Originally Posted by jezza2011
i see ok the tanks do this if you dont keep the vent open.
fuel gets pushed out as the tanks gets pressurised as the fuel expands/evaps.
fill it up.
check for leaks.
then add in the 2st oil. Simples.
as per previous posts (i) "Full" in 22.5L tank can be over 22.5L total volume so no closer to knowing the total volume; (ii) if genuinely full there will be no space for the 0.5L of oil.
Its always best to put the 2st oil in when you fill it up. that way you cant make the mistake of not putting it in later and having a seize up. !!!
Whilst you mitigate that risk you do introduce other potential problems: e.g. a spillage/leak like this is then an oily (slippy) spillage; if e.g. he never used the fuel (as has nearly been the case because of other issues) he had a load of fuel with oil in it - without the oil stick it in the car, with the oil? Does he keep it? Risk it in the car? Bring in back home (its probably dangerous goods on the ferry/chunnel) etc. Long term storage of premix is a problem compared to fuel on its own; Swapping fuel between boats with 2 str / pre-mix / autolube / 4 stroke [when cruising in company or at a club etc] is easier if you don't premix until the fuel is about to be used... ...so I would suggest it is not "always best to put 2 stroke in when you fill it" and certainly not much help to the OP now.
Vincent - unless you have lost a whole load of fuel you really aren't going to notice. If you add 400 mL of oil (the correct amount for 50:1 with 20L) and actually only have a ration of 37.5:1. If you've lost more than 5L you'll know by sight or lifting the can. More likely you have lost less than a litre. So your ratio would be 47.5:1 . Most people probably aren't that precise in measuring either the fuel or oil to get it any better than that anyway. Most engines will run on 25:1 but smoke like mad and possibly the plugs foul at low revs [indeed your engine may well have been run when new in at 25:1 for the first couple of tanks full]. Some engines being used in environmentally sensitive places will run on 100:1, and although most people in the UK are scared of that number in premix (my autolube seems to use a little less than this ratio averaged out); you could probably
get away with running a single tank at 100:1 on an engine that has been well lubed for all its life so far, with no ill effects. i.e. what I am saying is you probably have less that 10% error on your ratio, and even if it was 50% you would probably be fine.