John - My rull of thumb is: Crank it at the slowest speed it'll reliably start at. The colder the ambient temperature, the higher this speed will likely be, up to a maximum of half throttle or maybe a little less.
As for the operating rpm, I run the engine from a few hundred rpm below the peak torque rated rpm on up to maximum, depending on what I'm doing. If you need full PTO rpm, for example, you have to run it at PTO rated speed, which is usually pretty close to maximum rpm. It doesn't hurt anything, but running it at faster rpms than necessary will cause unnecessary wear, by definition. On the other hand, lugging the engine at rpms slower than the load dictates, does more damage a lot faster than the aforementioned wear, so choose your rpm accordingly.
One way to tell if you're lugging the engine is by seeing how much faster the engine runs if you remove the load. If you're running the engine below the peak torque rated rpm and the rpm jumps by more than several hundred rpm when you remove the load, you're probably running the engine too slow. Likewise, if you're running below torque rated rpm and you see more than a little black smoke, you need more rpm.
Mark
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Edited by MChalkley on 7/26/00 11:16 PM.</FONT></P>
