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Does the engine have the idle stabilizer on it?
How many turns are the idle mixture screws turned out?
Start and run the engine for a few minutes and immediately pull the spark plugs. Are you getting any water on the spark plugs, tiny dropplets?
What ignition tests have been done?
Look on top of the float bowls is there a small brass air jet?
What is the idle timing set at?
What is the engine serial number?
Do you know which high speed jets are in the carbs?
Remove the kets in the top of the float bowls.
Your motor didn't come with a low water pick up, but that wouldn't make a difference.
The milky substance sounds like you are getting water in the engine. This is a common problem. You will need to have the exhaust side plates replaced. When the engine was rebuilt did he install a new inner water jacket?
Try richening up the idle mixture to about 1 3/4 to 2 turns out from lightly seated.
OK, the inner plate will need to be replaced.
Here is are complete instructions to do it right so it won't happen again. You do have to examine the engine carefully for any cracks, although they are really uncommon.
Unless the techs at the shop are over 50 they probably have little experience with that engine.