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  1. #1

    Recipie for a Runaway Lean condition

    Was hoping to gather together a consolidated thread for myself and others currently experiencing a lean runaway condition. For those of you who have had air leaks before and found them before punching a hole in the bottom of your blocks, where was your leak and on what type of engine? Also, how did you diagnose and fix it? My personal search begins on my 1200 SUV tomorrow afternoon.

  2. #2
    Moderator OsideBill's Avatar
    Join Date
    Mar 2006
    United States
    Pressure test is the first test we do to determine if there is a leak in gasket or sealing surfaces on any 2-stroke ski.
    If this checks out then it is fuel delivery related and we look into that.

    Most common place for a leak on used motor is crank seals.

    leaks can also be found in pulse lines, intake gaskets, reed gaskets, case mating surfaces, etc.

    One thing to always make sure of when starting a ski that has been sitting a long time, or has had freshly rebuilt carburetors installed. Is the system full of fuel? I always take the plug wires off and crank the ski until I get fuel coming out of the return to tank line. Then I put the wires back on and fire the ski. This way it will not runaway due to lean while the carbs are trying to fill with fuel for the first time.

    If a ski runs away on you, pull the choke closed. or if you dont have a choke, pull the throttle wide open to get it to shut down. pulling the lanyard does nothing.

  3. #3
    Thanks Bill! I'm actually looking forward to building an engine winch today to yank it out and Chase the leak down. Glad you mentioned the pulse lines. I think mine may be cracked looking.

  4. #4
    Pulled the Carbs to check them for air leaks...nothing.
    Decided to replace them with a freshly rebuilt set off of my other SUV. while installing the pulse lines I dropped a worm clamp into the hull. Cursed a blue streak and then decided to replace the pulse lines all together. As I pulled the one off that connects to the front carb I noticed something inside obstructing it. You are not going to believe what I found...and I am not BS'ing here as impossible as this sounds. I blew into the pulse hose and no air would pass. I finished installing everything, primed the system and dry-fired the ski to see if it would run away. It did not seem to but i have to wait till tomorrow morning to put it on the lake. Then, I will put it back on the lift to see if she runs away.

    When I say that it "runs away" here are the actual symptoms:

    1. Low end bog
    2. No holeshot. She slowly climbs out of the hole but then eventually works her way up to 52mph on the GPS.
    3. On a trailer or the lift, I fire it up to blow the water out of the exhaust and it starts up fine but then in 5 seconds or so it climbs up to the 7000 rpm line. Some times it takes a short blast on the throttle to make her stick up at 7k. Most of the time it will kill no problem by pulling out the lanyard but every now and then you have to jam non the throttle to kill it.

    My question is this: if a pulse line was completely blocked off...would it give the runaway symptom? Also, does it make sense that it would get to normal high speed (eventually) with a pulse lines stopped up?

    And now for the photo of the debris inside the line. Talk about scratching my head when i saw this. It is a cap off of the end of a low speed screw on the Carbs. I have no friggin idea how I screwed this up and got it in there...especially since the pulse lines never came off the engine. Anyway, enjoy.
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  5. #5

    Join Date
    Mar 2008
    Dallas, TX
    That debris is part number 19 on the carb parts list. That is the rubber pin/bush that holds the fuel check valve (number 20) in place. You also have a check valve floating around in the fuel pump. This will cause that pump to put out zero psi causing that cylinder to run lean and runaway. You need to pull the carbs and rebuild them. Good Luck

  6. #6
    Just had the completely blocked off situation. Some run-away on the hose, but no run-away while running in the water. Just no power at all. Fixed the blocked off problem, all went normal. Until the other cylinder seized, 45 minutes later. Probably not related. Carbs f'd, probably just with ethanol tainted gas.

  7. #7
    Thanks for the confirmation on the runaway due to debris. But it really is the cap (part 23 on the Mikuni pdf). The picture makes it hard to tell and I do know exactly what bashing you are referring to. The round check valves are in place. I put a fresh set of Carbs on that I rebuilt for my other SUV just to rule out the Carbs as a problem. I did break down the ones that i had on when it was running poorly this moClick image for larger version. 

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ID:	275626rning. Everything looks great except for the popoff on those. I have some new 1.5 n/s's on the way to do OsideBill's recipes for success on the SUV. Here is a better pic of the debris.

  8. #8
    Oh no! Thanks for confirming the condition old-man. Yes, it ran away on the hose but not under load of water. Same same it seems. Can't wait for Sun-up tomorrow to see if that little damn worm clamp dropping in the hull saved the day. If it wasn't for that...I was just going to re-attach the last of the pulse lines and try to run it. I am pretty sure someone out there was looking out for me when they nudged that ring clamp off my line ha! Of course, they were probably making up for when they nudged the low speed cover into the end of that pulse ha! Sure hope that ring clamp made it to the bottom of the hull and not into the intake manifold .

  9. #9
    Shoots old-man, now you have me wondering. Here is a question for the higher skillset.

    Obviously I had a blocked pulse line. So changing the line should rid me of the runaway if that was the cause. The other significant symptom that may or may not be present tomorrow is that it bogged at low rpms (also should be resolved with the newly rebuilt/popoff tested Carbs.

    The last condition is the crawling out of the hole even though it registers high rpms. Like I said, it does get up to the programmed 52mph eventually after about a minute at wot but it just struggles to get there while sounding strong. The wear ring to prop clearance looks good and prop is in good shape (solas on the way anyway).

    Could it be that the front cylinder has been slowly melting down while running lean and if so...would the slow-out-of-the-hole be a valid symptom of a cylinder being forced to move with no or limited fuel/oil? I guess if she takes off like a rocket tomorrow morning I will cu the ride short and have to pull the head to check the health. Thoughts on the plausibility of a plugged pulse affecting hole-shot but not the top end speed?

    At the moment, we suspect cavitation caused by an un-filled pump plate (I think that is the term). Anyway...that mod will be done along with the rest.

  10. #10
    Moderator OsideBill's Avatar
    Join Date
    Mar 2006
    United States
    If that cap was in your pulse line, than that fuel pump on the carb was not able to work. The solution I showed you on the drilled return mod, would have allowed the other two working fuel pumps to feed this carb. Thats one of the benefits of doing that mod.

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