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Octane Rating and Timing Advance

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  • Octane Rating and Timing Advance

    Hello, I have a 1977 Johnson 70hp OB. Currently I am running 93 octane, and have the WOT timing advance set to 14 degrees. Factory WOT timing in 77 was 17 degrees, but have been informed on the forum that its recommended to lower maximum advance to 14 degrees. First question is, am I wasting money by running 93 octane over 87? Also can I run 93 and maybe bump the max advance back up to 17 degrees? Will I gain much of anything by going from 14 to 17? Let me know your opinions.

    Thanks!

  • #2
    Change your timing back to 17 deg and run 87 octane I doubt you will see any difference in the performance of that block by doing that. Remember you have a 49 cid block making 70 hp. That is pretty much at the top end of what that block will make for power and still be reliable. When OMC tried to get another 5 hp out of the same block they actually had to change the port configuration in the block and add 300 rpm to the WOT range and sacrificed some of the low end power. Later models of that motor are actually at 19 deg timing. Right now you are stuck running 93 octane if you ever can't get it what will you do then? I am not a fan of changing timing on 2 strks because there is no real value in it. But it is up to you it's your motor for me if I want more power i will get a bigger or newer motor..
    Last edited by flyingscott; August 24th, 2017, 09:02 AM.

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    • #3
      I had just read on here that you need to take away a few degrees of WOT timing in order for it not to ping due to lower quality of gas now days. Is this not true? If not ill go ahead and set it back to 17 degrees, and next tank will be 87 octane.

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      • #4
        Very seldom does timing get changed on outboard motors. I run 87 octane in all my motors unless the owners manual states differently. Do you have the owners manual look in there for the octane recommendation. I would bet your motor may need to run the higher octane at 17 deg so advancing it more may cause problems. I would try to find out if there is a service bulletin for timing adjustments to deal with the unleaded fuel. Definitely find the owners manual or call a dealer.
        Last edited by flyingscott; August 24th, 2017, 11:13 AM.

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        • #5
          What have read on the forums is pure nonsense speculation, max carb opening must match max timing advance as factory stated. If need to work at 17° re adjust it back to specs. It's bad for 2 strokes OB's to run at max wot range with less timing advance.

          Happy Boating
          Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

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          • #6
            Originally posted by Sea Rider View Post
            What have read on the forums is pure nonsense speculation, max carb opening must match max timing advance as factory stated. If need to work at 17° re adjust it back to specs. It's bad for 2 strokes OB's to run at max wot range with less timing advance.

            Happy Boating

            Not as bad as when the timing is advanced past the specified value.

            To the OP, set the timing for 17 degrees BTDC, unless there is a service bulletin saying to retard the timing.

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            • #7
              Not as bad is not Optimum. We've had cases on out of the box OB's where timing advance was 2-3° off from factory specs, once installed and tested OB responded bit sluggish, put back to specs and made a difference. Have read lots of OB's Service Manuals, all of them stated max 10% E content, none stated to retard timing advance less than factory specs. If there isn't any Service Bulletin specifying so, then it's word of mouth, written speculation & disinformation..

              BTW, timing advance set slightly passing factory specs along a less pitch prop maximization.can benefit any 2 strokes OB to perform much better at high altitudes.

              Happy Boating
              Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

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              • #8
                Hi JC
                If you are he same fellow I am thinking of...you are over here in Scotland and run your boat on Loch Lomond ?
                If so...I wouldn't bother too much with anything like that.
                Normal unleaded (although higher octane than our fellow US boaters) is just fine for your engine. It's never going to be a super Performance boat. I'd just get on with enjoying boat and the lifestyle it brings !

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                • #9
                  Why do you say your octane rating is higher?

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                  • #10
                    Hi Ovandr

                    Simple answer is just because it is.
                    Not as simple as most will think and I am aware of "Ron" and "mon".
                    Truth be told there isn't a great difference and the numbers stated on the pump in us and uk are not directly comparable.....but in essence they are higher here in the uk and eu. By doing a few sums...and trying to be comparable...on average about 1/2 ratings per each level of fuel (regular and super).

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                    • #11
                      With the exception of our supermarket fuel !

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                      • #12
                        I've never done the math, but I'll take your .5 higher rating, but we can get the same thing here in many places, multiple levels that go higher than normal if you want it.

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                        • #13
                          Mine 48 spl runs best on 89. Nice and smooth, idles great and starts with one bump. I'd time yours like this. Set it a 15 then run it wot with the trim all the way down. Note max RPM. Tweek the timing screw manually up a little and down a little until you find the sweet spot and max RPM. It won't be a dramatic change but you will find a happy place. After doing them this way I never use the book. Speed will fall off if too high or too low so if you go easy with the tweeks you can't screew it up.
                          Arizona

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                          • #14
                            I feel zero performance advantage running a hi octane fuel in my 1995 Mariner 115 HP, at least none I can quantify. I do fill with hi octane when I put the boat away for the winter as I was told or read that the higher octane prevents phase separation in the gas but I do run hi octane in my 15HP kicker as I don't use it much all year 1/2 tank) and I do see a difference in cold starts on the baby. My wife said when I put on a the matching 15 HP Mariner that I was lucky to find "Look, the big Momma motor had a baby!"
                            Experience is a lifetime of mistakes, wisdom is not making them again.

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                            • #15
                              Sign up today
                              If you run 2 strokes OB's at wot with less timing advance than factory specs while carb is fullly opened, besides feeling it bit sluggish, expect more fuel/oil mixture draining unburned through exhaust & prop. If wanting OB to deliver full HP and max RPM go for a prop maximization instead of fingering timing advance/ carb synch away from factory specs.

                              Happy Boating
                              Last edited by Sea Rider; September 8th, 2017, 11:24 PM.
                              Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

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