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I don't think it's really the enrichment's fault. You have to be giving the engine extremely high load to cause enrichment. If you look at any BSFC chart you'll notice that the efficiency gets really terrible at low rpm (like 1000), even at middle load. I think the reason for this is a combination of:
1. Slow intake air speed, poorer fuel atomization
2. Greater proportion of heat rejection into coolant, oil, pistons, etc.
3. Higher piston skirt friction, possibly bearing friction due to lower proportion of hydrodynamic lubrication
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