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  1. #1

    HELP! My SLX is Melting Down!

    96 SLX, bought a couple months ago. had good compression on all three when I got it, but ran bad. Rebuilt carbs and installed new reed block, head, and carb gaskets. Ran great and had great wash.

    Havent run it in a few weeks.

    Today took it to the ramp to move it from a single trailer to a tandem with the SLX780 I just rebuilt. Took her for a quick blast, ran good, but RPM was way high, hitting 6600, so I backed off WOT and ran 3/4 most of the time. 5 minute run tops, up the creek and back. On the way back, took a hard high speed right turn and she shut down. Was very hard to get going again, but she did, and ran OK back to the ramp. Put her on the trailer.

    Ran the SLT a bit more, man I love that boat!

    Put the SLT on the trailer and decided to run the SLX again.

    Ran fine this time. About 5-10 minutes, no long high speed runs but I beat the crap out of her, power jumping, donuts, generally acting like a hooligan. No issues this time. Back on the trailer.

    Got her home, flushed, and checked compression for giggles.

    M-C-P are now 115-135-135. Mag is down 20 pounds.

    Pulled the heads. Mag piston is not holed, but it started to melt down, the "78" on the face is gone and there is a quarter sized rough spot in the center.

    Completely bummed here.

    I richened up that carb 1/2 a turn, and the other two 1/4 turn just to be safe. There is now no wash on the Mag and minimal wash on the C and P. I had great wash last run.

    I know when I pulled the heads last time the coating on the gaskets looked fine so I didn't spray them with copper. They had about an hour or two on them. This time the black coating is definitely hurting so I will spray them up before installing.

    Heading to the lake tomorrow for a week with the SLT and the SLX.

    This boat was running great with good wash, and freshly rebuilt carbs all set to spec, lever height checked, genuine Mikuni diaphragms, etc.

    Could the head gaskets being reinstalled twice (I checked wash twice after my carb rebuild and gasket replacement) create an air leak causing me to run lean?

    I need this boat to make the week. I can replace the mag piston and rings if I need to over the winter.

    Advice here? I'm literally hooked up in the street and leaving with two trucks, camper and tandem ski trailer tomorrow. I'm bringing tools

    I don't want to kill the SLX.

    I know I need a flexible light and maybe an android borescope. I am never pulling heads without a necessary reason again. My SLT is running like a champ and I do not want anything to happen to her.


  2. #2
    Click avatar for tech links/info K447's Avatar
    Join Date
    Jul 2007
    near Toronto, Canada
    Hmmm. Going from good wash across all three to no wash and also piston burn/melting, something has changed

    Did anything change between last outing and now? Different fuel supplier, different octane gasoline? Different altitude or weather?

    Fuel selector valve not turned all the way open?

    Seat off vs seat on?

    When the engine stopped by itself you may have hot seized a piston, which can score the cylinder walls and/or transfer aluminum onto the cylinder walls. This will reduce compression.

    Make sure the cooling system is working properly. While the exhibits temp sensor is supposed to worn you, a restricted water flow can raise engine temps enough to make a difference. Check the plastic mesh intake screen for clogs.

    Check inside the thermostat housing, especially the pressure bypass valve. If that plastic tip or long spring gets jammed closed the engine will run hotter at speed.

    These high output 2-stroke carburetor watercraft engines really need the fuel system to be 100% right. You know that, of course.

    As you discovered, if it is running 'too strong' and the wide open RPM is higher than it should be, it is probably running lean. That is not the time to keep riding it at speed. Even 3/4 throttle is too much when something is not right. Just walk it back to the launch or dock, maybe barely on plane.

  3. #3
    Absolutely nothing has changed. Seat on, same gas, etc. Only variable, I pulled heads to check wash. I am stuck on that having to have something to do with it.

    I have the heads off, walls are thankfully perfectly smooth. No visible damage or anything I can feel with a finger. Piston does not wobble, etc.

    I have left over anaerobic sealant from my rebuild, would that be better than copper for the head gaskets since it's open now anyway? Since the coating on the gaskets is coming apart, should I clean it all off? Just get new gaskets?

    Thanks for the help as always.

  4. #4
    Click avatar for tech links/info K447's Avatar
    Join Date
    Jul 2007
    near Toronto, Canada
    Head gaskets put on upside down or incorrect tab orientation?

    Just speculating here. Coffee not fully in effect yet...

  5. #5
    They're definitely right side up (78 ) but maybe they're spun in the wrong spot... I'll have to double check. Heads are temporarily back on and we're getting on the road.
    Last edited by K447; 08-27-2016 at 08:42 PM.

  6. #6 much for the 780? stimpsonjcat's Avatar
    Join Date
    Apr 2012
    Georgia - Lake Lanier - near 6TH
    Yeah 6600 RPM should have smacked you between the eyes as a bad thing.

    I keep mine at 6250 max.

    I never reuse head gaskets. You need to get a light or cheap borescope for checking wash and stop taking the heads off to check it.

  7. #7
    RLACEMAN's Avatar
    Join Date
    Aug 2011
    Random Lake WI
    Was it a cooler day when this occurred? or cooler water temps? 6500 is normal for an SLX. They usually runs the best right before they seize, mine did the same thing, put a dimple in 2 of the pistons. I also wouldn't reuse head gaskets, but I don't think that was your problem, if there is a leak because of that, there would most likely be a small water leak you could see or small amounts of water could be sucked into the cylinder, which then would cause the motor to run like crap & you wouldn't have been getting peak rpms as you were.

  8. #8 much for the 780? stimpsonjcat's Avatar
    Join Date
    Apr 2012
    Georgia - Lake Lanier - near 6TH
    Oh..and not to be a biatch...but I took my 780SLX hull to the junkyard this morning.

    Another SLT780 is on the trailer and in the garage right now.

    Shirley there is an unloved SLT hull you could bolt that engine into somewhere nearby?

  9. #9
    Update... checked the orientation of the gaskets, they're good. Cleaned them up and reinstalled with copper spray.

    Splashed her at the campground marina Monday with the slt. First test run was great, 6300 rpm wide open and no signs of losing power or shutting down.

    Ran the next day on a decent run, maybe 5 miles. Daughter was on the slx and me on the slt. About 3/4 of the way home, it shut down on her. Started back up ok. I test ran it myself when we got back to the dock and we're back at 6600+ top end. Obviously a problem somewhere.

    Good news, I asked her to keep it at or below 5000 all week, and so far so good. We've been running her 5 days, about 12 hours so far, and she has not come apart.

    Worse news (kinda lol), I love the SLT so much! My plan was to scrap the slx over the winter and find a slt to put the 780 in after I rebuild it. But the kid loves the slx, she is hooked bad she told me it's her boat now.

    I will be pulling and tearing down the 780 over the winter regardless of what hull it will be ultimately destined for, I plan to pressure test before teardown so I can see if I had a leak, and where.

    Just gotta make it thru labor day weekend hereally! Wish us luck!!

  10. #10
    So we made it through the week, there were no other issues all week with the SLX.

    Got her home and compression is actually UP 5 pounds in the MAG hole, at 120 now. Who knows. I also got an android borescope and the piston still looks the same as it did when I had the heads off earlier, but there is a bit more wash on all three cylinders now.

    I also forgot that about half way thru the trip I was poking around making sure everything was snugged as best I could (intake, etc) and the nuts on the Mag carb were snug, but I was able to get about another 1/4 turn on them without a terrible amount of effort, so they might have been just a bit loose, perhaps that was the issue. Snugged everything else up that I could reach as well.

    I posted the SLX for sale locally yesterday, so I'm not going to worry about it too much at this point. The wash is looking better so I'm comfortable selling it now in good conscience. If it was still giving me issues I would probably either part it out, or rebuild the 780 to keep as a spare motor.

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