Had same problem and turned out to be the fuel regulators were full of oil. We were using a low grade LPG then went to better grade and haven't had that problem lately. Still had to rebuild the regulators. Cost was a little over $100.00 dollars but they ran great after that. Not saying that is your problem but just something to check.
There should be a plug/screw at bottom of regulator to drain out oil if any is in it.
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