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.
Iām 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.
Chrysler 2-barrel carbs on the 318 engine had a similar problem in the 1975-77 timeframe. Chryslerās 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 didnāt cause float bowl draining, but I can see how itās possible on other carbs.
The problem solving and testing has been going on for nearly a year and, according to the internet, people tend to notice it after rebuilding the carburetor.
Thanks for your ideas and I will give them a try.
Iām late to this topic but recently encountered something similar on a rebuild of an Aisan carb on an 87 Toyota pickup, CA emissions. To investigate I took the carb off the truck, filled the bowl to the middle of the sight glass window and waited. 8 hours later - very little level change. Perhaps a 1 mm drop over 8 hours, presumably due to evaporation and wicking through those horrible paper gaskets that came with my cheap rebuild kit.
I put the carb back on the truck for another test and after running & shutting down low and behold - the level dropped below the window inside of an hour! Soā¦I surmised in my case the rapid bowl level drop was either (a) back pressure from the carb vent control line into the carbon canister, (b) elevated temp from engine heat causing more fuel evaporation or (c) a combination of (a) and (b). I tested it with the carb vapor canister line disconnected and voila! - I observed the same minimal drop that I saw with the carb off of the truck.
I also checked that the carb vent valve was open after ignition shutoff and how much pressure it took to blow air through the carb evap canister. Not much, but evidently sufficient to force enough fuel through the carb after engine shutoff with the vent valve open to lower the bowl level below the sight window before equalizing to atmospheric pressure.
So, Iād advise checking your carb canister. Mine wasnāt by any means completely plugged but I plan to replace the carbon with some coarser diameter media to eliminate as much back pressure to the carb as possible without deleting the evap system entirely.
Also - I didnāt have any hard starting symptoms so assume in my case there wasnāt enough back pressure to completely empty the bowl in between starts. Just enough to cause that mysterious disappearance of the fuel level from the sight window.
That was an excellent way you narrowed it down to the carb vent/evap canister @lwb11211121_187632, thanks for sharing. I experienced a similar problem on my 50 year old Ford truck. In my case somebody (guess who??? ā¦lol) replaced the carb at some point, & decided the replacement carbās vent port would work better if it just had a plug over it. I narrowed the fuel bowl level drop to the vent the same way, test by allowing the fuel bowl to vent freely.
I think you already had pinpointed the issue I was having in your previous post with similar methods. What I didnāt realize was how little back pressure from the canister it would take to create this effect though (especially when shutting off a warm engine). I can still blow through my canister with just a bit of effort and had replaced the carbon (fine mesh stuff) not that long ago. I think part of my problem is just being OCD about seeing the fuel level after shutoff. My carb may have done the same thing when it was new 36 years ago but when I bought it the floats werenāt adjusted properly so I couldnāt see the fuel level when it was running either.
There are carbs (Webber, Eldebrock, Holley) that might make a carb with all the emission crap so it will pass inspection. Iāve done a Holley replacement more than once. But it depends on the vehicle. Or just get a rebuilt one (probably the easiest). Why spend all this time on a fix that may never work.
Fuel bowl venting is still somewhat of a mystery to me. Does anyone here know the history of how carb fuel bowls have been vented over the years? It appears from what I can tell by looking at the parts diagram, one early carb, the Autolite 1100 I think, thereās a tube from the top of the fuel bowl chamber, presumably above the fuel level, that goes right to the carbās air inlet.