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  1. #1

    Break-In Fuel/Oil Mix ratio on 1993 SL650

    I'm on about hour 6 of my "93 Sl 650 break-in. New top end.

    I've gone through about one full tank at 32:1 (Maybe a little richer)

    After about 2 hours, and then again at 4 hours, I had to clean the plugs because it started loading up a bit when giving throttle. It would run real good (sticking to no full throttle, etc re: break-In) but every once in a while it would not accelerate and I had to let it idle for a bit and then it would take off again. I am running the screws rich!

    This morning, to check the wash, I pulled the plugs and took a look. All 3 are still VERY clean, with just a slight hint of carbon. I turned the screws, low and high, in to exactly factory settings and will take it aout again. Can someone tell me how long before I should expect the carbon to appear after screw changes.

    I'm trying to "ease" my way to the proper settings because I'm doing my best to avoid a burnt piston!
    Last edited by K447; 09-22-2012 at 09:47 PM. Reason: Fuel/air typo


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by ltmark View Post
    I'm on about hour 6 of my "93 Sl 650 break-in. New top end.

    I've gone through about one full tank at 32:1 (Maybe a little richer) ...
    I think you are missing the 'break' part of the process.

    By now you should be applying full throttle for increasing periods of time. The rings achieve most of their wear-in against the cylinder walls when you are applying full throttle.

    The very first few heat cycles are fairly short run time duration. Run the engine, moderate and varying throttle, then let it cool. Do that again for somewhat longer time and more throttle.

    After just a few heat cycles you want to be dipping into full throttle. Initially for just a few seconds at wide open with cruise time in between (varying throttle), then work towards more often and for longer moments at wide open.

    By the end of a full tank you should be running full throttle for significant time periods.

    Don't excessively baby the engine or you will not achieve optimum break-in. The engine will still run just fine almost regardless of the break-in process you use, but for optimum power and piston ring sealing you want to get those rings forced against the cylinder walls, which requires heavy throttle.

    The primary reason for not applying full throttle right from the zero hour is that the initial friction between the fresh piston rings and the honed cylinder walls is high. Friction creates heat and wear. The extra oil during break-in plus the short periods of heavy throttle allow the cylinders and rings to get comfortable with each other, yet not overdo things.

    As the break-in proceeds (cylinders get smoother and rings get better seated) there is less excess friction and additional throttle is needed to continue to force the rings outwards and complete their wearing process against the cylinder walls.

  3. #3
    Thank You. Seems like you are the one always helping me out. What about the screw issue. I'm assuming that when it runs real good and then seems to be loading up a bit and the plugs are oil soaked when that occurs, it means I'm too rich? Should I go to 40:1 now and will that alone, possible change the plug issue if the screws settings are close?

  4. #4
    Click avatar for tech links/info, donation request K447's Avatar
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    I will let the Fuji engine guys comment on carb settings and piston wash.

    By now you are probably safe moving to a 40:1 oil ratio (pre-mix, I presume), given that you still have some extra oily fuel in the tank. And you want those piston rings to actually get fully worn in! Extra oil isn't going to help with that at this point (6 hours run time already).

  5. #5
    Thank you.....

  6. #6
    OK K447...how 'bout a little more help? I'm at the end of my first 10 gallons, but still bothered by a few smaller things, primarily, my idle mixture right now. The manual say 1/4 turn out on all three carbs. I am right there, but if I clean the plugs and start the ski for just a short bit (10 sec max) and then pull the plugs, they are oily. During my last outing, it would run great, with rapid throttle response, but a few times, it wouldn't die, but seemed to be loading up and then after 15 sec or so, it would take off. Question, If I crank the lows in further, close to being fully closed and it still starts and idles, then I'm leaking gas into the engine (also would account for the very clean pistons and wet plugs). All 3 mikuni 38's have been rebuilt with OEM kits and I have gone thru them several times, double/triple checking everything. The book says 10-16 pop-off, which I have! Although my 93 did not have a return restrictor from factory, I installed one in he return. I read that the small clear plastic check valve can cause leak thru. If my plugs keep oiling up, do you have any thoughts from what I have posted here? I was running 32:1 but will be at 40:1 with this current tank when I go out Fri. ++last thought, can lean lows cause engine damage or just shitty idling and initial throttle response?

  7. #7
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    Your low speed screws don't account for much past idle. The plugs will be wet / oily with idle only or extended "no wake" riding. Best thing to do is set you idle high ~1500 rpm with the idle speed screw. Then slowly open the lows the same amount each time until the rpms peak then start to drop. Once they start to drop, close the needles a little bit until the best rpm is reached. Then you can return the idle speed screw to the correct idle rpm.

    I think it is odd that the carbs will leak gas when you close the low speed needles (screwing them IN all the way). Did you replace the clear plastic check valves with your rebuild kit?

    Have you read through the carb tuning tips: http://www.greenhulk.net/forums/show...t=15473&page=2 Post 13

    I would think the amount of time you spend idling in a lean condition would not cause extensive damage. However, you will immediately know you are lean. The motor will fall flat on its face and die when you go from idle to WOT. On the other end of the spectrum, the motor will gurgle and stumble and eventually die if you are way to rich.

    Do you know what your jetting looks like? Are these the original carbs, or from another parts ski?

  8. #8
    Other than the suggested changes (removing petcock, etc etc) everything is stock. I installed the complete OEM carb kits, including the check-valves I bought the ski new in '93 so nothing has been done to it that I didn't do. It MAY be fine, I am just being overly cautious after my top-end rebuild. Some of it may be too much care during the break-in. I payed particular attention to the Varying throttle procedure so possibly I was cutting the throttle too often for my up and down RPM, allowing for some fuel build-up in the carbs . Also, the lake I go to nearby has a very long distance from the beach to where I can open it up...probably 150 yrds or so, so there is quite a bit of wake-free before I can open it up. Also, after the first tank, I looked at the wash and there is just some "slight" carbon speckling in the center third of the piston tops. Very shiny clean, otherwise. I'm trying to ease in to a good safe tune since I rebuilt it for my grandkids and when it's ready for them, they only know three speeds...fast, faster and hyperfast!

  9. #9
    By the way, specifically, the jets are original. Right now, I have the lows at 1/4 out, and the highs at 7/8, 3/8 and 5/8...Mag to PTO, per the manual. The only mod I have done other than the suggested changes (triple pump, etc., ) is I cut the rubber air cleaner periscope back about half way to allow a little more air intake. A very minor mod! I have the pop=off in and t-Stat out. Seems to be running fine re: heat.

  10. #10
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by ltmark View Post
    ... I have the pop=off in and t-Stat out. Seems to be running fine re: heat.
    My recollection of the water cooling flow on the early Fuji engines is rusty, but if it works like the domestics then having either the thermostat or the pressure bypass valve removed is almost the same as having them both removed.

    Increased engine cooling at low RPM compared to higher RPM, if my memory is correct.

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