1. Linkage Problems:
* Secondary Linkage Adjustment: The most common cause. The linkage connecting the throttle lever to the secondary throttle plates might be out of adjustment. It needs to be correctly adjusted so the secondaries open fully when the primary throttle plates are almost completely open. This usually involves adjusting a rod or linkage. There are often adjustment screws or points where the linkage attaches. Consult a shop manual or online resources specific to your carb for the proper adjustment procedure.
* Bent or Broken Linkage: Carefully inspect the entire secondary linkage for any bends, kinks, or broken parts. Even a slight bend can prevent full opening.
* Binding Linkage: The linkage might be binding due to rust, corrosion, or incorrect routing. Lubricate moving parts with a suitable carburetor lubricant.
2. Vacuum Issues:
* Vacuum Diaphragm (Secondary Air Valve): The secondary throttle plates are typically controlled by a vacuum diaphragm. If this diaphragm is torn, punctured, or leaking, it won't provide enough vacuum to open the secondaries. This is a relatively common failure point. Inspect the diaphragm for damage and replace it if necessary.
* Vacuum Leaks: Leaks in the vacuum lines leading to the secondary air valve will also prevent proper operation. Check all vacuum lines for cracks, holes, or loose connections.
3. Secondary Throttle Shaft:
* Sticking or Binding: The shaft that the secondary throttle plates rotate on might be sticking or binding due to dirt, corrosion, or wear. Try lubricating the shaft carefully. If it's severely worn, it might require replacement or professional attention.
4. Secondary Throttle Plate Issues:
* Sticking or Binding: The secondary throttle plates themselves might be sticking due to corrosion, dirt, or build-up. A thorough cleaning of the carburetor might be necessary. Make sure the plates move freely.
* Worn or Damaged Parts: In severe cases, the plates, shafts, or related components might be worn or damaged beyond repair.
5. Choke (Less Likely):
* Choke Linkage Interference: Though less likely at full throttle, check that the choke linkage isn't interfering with the secondary throttle linkage.
Troubleshooting Steps:
1. Visual Inspection: Start by carefully inspecting the entire linkage and the secondary air valve and diaphragm. Look for anything obviously broken, bent, or damaged.
2. Manual Check: With the engine off, try manually opening the secondary throttle plates. If they don't open easily, you've identified a problem.
3. Vacuum Check: Use a vacuum gauge to check the vacuum at the connection to the secondary air valve. A significant drop in vacuum could indicate a leak.
4. Clean or Rebuild: If the above steps don't reveal the issue, a thorough cleaning or even a complete rebuild of the carburetor might be necessary. This is often a complex task and may require specialized tools and knowledge.
Important Note: Working on a carburetor requires care and precision. If you're not comfortable working on carburetors, it's best to take your vehicle to a qualified mechanic. Improper adjustment or repair can lead to poor engine performance or damage. Also, consult a shop manual specific to your 1984 Oldsmobile 307 with the Rochester Quadrajet for detailed diagrams and instructions.
