Most of the HTR's are mounted on the endgate of a truck and as a result get bounced around alot, resulting in a worn inlet needle. If you are experiencing fuel in the crankcase this could be at least a part of your problem.
When the unit runs for the 20 seconds or so and slows down, is the engine blowing black smoke? If so, it may be overfueling due to a worn inlet needle. Have you checked the oil level in the crankcase?
What are the engine numbers? I'm assuming it's a Briggs Vanguard, at least that's how most all of the HTRs were equipped.
If the unit was working fine for two jobs and you're not seeing any smoke, it's my guess that the problem is an obstructed carb.