Drivetrain Issues (Most Likely):
* Torque Converter: A failing torque converter is a prime suspect. It's responsible for smooth power transfer between the engine and transmission. If it's slipping, the engine will rev up but the car won't accelerate properly. This is very common in older vehicles.
* Transmission Problems: Issues within the transmission itself, such as low fluid, worn clutches, or internal problems, can cause similar symptoms. Check the transmission fluid level and condition. A low or burnt fluid is a bad sign.
* Drive Shaft Issues: A problem with the driveshaft, such as a bent shaft, u-joint failure, or worn bearings, could be causing power loss under load.
* Rear Differential Problems: Similar to the driveshaft, issues in the rear differential (low fluid, worn gears, etc.) will rob power and cause sluggishness.
Engine/Carb Related Issues (Less Likely Given Symptoms):
* Carb Tuning: While the carb conversion is *not* likely the main culprit given the fact it idles fine, it needs to be properly tuned for the engine and the additional load when driving. The idle mixture might be fine, but the acceleration and power enrichment circuits are likely off if it's stumbling under load.
* Ignition System: A weak ignition system (worn distributor cap, rotor, wires, or coil) can cause misfires under load when the engine demands more spark energy.
* Fuel Delivery: While less likely if it idles fine, a fuel pump that struggles to keep up with demand under load or clogged fuel filter could be contributing.
* Vacuum Leaks: Vacuum leaks can affect engine performance and especially become noticeable under load.
Troubleshooting Steps:
1. Check Transmission Fluid: This is the *first* thing to do. Check the level and condition of the transmission fluid. Low, dark, or burnt fluid indicates a problem.
2. Listen Carefully: Pay close attention to any unusual noises coming from the transmission or rear end when driving. Grinding, whining, or clunking sounds are telltale signs of problems.
3. Check for Codes (if applicable): If your truck has a diagnostic system, check for any trouble codes that might provide clues.
4. Test Drive with Observation: Have someone watch the engine compartment while you drive to look for anything unusual (e.g., vapor from the transmission, fuel leaks).
5. Inspect Drive Shaft and U-Joints: Look for any obvious damage or excessive play in the driveshaft and u-joints.
6. Carb Tuning (if other issues ruled out): If drivetrain issues are ruled out, then focus on carb tuning. This might require professional help, especially if you're unfamiliar with carburetor adjustments.
In short: The drivetrain (torque converter and transmission) are the most likely culprits. Address those first before getting into more involved engine diagnostics. Ignoring drivetrain problems could lead to more significant and costly repairs down the road.
