Fuel pump problem

[Markus Bekkelund]

I have a 1940 cadillac with a fuel pump problem. The car is running fine when cold, but after beeing driven a while, or idling when hot, the fuel in the "glass cup" starts bubbling. After a while the pump will not manage to pump fuel at all, and the glass empties out. Then after a few hours, it runs fine again. The fuel pump has recently been rebuilt by myself, but is still not working right. Maybe I have done a bad job rebuilding it, or does the fuel boil?

Here is a video of the fuel pump: https://sendvid.com/sprk7x96

Advice much appreciated

- Markus

[Walter Youshock]

When you rebuilt the pump, did you hold the pump eccentric in the position of a full fuel stroke and then tighten the diaphragm screws?  Also, when installing it, the pump rod should be in the full stroke position. 

[Markus Bekkelund]

No, I did not... Thank you very much. How do you install it in the full stroke position?

[fishnjim]

I hear people having this problem and blaming it on "cheap" gas but I have not experienced it.   There's either air (in leak) or vacuum getting to the fuel system to cause this. 
There's an anti-syphon valve on the early carbs that can foul/stick.   So it's probably not the fuel pump but need to recheck.   Get the manual for your model carb and check it out.
Old cars need to have the fuel system cleaned and the carb and fuel pump rebuilt with ethanol resistant materials prior to use today.   
In order to "boil" the fuel, you'd have to have one extremely hot engine and it would not run like that.

[Glen]

In order to "boil" the fuel, you'd have to have one extremely hot engine and it would not run like that.

The boiling point of fuel (or any liquid) is determined by temperature and the pressure it is under.  The lower the pressure the lower the boiling point. 
The mechanical fuel pump on these older cars puts the fuel under very low pressure during the intake stroke. The fuel can then flash into a vapor.  When that happens the vapor prevents any more fuel from entering the pump.  The pump lever then comes off the cam lobe and the pump returns to normal pressure and the vapor condenses back into a liquid.  But there is not much fuel in pump so it has little to send to the carb.  On the next intake stroke the pressure in the pump again goes low enough to cause the fuel to again flash to vapor, and the cycle repeats.     
That’s called vapor lock.