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  1. #1

    A/F Ratio info on 2-strokes

    I'm trying to help out a buddy of mine. He has a two stroke outboard that has blown twice in the last month (fortunately, the second time covered by warranty). He was told the reason it went ka-boom was because he ran too lean, causing the piston to sieze. It's not an oiling problem since he does strickly a pre-mix in his tanks.

    I've seen a lot of info on A/F ratios for 4-strokes (referring to the threads on the sea-do side of this forum........but was curious how that info compared for 2-smokes. And does the A/F meters you can buy also work with the 2-strokes?

    Thanks for any help.


  2. #2
    All hail the Chief! fullboogie's Avatar
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    First, no, a good A/F meter (meaning a wideband using the LSU-4 sensor) will absolutely not work in a 2 stroke. The oil in the exhaust would foul the sensor in a matter of seconds.

    In fact, I'm not sure how one would measure A/F ratio on a 2 stroke due to the fact that lubrication comes from the inlet charge, therefore "contaminating" the exhaust. In addition, I would think that the "indicated" A/F ratio on our imaginary meter would be thrown off by changes in oil ratio. I think the only way to go is with an EGT sensor(s).

  3. #3
    Happily Self-Employed WFO's Avatar
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    Lean is a very broad term on a two stroke engine and I hate it when people just calmly state "its too lean".

    Lean on a 2 stroke can mean:
    -not enough oil
    -not enough fuel
    -too much cylinder temperature (compression, insufficient cooling, timing problems)
    -air leak(s)


    I have been searching and inquiring about a sensor to run on my EFI ski for a while now. The problem with trying to monitor A/F mixture on a 2 stroke, is finding a sensor that will hold up and read correctly with the 2 stroke exhaust. No one will guarantee that it will work.

    I personally dont know too much about the outboards. Tell your friend to try another shop to see if possibly they have seen the same type problems and maybe they can tell him why it keeps blowing up.

  4. #4
    r33pwrd's Avatar
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    maybee a EGT gauge? You would need a good baseline # but on my skyline motor I only used a EGT gauge to tune my electromotive!

  5. #5
    Old Ski Insanity w/rpm500's Avatar
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    We used a funky unit with the webers and delardo carbs on the VW engines... was a ball type flow meter on a spring loaded gauge .. as you adjusted the needles ,the flow at the venturi would indicate on the meter.. idealy, to get them all as high as possible and evenly balanced out.. It was not for a/f ratio per say,thats still good old plug readings,soot levels at exhaust and seat of the pants... this unit really indicated flow efficiancy well while adjusting needles, to get mulit carbs matched and would help show the improved or poor flow levels when changing jets and needle's.
    I do realize this was for a four stroke carb motor and always wondered if it would work to some effect on a 2 smoke'r
    Last edited by w/rpm500; 08-24-2006 at 04:29 PM.

  6. #6
    All hail the Chief! fullboogie's Avatar
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    Talking

    Wow! The wrong tool for the wrong carb off an engine that's not in his ski - that's helpful!
    Last edited by fullboogie; 08-25-2006 at 02:22 PM.

  7. #7
    txgp1300r's Avatar
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    That may be , but he was thinking out of the box ,and he was trying to be helpfull, which what this place is supposed to be about.......

  8. #8
    Old Ski Insanity w/rpm500's Avatar
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    Smile

    ok.. wrong tool by your vast knowledge ... and you offered besides one sensor? ...one, I THINK..., one IN FACT, I'M NOT SURE... and an additional I WOULD THINK....?????? Not to be pissy, as Your reply is.... but have you personally Ever actually used one???Seen one??? To know it wouldn't work????

    http://www.cbperformance.com/catalog.asp?ProductID=572
    Last edited by w/rpm500; 08-25-2006 at 04:23 PM.

  9. #9
    LOL, The tool you are refering to is a manometer. AKA carb sync tool. It is very useful (just not for this)

    1350f Is the max sustained temp you want to go on turbo engines due to melting the vanes in the turbo, therefore I would think staying below 1300 for long WOT runs would be safe. Be sure to get a better baseline than that though.

  10. #10
    txgp1300r's Avatar
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    I had a blown roadster w/ two holly's on a bds 6-71 and i used a tool like that to sync the carbs..really worked very well to even out the mixture and low speed drivability...

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