Toyota ASIN Carb. Rebuild; Float Bowl drains after shutting off engine

Vapor lock. The fuel boils in the float bowl and raises the float tightly against the needle/seat. The pressure then forces gas past the check ball in the accelerator pump circuit, out the accelerator pump discharge tube, and into the manifold past the throttle plate.

With the engine hot and then shut off, wait a 4-5 minutes and eyeball the sight glass. You may see the gasoline in there boiling like coffee in a pot. Other Asian brand cars used carbs with sight glasses and boiling gas is a problem there also. Also eyeball the accelerator pump discharge tube at this time and note if gas is dribbling from it.

Vapor lock can be tamed a bit by using a rubber fuel line instead of a steel one, adding a thick fiber insulator block between the carb and manifold, etc.
Some old Toyotas back in the day actually came with a tiny (2 inches or so diameter) fan that would blow air against the carburetor float bowl when the engine was hot. Kind of like the radiator cooling fans staying on after an engine is shut down.

I’m never been that big into Toyotas but one possible area that afflicted carbs on old Subarus was warpage of the throttle body and the float chamber sections. They would leak gas internally or suck air; or both. To check this would require disassembling the carburetor and running a flat file over both mating sections.
We used to get some old Subarus in that were horribly warped; some as much as .020 of an inch. Repeated repairs/rebuilds got the owners nowhere except more headaches which were cured by a flat file being used on the mating surfaces of both sections. Hope that helps.

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Chrysler 2-barrel carbs on the 318 engine had a similar problem in the 1975-77 timeframe. Chrysler’s warranty repair procedure was to lay a sheet of fine sandpaper on a glass surface, wet it, and then take each warped half and swirl it around on the sandpaper until it became flat again.

The symptom of a warped carb on those cars was roughness at idle and low rpm range. It didn’t cause float bowl draining, but I can see how it’s possible on other carbs.