My new hobby car (Fiat 126p, 2 cyls air cooled, rear mounted) is not doing correct fuel economy. Spark plugs looks ok. Fumes, at cold start are black but as it warms up, it looks ok. The only suspicious thing here is that one of the intake valves doesn't hold calibration. The rocker arm has some lateral displacement (about 3mm) because the center support for the rockers' rod, which is aluminum and is the oil feeder to up there, got worn by the side of that rocker arm. So if I put the rocker arm in position, I can calibrate, but by just sliding it the clearance will get much bigger. I know the solution is to simply add a spring steel calibration washer between the parts, to obligate the rocker arm into the right position. Obviously, this makes the engine rattle and I should take care of that or the arm tip will get damaged, etc.
However I wonder, and that's the meat of this question: would having such damage, which prevents that valve to maintain a correct clearance, be enough to influence fuel consumption? It should be doing about 15 km/L but it is doing about 10 km/L