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  1. #1
    Ziggy's Avatar
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    Super Jet project - what octane gas?

    Putting the finishing touches on my '90 SJ SN restore project. The ski is a 701 62T conversion with a Factory B pipe. Not much else performance-wise.The engine compression is 190 in the front an 187 in the rear, which seems pretty high to me, but I don't have much experience with these engines or Yamaha in general. Can this engine be run on pump gas, I didn't want to have to run high octane fuel?
    Thanks!


  2. #2

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    Thumbs up

    170-175 psi is high as you can go with 93 octain gas on these motors, You could get by with a 50/50 mix race gas 110 octain and 93 octain gas up to 190 psi over that you need 110 race gas only. Tommy Jordan

  3. #3

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    Cool

    Quote Originally Posted by Ziggy View Post
    Putting the finishing touches on my '90 SJ SN restore project. The ski is a 701 62T conversion with a Factory B pipe. Not much else performance-wise.The engine compression is 190 in the front an 187 in the rear, which seems pretty high to me, but I don't have much experience with these engines or Yamaha in general. Can this engine be run on pump gas, I didn't want to have to run high octane fuel?
    Thanks!
    you might double check with another gauge

    later cd

  4. #4
    Stop staring at my tits NJJer's Avatar
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    195 in one of my 61X's and never a problem with 93 octane.

    Cranking PSI is not an exact way to determine what octane to use. It is just a general "rule".

    Just what is done to your engine that is has that high cranking PSI?

  5. #5
    L Carlson's Avatar
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    Quote Originally Posted by NJJer View Post
    195 in one of my 61X's and never a problem with 93 octane.

    Cranking PSI is not an exact way to determine what octane to use. It is just a general "rule".

    Just what is done to your engine that is has that high cranking PSI?
    +1 on all stated

  6. #6
    Ziggy's Avatar
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    Quote Originally Posted by NJJer View Post
    195 in one of my 61X's and never a problem with 93 octane.

    Cranking PSI is not an exact way to determine what octane to use. It is just a general "rule".

    Just what is done to your engine that is has that high cranking PSI?
    If I remember, the seller said the head was cut to bump compression, but that was over 4 years ago & I've slept since then.

    There's a very good chance the high compression is a result of all of the oil that was put into this engine before it was put into long-term storage. I forgot about that. I'll run it on the hose for a few minutes with 93 @ 40:1 & clean it out. I'll do another check after & see what it is.

    The gauge is OK - calibrated to +/- 0.5% FS which is an error of 1 psi @ a reading of 200psi. Durned good Wika gauge.

  7. #7
    Moderator DrewNJ's Avatar
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    I've run with a few SJ's with compression just under 200 psi on pump gas.. But they are not WOT for any more than a few seconds at a time. Straight up freestyle skis.

    I ran my kids 701 right at 150psi.

    Also, I was under the impression that you could not effectively cut a stock 701 head due to the domes/squish area...

  8. #8
    zpeters's Avatar
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    more info on the domes/squish is a good idea to find out if they were cut etc..
    I use 93 on my sj at 190 psi

  9. #9
    Ziggy's Avatar
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    Honestly, I can't rely on my memory after 4-1/2 years! I could have been thinking of any number of engines, and have to admit I really don't know how the compression was achieved on this one. I'm just happy it has compression at all and it's that close cylinder to cylinder.

  10. #10
    Moderator DrewNJ's Avatar
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    Quote Originally Posted by Ziggy View Post
    Honestly, I can't rely on my memory after 4-1/2 years! I could have been thinking of any number of engines, and have to admit I really don't know how the compression was achieved on this one. I'm just happy it has compression at all and it's that close cylinder to cylinder.
    Great Point!!

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