Classic/Antique Car Repair: Hard starting when engine hot, Hot starting problem

Question
Welcome back from vacation Dick. I know this topic has been covered before, but please bear with me or else direct me to an applicable thread. My 1948 Buick Super starts easily when cold. After driving (engine hot) it will start up immediately after engine is shut off. However, after setting for 5-10 minutes plus, it will NOT restart until engine has cooled. Starter spins engine easily, but engine never catches like either no gas or flooded. I have tried "engine flooded" procedure (accelerator pedal to floor) but that does not help. Once engine has cooled (an hour or so depending on weather) engine again easily starts. I have been told that it is evaporation of gas in carb, 10ethanol, vapor lock, percolation of gas, etc. These may be true, but if so, can anything be done to correct problem? Engine has been rebuilt and is in excellent shape, Stromberg carb rebuilt and car runs great once it starts. I have a 6 volt pusher elec. fuel pump in line with the original mechanical pump. Starter switch is the original accelerator pedal one.  If problem is the fuel, then it is interesting to me why my 1949 Packard Super with same fuel, same accelerator pedal starter switch and identical 6 volt electrical fuel pump in line with mechanical fuel pump does not have same problem. Old' Packard starts easily hot or cold. Thanks for your help. Have a great day.
Charlie Cain
North Texas {where weather IS HOT}
PS I also have a 1956 MGA that does not have hot starting problems. Just the Buick id afflicted with this modern day malady

Answer
Thanks for the welcome back.  We had a great time on the beach with our grandkids - and no phone, TV or other distractions!  Also, got out of this heat (it has been in the 100s here for a couple of weeks now - which is not unusual for August and September, but is rare for this early in the summer)

Anyway, this does not sound like vapor lock to me, although it is possible.

Before throwing money at this, let's figure out whether it is lack of fuel or lack of spark:  The way to do this, after you set up the situation that produces the problem,  is to use ether starting fluid (which I'm assuming you are familiar with - available at your parts store), or WD-40.  I prefer the WD-40 because it is a little easier on the engine bearings, but starting fluid is a little more volatile, and sometimes an engine will start on that when it won't start on WD-40.  So try the WD-40 first.  The way to use either of these is to have a helper crank the engine as if to start it while you give the carburetor air intake a very brief whiff of either fluid - it doesn't take more than a whiff of it to get the engine to fire, at least briefly.  If that happens, we know you have spark.   If it doesn't, we need to go down that road.

If you determine it is lack of spark, check the voltage at the + terminal of the coil (the one that comes from the ignition system) while the engine is cranking when it is hot.  It is possible that the starter is drawing too much current when it is hot causing the ignition voltage to drop below normal.

If you do succeed in getting the engine to fire, the next question is will it keep running, or will it die out quickly. If it will keep running once it fires, that is a symptom of low compression.  I know you said the engine is in fine condition, but I would still want to do a compression check on it if that symptom occurs.

Another thing to check (and do this before the compression check because it is easier) is the fuel/air mixture.  Do this by manually holding the choke flap partly closed while you go through the starting routine - see if that will keep the engine going.  If it will, I suspect a vacuum leak causing a very lean mixture.

The way to check for a vacuum leak is to get the engine running (when it will start and keep running), and then slowly move your hand or a piece of cardboard over the top of the carburetor in such a way as to partly block off the air supply to the engine.  If at ANY point as you start to restrict the air flow, you notice the engine speeding up, you have a vacuum leak.

I'll stop typing now, and wait to hear back from you with some results.  Use the "follow up question" link so you don't run into my daily maximum number of questions limit.

Dick