Hi FIM
A few reasons below why they can flood
Blocked return/spill-back port (trapping the fuel instead of letting it flow back to the tank).
Thermal tank pressure / bad venting (pushing fuel past the pickup tubes continuously).
Mismatched or incorrect diaphragm spring tension (altering pump stroke/travel).
Mixture screw backed out too far (over-delivering through the low-speed circuit).
Choke butterfly staying partially closed (pulling a heavy siphoning vacuum).
The Return Hole Got Blocked: Inside the casting under that side cover, there is a tiny overflow/return port designed to let excess fuel spill back into the tank once the internal well is full (acting just like a float bowl overflow). If a gasket edge gets misaligned slightly, or if pattern-cut aftermarket gaskets cover that tiny return hole, the fuel has nowhere to go. It keeps pumping fresh fuel into the well until it overflows straight out the throat and down the valves.
Thermal Siphoning / Vapor Lock: As the metal tank and carb soak up engine heat for a couple of minutes, the air space inside the sealed tank expands. If the tank cap vent isn't clear, that pressure forces fuel up the pickup tubes continuously, overwhelming the metering wells regardless of the diaphragm's pumping action.
Checking that the return spill-back port is completely clear and unobstructed by the new gasket usually stops them from filling up plumb-full and drowning out.
the small spring tucked under the diaphragm is what controls the tension and travel of the pump paddle/actuator.
If the spring is too long or too stiff, it keeps too much constant pressure pushing the diaphragm inward, which can force the pump to stroke too aggressively or hold a valve open longer than it should. On the flip side, if it's incorrect or mismatched, it upsets the delicate pressure balance against the crankcase pulse, altering the stroke and letting fuel continuously creep or flood into the well instead of metering correctly.
Getting that exact spring tension right—or making sure an aftermarket kit didn't supply a generic spring that's throwing off the pump travel—makes all the difference in stopping that warm flooding.
1. The Mixture Screw (Idle/Low-Speed Adjuster)
How it affects flooding: On these carbs, the idle mixture screw meters the fuel drawn up from the idle well. If the screw is backed out too far (too many turns open), it creates a wide-open path for fuel to feed through the low-speed circuit.
Combined with a warm engine or tank pressure, excess fuel rushes through that circuit unchecked, loading up the cylinder and contributing to the flood. (Standard initial setting is usually seated lightly and backed out about 1.5 to 2 turns).
2. The Choke Butterfly Staying On
How it affects flooding: If the choke butterfly is even slightly partially closed (or if the linkage/stop is bent so it doesn't open 100% when pushed off), it acts like a permanent choke.
The Result: As the engine runs, a partially closed choke creates a massive vacuum pull right at the throat. That high vacuum acts like a siphon, drawing an enormous amount of raw fuel straight out of the jets, instantly flooding the engine and killing it within a couple of minutes once it's warmed up.
Cheers
Max.
