Mixing 2 stroke fuel ratios for dirt bikes (premix) can be a royal pain in the jaxi, and more annoying than having to listen to Britney Spears on the office radio. However, it's essential for two bangers and it's something you need to do correctly or you'll damage or even destroy the engine.
A lot of people seem confused about what ratios to mix the fuel and oil at. The safest, easiest and best answer is mix it at the ratio recommended by the manufacturer of your dirt bike. They are the ones who know the engine better than anyone else including your local mechanic.
Keep in mind the ratios the manufacturers recommend are for a high performance dirt bike that is designed to race at its full capacity. If you have skills like Ryan Dungey and you ride your bike at full noise most of the time you need the recommended petrol to oil mix to keep the piston and rings properly lubricated. On the other hand, if you are just pottering around and spending most of your bikes time in the low to mid revs range, that extra oil is probably an overkill and not entirely needed.
The problem with veering away from the recommended fuel/oil ratio is the carburetor is set up for the right amount of petrol/oil/air mixture, so if you significantly alter this ratio you will need to change the jets and carby settings to suit.
My YZ 250 runs at 32:1 which is what's written in the owners manual. If you don't have an owners manual, consult your local (Suzuki, Yamaha, KTM etc) dealership and ask them.
A lot of riders run their bikes at 40:1. If you aren't pinning it everywhere you will get away with this. Just keep this in mind: You will not damage your engine by mixing too much oil with your gas. The worst it might do is foul plugs, although a gunked up carburetor or incorrect carby settings are a far more likely cause of fowled plugs and a smokey exhaust. However, if you forget to mix oil in at all, or consistently don't use enough you risk wearing out the engine prematurely or seizing it.
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