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I've never seen an inline run too cool. They run on the hot side if anything.
Remove the idle stabilizer, they are prone to failure.
Remove the air jet in the top of the float bowl, leave it open.
Richen the idle mixture, about 2 turns from lightly seated.
What jets are in the carbs? Which spark plugs are you using?
What is the timing set at?
What is the idle speed?
Double check nothing is wrapped around the propshaft.
Run on the hose for about 5 minutes, shut engine off and quickly pull all the spark plugs, at least the top 5. Check for any tiny drops of water on the plugs or a milky look.
Which jets and spark plugs are you using?