I'm working with the newly rebuilt 20R in a Chinook camper, after about 30 miles of flawless driving the carb has suddenly developed a problem that's got me scratching my head. It will start and run just fine until a few minutes after the engine reaches operating temperature. It suddenly starts dumping fuel down the carb to the point that it won't idle without revving it constantly to clear it out. If I let it cool down it will start back up and run good again. It has the stock Aisin carb, I took it apart after the engine rebuild and inspected everything (which looked really clean) replaced the accelerator pump, top gasket, and float. I can see by the bowl window that the fuel level is staying where it should while this is happening. I do have the choke wired open for the time being but I can't see how that could make it run rich. If I look down the throat I can see fuel pooling and boiling on top of the secondary butterfly. I'm getting ready to pull the thing off an take it apart again but I really don't know what I expect to find. Anyone have any suggestions or insights?

 

It sounds like it has to be a problem with the needle and seat or the float level. If the float is set correctly and the needle is seating properly then the flow of fuel should pretty much be cut off from the pump at idle.

 

I don't understand how that could be the case if the fuel level in the bowl is staying right in the center while it's happening.

 

Is it possible that you missed something while rebuilding the carb?

A vacuum leak would also cause some of the symptoms you're having. Specifically, that this happens after it warms up and that it was fine for a while and now it's got this problem.
Something could be wrong with the carb/manifold gasket or one of the numerous vacuum circuits in the emissions equipment. Get it warmed up and spray carb cleaner at a few places to see if you get a change in idle.
Replace any old cracked hoses and make sure they're all routed to the correct ports.