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Thread: Fuel mix ratio

  1. #1

    Fuel mix ratio

    Morning crew,
    My boat is fitted with a 25hp Yamaha 2 stroke CV, think its around 05 model. Hadn't had it for long and the motor is running a bit rough. I have replaced plugs as once I had removed old ones they looked shot but were also covered in oil and oil also present around plug housing. I have been using a mix of 100:1 as there is a sticker under the choke that states this but several people have told me that they have never heard of this motor running on this ratio and when on the water, she does smell like she is running a bit rich. Can anyone confirm on what ratio i should be def mixing. It is def running better with the new plugs.....for now?
    Any info would be greatly appreciated. (also using the dearer fuel i should mention.)
    Cheers,
    JN

  2. #2

    Re: Fuel mix ratio

    Ok so now I am more confused.! I spoke to a Yamaha dealer and he said I should run it at 50:1 which means to me unless I am completely wrong, i need to add 200ml of oil to 10litres of fuel......I had been running at 100:1 meaning 100ml of oil to 10 litres This doesn't make sense to me as as mentioned above, the old plugs we covered in oil and excess or unburnt oil could be seen around the spark plug housing once i removed them, and the motor was a tad smokey and running rough at idle........I was under the impression i was mixing to much oil causing this and by adding more oil, would make the situation worse....am i completely missing something hear or could the oil/fuel mix not burning correctly due to the stuffed plugs to begin with.....?

  3. #3

    Re: Fuel mix ratio

    Lots of smaller HP Yamahas were suggested as 100:1. They seem to run OK at that, however lots use 50:1 because it's what everything else uses, low speed operation will oil plugs, make sure you give the motor a bit of high speed work.

  4. #4

    Re: Fuel mix ratio

    The other issue is you might need a hotter plug than the ones that were in there, go up one grade. 100 to 1 is fine for that motor, adding more oil is going to make the situation worse.

  5. #5
    Ausfish Addict disorderly's Avatar
    Join Date
    Sep 2006
    Location
    In the Jungle/Mission Beach Hinterland

    Re: Fuel mix ratio

    I use 100 to 1 on my Yammie 15hp on my little explorer..it was mentioned that to use 50 to 1 might result in oiling up the plugs...dont know it thats true or not but the motor has been run at 100 to 1 since new in 2007 and still starts and runs like new..

  6. #6

    Re: Fuel mix ratio

    Current plugs are BH7s10.....

  7. #7

    Re: Fuel mix ratio

    Looks like an NKG brand. The number 7 denotes the temp of the plug, I would try an 8

  8. #8

    Re: Fuel mix ratio

    Yeah mate,
    NGK. I might do as you have suggested and see how it performs. If no good, i will just go and get it serviced. Thought i would just try a couple of things before i dish out money for a service it may not need.
    Thanks,
    JN

  9. #9

    Re: Fuel mix ratio

    When u pour the oil into the fuel tank u need to shake the tank to mix the oil in

    100/1 means 100 litres of fuel to 1 litre of oil

    If your putting in 10 litres of fuel u add 100mls of oil also use a mineral or semi synthetic oil as full synthetic oils are gor extreme conditions running hours on end at wot or racing

  10. #10

    Re: Fuel mix ratio

    Yeah,
    I need to change the way i do that. Normally i just put in x amount of litres in the tank then add the required oil. Don't shake it as tank is under the floor. From now on i will put the required oil in the tank first then add fuel in an effort to have it blend through better.
    Thanks mate.

  11. #11

    Re: Fuel mix ratio

    Quote Originally Posted by JEWIENEWIE View Post
    Yeah,
    I need to change the way i do that. Normally i just put in x amount of litres in the tank then add the required oil. Don't shake it as tank is under the floor. From now on i will put the required oil in the tank first then add fuel in an effort to have it blend through better.
    Thanks mate.
    That might be why you're having problems. If the oil is sitting on the bottom your engine will be copping too much oil initially. More worryingly though is when the oil goes through then your engine will be under lubricated and this could end up destroying it. Putting the oil in first might not be much better. You should be mixing oil plus fuel in a container and then putting the mixture into your tank.

  12. #12

    Re: Fuel mix ratio

    Good call Billfisher,
    i only thought of that after i made the post.....a case of the simple things done properly here I think. I have learnt a lot here in a few hours. Will see how it all goes with a few of these changes put in place...

  13. #13
    Ausfish Addict Chimo's Avatar
    Join Date
    Jun 2006
    Location
    Gold Coast

    Re: Fuel mix ratio

    Also consider running the better mixed fuel (100 : !) ratio through a Mr Funnel filter to make sure you are adding no water to the tank.
    In fact it it was mine I would syphon all the fuel out of the tanl and run it thru a Mr Funnel or equivalent.

    My OMC mechanic told me recently that he often has to do a good total clean on Yamaha carbs as the jets are so small that contaminants inc water tend to block things up. OMC jets seem to be less sensitive..

    I just got a a little 2 stroke Yam for a tinny and the first thing I did was empy the tank and put what came out thru the Mr Funnel equivalent filter.and the 100 : 1 mix I brew for it will also go thru the filter too.
    Attention to detail is king!

    https://www.mrfunnelaustralia.com.au/video.html
    What could go wrong.......................

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