Too much pre-heat?

It sounds like you boiled the fuel out of the lines. Do you have a return line and a electrical fuel pump to clear it.

To heat the entire engine more effectively, I’m wondering if it would be better to place your heat source at the lower aft opening in the cowling near the exhaust & place a blanket over the upper cowling.  I believe it’s also important not to judge your pre-heat based on oil temperature alone. It is more important to ensure that the entire engine is heated uniformly.  Garrett may be correct in that your current method may be creating vapor lock in your fuel system.  
Before I installed a Tanis pre-heat system on my 912 ULS, I hung a 100 watt work-light in the lower cowling and covered the upper cowling with a blanket.  This method worked well, but I worried about the possibility of fuel dripping out of the carburetors… for all of the reasons fuel drips out of carburetors… and causing a fire hazard.  

A few years ago EAA put out a magazine article of a home made hot air heater. The heater ended up starting a fire in which they believe a fuel leak dripped fuel directly into the heater resulting in a fire destroying 10 aircraft including the hangar building, the fire went from aircraft to aircraft as the fuel tanks burned thru. The article was retracted in the next issue, its best to use an approved/safe heating method like a Tanis system.

Heating the block up is a good thing,

Heating the Carbs up to the same temp is not so good.

The hair-dryer is an economical source of heat but should be monitored all the time.

Do not leave the heater unattended while heating. If you can't see and hear it, you are too far away.

It would be better to place the Hair-Dryer at the far end of a metallic (Not Plastic) Cloths-Dryer vent hose and install a thermostat to limit the cowling temps to ~80°F.

Keep the heat source well clear of the Aircraft.