* Specific carburetor model: Dodge used various carburetors on the 318 in that year (likely a Motorcraft or Carter, possibly a Holley depending on options). The adjustment screws and their function will vary slightly between models.
* Altitude: Higher altitudes require different adjustments than sea level.
* Engine condition: Worn engine components will affect the ideal settings.
* Fuel quality: The quality of the gasoline used influences the mixture.
Instead of aiming for arbitrary numbers, the proper procedure involves using a vacuum gauge and a tachometer to fine-tune the carburetor. This is the only accurate way to ensure proper operation.
General Procedure (this is a guide only; consult a repair manual specific to your carburetor model):
1. Locate the adjustment screws: Typically, you'll find idle mixture screws (often two, one for each barrel of a two-barrel carb) and an idle speed screw. They are usually located on the side of the carburetor.
2. Warm up the engine: Let the engine run until it reaches normal operating temperature.
3. Start with the idle speed screw: Adjust the idle speed screw (usually on the throttle linkage) to set the engine RPM to the manufacturer's specification (consult your owner's manual or a repair manual). This should be around 750-850 RPM, but double check your documentation.
4. Adjust the idle mixture screws: This is where the vacuum gauge becomes essential. Connect the vacuum gauge to a manifold vacuum port. With the engine warmed up and the idle speed set, slowly turn each idle mixture screw in small increments (1/8th to 1/4th of a turn at a time), observing the vacuum gauge. The ideal setting will result in the highest and most stable vacuum reading. If you don't have a vacuum gauge, you'll have to rely on visual cues like smooth idle and exhaust color, which is far less accurate.
5. Fine-tuning: Once you've adjusted one mixture screw, adjust the other. Go back and forth until you achieve the highest and smoothest idle.
6. Check for leaks: After adjusting, check for vacuum leaks around the carburetor and intake manifold. Leaks will significantly impact your adjustment efforts.
Crucially: Without a vacuum gauge and a repair manual specific to your carburetor model, precise adjustment is very difficult and you risk causing more problems than you solve. Consult a repair manual for your specific 1987 Dodge D100 with the 318 engine and the exact carburetor model number. This manual will provide diagrams, specifications, and procedures specific to your vehicle. This is the most reliable way to avoid damage to your engine. Alternatively, a qualified mechanic can adjust your carburetor efficiently and correctly.
