problems.
Before you start:
* Warm-up the engine: Let the engine run until it reaches normal operating temperature. This is crucial for accurate idle adjustment.
* Consult a shop manual: A factory service manual for your specific year and model Cadillac DeVille is invaluable. It will provide diagrams, specifications, and detailed procedures for adjusting the idle. You can often find these online or through classic car parts suppliers.
General Steps (These are *general* and might vary slightly based on your specific carburetor):
1. Locate the idle speed screw: This is usually on the carburetor itself. Your shop manual will show you exactly where it is. It's often a small screw with a slotted head.
2. Locate the idle mixture screws: These are also on the carburetor, often two of them. They control the air/fuel mixture at idle. These are often a bit more difficult to find than the idle speed screw, and your shop manual is crucial here. Do not adjust these yet. Adjusting them incorrectly can cause serious problems.
3. Initial Idle Speed Adjustment: With the engine warmed up, carefully turn the idle speed screw to adjust the RPM. Your shop manual will give you the proper idle speed specification (in RPM). Generally, it's between 600-800 RPM in park or neutral. Small adjustments are key; make tiny turns and check the RPM.
4. Idle Mixture Adjustment (Advanced): This is where things get tricky. Once you have a reasonable idle speed, *then* you can carefully adjust the idle mixture screws. This involves a process of adjusting one screw slightly, then listening for changes in engine sound and smoothness. You'll want a smooth, consistent idle with no rough running or hesitation. This requires experience and often involves using a vacuum gauge to find the optimal setting. Again, this is best left to a mechanic unless you have extensive experience.
5. Check for Vacuum Leaks: A vacuum leak can significantly affect idle. Inspect all vacuum hoses and connections for cracks, looseness, or leaks.
Why a Mechanic is Recommended:
* Carburetion is complex: The carburetor is a precise mechanism, and incorrect adjustments can cause damage.
* Proper tools and gauges: Accurate adjustment often requires specialized tools and a vacuum gauge.
* Experience and knowledge: A mechanic has the experience to diagnose and solve related problems that might not be immediately apparent.
Attempting this adjustment without the proper knowledge and tools risks causing more harm than good to your classic Cadillac. The cost of professional help is a small price to pay compared to the potential repair bills from a botched adjustment.
