When you run it in the drum, try to have the motor in the water so the exhaust water outlet is not more than a few inches out of the water. At least the bypass plate on the side needs to remain submerged while running and not just the snout. I use a plastic 55 gallon drum as a test tank also. I may upgrade sometime but it does fine most of the time. Sometimes I have to use a big shop fan to blow exhaust gases away from the carburetor if there’s not much breeze but as far as cooling, it does OK. A fan is needed with any tank no matter the size if there isn’t enough breeze to evacuate the exhaust.
As far as the impeller vanes righting themselves when the engine runs even if they get turned backwards, sometimes they do and sometimes they do not. I have had several motors apart OMC and Mercury with various sizes and shapes of rubber vane impellers and have found a number of them with some of the vanes bent backwards after being run. The latest one was a ’73 Johnson 25hp. It was still pumping OK, but I changed the impeller as a preventive measure and found this when I opened the pump housing. This motor had been running on a coworker’s pontoon for 5 years and was brought to me for service. I ran it in my tank before servicing the pump also.
-Ben
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