I've got a air cooled Deutz diesel that does the same thing. It came in the case trencher that I made the back hoe from. Before I cut it up I rigged the engine up and hot wired it to start. It started right up but was blowing out out the exhaust like crazy. I was running around in a rain shower of oil pulling levers and trying the hydraulic like a chicken with my head cut off. Seemed like 5 minutes but it was probally only 30 seconds.

Sure was an exciting 30 seconds. When I hit the kill lever it kept running for a few Seconds, thought it might run away. I have the engine sitting out under the shed but haven't messed with it since.

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