Best pre mix ration for dtr? Fresh bore and piston


  • As the tits says just so I can check my bike runs and to tide me over till I can sort my pump?


  • I was at 333ml oil / 10L petrol . There is an app in google play store - castrol calculator - handy premix calculator , not sure if availible for iphones


  • Thanks, I just done my first start on 155 to 5l


  • The general rule of thumb is you want to sit at around 2%. If you want to be very picky (like me) then you can mess around with the ratios until you feel your bike is happy. Which to me means no smoke when warmed up and idle, oily smoke when accelerating but no thick oil spitting or dripping off the pipe. My GasGas's manual recommends 2%, but I only ever put 1.8%.

    Also if you have mixed fuel in your tank and the bike sits still for some time, give it a shake to make sure that fuel and oil is mixed up nicely.


  • Remember, that any oil you do add to the fuel, will weaken the mixture and as such may not run right.

    It will now start to run lean as you add oil into the fuel.


  • @Calum sorry to sound dumb but why would I add more oil in? I think this mix is perfect not that I've ever pre mixed before and I've never actually ridden a 2 stroke😬


  • @declan Try reading what I posted again...

    I said remember, any oil that you add will weaken the fuel mixture.

    The DT's are setup to run an oil pump.

    The oil is added AFTER the fuel has been drawn from the carb.

    If you add the oil to the fuel, it is now drawing that mixture through the main jet, which was designed to run fuel only. Now you are drawing fuel and oil through there the mixture is going to be leaner. Since it's designed to draw through a prescribed amount. So it will draw in the correct amount, but now this amount holds the fuel AND oil, so the Air Fuel Ration (AFR) is going to be incorrect.


  • The difference is so little that its not an issue in the real world on our bikes. But the theory is correct, and was a much bigger issue back in the day where people would go as big as 6% oil ratio for flattracking and roadracing.

    If you do the math: 240 main jet, + 2 percent to compensate for the oil = 244.8 main jet. Temperature makes a bigger difference than that, and I dont see people changing jets several times a day because of that.


  • @Jens-Eskildsen You would be surprised though.

    I remember when I was running premix, you never actually drained the tank out completely, else how did you make it to the petrol station.

    So adding 300ml oil every time eventually leans out the mixture. On a daily commuter this is easier to do than you might think.


  • @Calum This is why I never mixed my fuel at the pump. I've got 2 10litre jerry cans I fill up when in town and mix at home in the garage.


  • @Calum I get what you mean but I mix mine in jerry cans and it's temporary till I get my pump in shape


  • Sure, if you add more and more oil to the gastank, you will lean it out, but thats just stupid. Never heard anyone do it like that before...

    The few people i know that still mixes and ride street, fill the bike from home from jerrycans and have small containers of oil that suits a certain amount of gas for when filling up during a trip. Like a 150ml bottle of oil, and fill up with 7,5 ltrs of gas as an example. Sure, the gastank might take more gas, but they only put in a certain amount so the ratio is allways right. That will get them home on a long trip, or atleast to a gasstation closer to home, where you can do the same trick. So you add a 2% mix, to the 2% mix already in the tank. Find a suitable container depending on your mpg/how far/often you ride ect ect.


  • @Jens-Eskildsen Genuinely never done that. Neither has anyone I known. For a road bike. Obviously that's the done thing for crossers and what not. But if you are out on the bike all day you sre unlikely to go home to fill up.

    We tend to literally just put in 10 litres and 300 mill.

    You are right though 3% is so small it wasn't even worth mentioning it.