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Look at these two graphs which display my test results with my 14mm and 18mm carb (correct size for my 48cc). The goal was to see what the best top RPM would be with the two carbs, both trying mineral oil premix (Amalie SynPlus) and synthetic (Maxima Formula K2). If the common beliefs are correct then the 18mm will outperform the 14mm no matter what because premix is supposedly only for lubrication and doesn't affect the fuel or combustion.  What's the first thing you notice? * the small 14mm allowed the same top RPM as the 18mm when the premix was synthetic. Ah, one more motorcycle myth in the trash. With correct premix selection a smalll carb can offer the same performance because premix oil does affect the fuel and combustion. What's the 2nd thing you noticed? * The 14mm and 18mm had opposite premix preferences.  WHY? Let me explain. Synthetic oil reduces fuel atomization. Atomization increases with higher air speed in the carb. The smaller the carb, the higher the air speed. Look at this graphic which displays my idea of how the fuel droplet sizes differ with different size carbs. The middle graph produces the best power. But a 14mm carb that causes the left graph using mineral oil premix can be shifted to the right by reducing atomization with synthetic oil. And a 18mm carb with mineral oil gives the middle graph which shifts to the right when switched to synthetic oil.  So in my case I was able to get the 14mm carb to perform the same as the 18mm carb by using synthetic oil. I made a couple videos explaining this: #1 #2 I then decided to retest after repairing my expansion chamber that had a growing crack at the beginning of the diffuser cone which could of effected the results. Then the results were even more dramatic with the power difference being 9% between mineral oil and synthetic. This second test was only with my 18mm carb so I'd have time enough to also test semi-synthetic and a castor/synthetic blend that's popular.  This test also showed that my engine with this 18mm carb prefered mineral oil. It also showed how a semi-synthetic gave results 40% from mineral oil to synthetic although the synthetic and mineral oil were blended manually 50/50. And Castor 927 which is a blend of castor and synthetic gave the same results as the synthetic oil. How do you know how your carb is sized? Just get my inexpensive carb size calculator which will show ideal sizes depending on which premix oil you use so you can compare your carb size to the calculated size. A normally correctly sized carb will produce between 50m/sec and 63m/sec average air speed. But this is crazy different from what people currently think! So? I don't care. Test results don't lie. Your choice is to be correctly informed or be popular. How do I know the correct fuel/oil ratio to use? Just get my inexpensive fuel/oil ratio calculator for use with Excel or Libre Office (free download). But isn't mineral oil inferior to synthetic? Actually Group 2 mineral oil is almost as good as synthetic. I have a hot running air cooled engine and it does great with Amalie SynPlus (which despite its name implying it has synthetic, it has none, only group 2 mineral oil.) |