I doubt any additives would be a problem, but if it really is vapor lock [unusual for a stovebolt] putting some heat reflective material around parts of the fuel line closest to the engine should help - or the old Ford trick of wooden spring clothes pins on it for a heat sink

Bill


Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics