Normally when tuning we would aim for a mixture around 12.8:1, this seems to make the most power whilst being without much risk of seizure. On a 50cc moped we find from experience that once the mixture drops much below 12:1 it can start to cause spluttering and significant power loss. Running rich isn't as potentially catastrophic as running lean because the excess unburnt fuel acts to cool the combustion chamber, but again you won't be reaching the full power potential of the engine and wasting fuel. This could be caused by for example a restrictive or dirty air filter or a jet that is too large. In contrast if the mixture ratio is much less than 12.5:1 the bike is said to be 'rich' whereby there is not enough air present for the fuel to be fully burnt. On a 50cc moped we find that once the mixture starts to approach 15:1 this is the point where there is a real risk of seizure and engine damage especially on long full throttle rides. This can cause seizures (where the piston expands from excess heat so much that it no longer will move within the cylinder) or can even melt the piston. The problem with a bike running lean is that not only are you down on power but more seriously the engine can run much too hot. If the mixture ratio exceeds this (perhaps the jet is too small or there is an air leak) then the engine is said to be running lean, or in other words the fuel/ air ratio needs to be altered so as there is more fuel in relation to the mass of air. Roughly speaking it's necessary for the mass of air to be approximately 13 times as much as the mass of fuel for the engine to burn all the available fuel and develop it's full power potential (and run safely and reliably). 'Jetting' your bike means tuning or adjusting the carburettor so that it is always supplying the correct ratio (mixture) of petrol/ air at all times to the engine.
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