[a big scary monster]

Glad you enjoyed your run, is that the first time you have driven it? This post is not that much help as it is not a solution but maybe help find the cause, 25 years ago my first boss used to let me drive his 1970's range rover with giant trailer during hay making. Before we started we used to take the bonnet and drivers door off, door so you could jump out throw a bail on and leg it back in whilst the RR ticked over in 1st gear low box and kept moving, bonnet used to come off to let the heat escape to reduce the chance of a fire and so you could glance in to see if hay and flammables had built up in the engine bay, the downside of this was the RR had to be left running all day even over lunch as it would not start whilst warm when run with the bonnet off, we had always assumed the bonnet aided air flow and cooling over the top of the engine/ carbs and it was simply boiling the fuel out of the carbs once turned off.
He swapped the RR for a defender when he fired a con rod out of the block on the m62, he got out picked the bits off the road chucked some oil and water in being a farmer he always had and still has 25 litres of each in the back and carried on on what cylinders remained. He drove it like that for about 15 miles on and off road with many stops for stream water and it finally gave in about ¼ of a mile from the farm. He then walked down his fields and caught a bus to Copley motors land rover and drove home in a brand new defender it was a G plate so 1990 his wife was suprised as he had only gone out to take his brother to Manchester airport.
He is certainly a character Oddly I saw him yesterday. One day I'll type you the story of the Shetland pony and the JCB .
My point being you have a very similar engine and have completely altered the air flow around it. Ethanol evaporates at 77.7°c if I recall.
I boiled the fuel yesterday in a chainsaw and mower it still feels cold when you put your finger in but bubbles a lot. I do recall the RR used to get quite warm lugging the trailer up and down the Pennine fields. Ed.