Stalls under acceleration

[Overk1969]

1986 c10 w 4bl 305
It runs fine while in neutral, throttle is very responsive. Once I place it in drive and push the throttle it starts to bog and if I floor it, it dies. I have rebuilt the carburetor to include the accelerator pump, the wires and plugs have been changed, I’ve checked all the vacuum lines and no luck. I’m at my wits end, please help.

 

[Sad Sack]

Based on your information:

1. Fuel Delivery Under Load​

• Fuel Pump Weakness: A pump can supply enough fuel at idle but fail under load. Check fuel pressure while in gear and revving (or simulate load if possible).
• Clogged Fuel Filter or Line: Even partial blockage can starve the carb when demand spikes.
• Float Level: If the float is set too low, the bowl empties quickly under acceleration.

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2. Ignition Timing​

• Base Timing: Verify with a timing light. If it’s too retarded, the engine will bog and die under load.
• Advance Mechanism: Check vacuum advance and mechanical advance in the distributor. If they’re stuck or not working, you’ll lose power when you need it most.

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3. Transmission Load​

• If this is an automatic, a torque converter issue or excessive drag could mimic engine bogging. Does it feel like the engine dies instantly or struggles first?

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4. Carburetor Secondary Circuit​

• If it’s a 2-barrel or 4-barrel carb, the secondaries might not be opening or are flooding. Check linkage and vacuum diaphragm.

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5. Vacuum Leaks Under Load​

• Sometimes leaks only show up when the engine torques over. Inspect hoses and gaskets for cracks that open under movement.

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6. Exhaust Restriction​

• A clogged catalytic converter or collapsed muffler can choke the engine under load. Quick test: disconnect exhaust before the cat and see if it runs better.

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Questions for you:

• Have you checked fuel pressure under load yet?

 

[Overk1969]

I’ve replaced the accelerator pump with the carb rebuild. Timing is set at stock or little above. I have not check fuel pressure. Thank you

 

[legend57]

I agree with Sad Sack's recommendations. I recommend you invest in a few tools before investing in parts.
* Get a fuel pressure gauge and plumb it between the pump and the carb. It should be between 4-6psi and not move much. (~$50)
* Get a timing light with an advance function. Check initial and total timing. Lots of youtube videos for how-to. ($50)
* Get a vacuum gauge with a long hose so you can connect it and route the gauge so you can see it through the windshield with the hood shut. I use the wipers to clamp on the hose. Monitor the vacuum as you drive. It should decrease as you open the throttle but not go to zero. ($20)
Report back with findings.