Troubleshooting Flat Spot
Things to Look For:
If an engine does not respond promptly when the throttle is opened quickly it (or the carburetor) is said to have a flat spot. This is usually caused by any of the following:
Accelerator pump piston (or diaphragm) leaks.
Accelerator pump valves leak.
Accelerator pump stroke too short.
Accelerator pump passages restricted.
Fuel volatility too low or too high.
Float level too low.
Fuel pump pressure too low.
The anti-percolating valve (on some carburetors) may open too soon when throttle is closed. If so, carburetor may have flat spot next time throttle is opened when engine is hot.
Fuel too hot due to hot engine and hot weather (see Vapor Lock).
If carburetor has a metering pin operated by throttle linkage and also a vacuum piston linked to the throttle to give a rich mixture at part throttle and moderate engine speed, a flat spot will be noted if the device fails to function properly because of stuck piston, vacuum leakage or restricted vacuum passages.
If carburetor has vacuum piston which provides richer mixture at part throttle and moderate engine speed by opening an additional passage or jet within carburetor, a flat spot will occur if fuel valves fail to work, or fuel passages are restricted, or if piston does not function because it is sticking, vacuum leakage or restricted vacuum passages.
Late ignition timing.