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  1. #1

    Having problems with my 780

    I have had this 95 Polaris 780 for about a year. I have rebuilt the carbs, fixed the fuel lines, replaced the fuel pump (3 output), CDI box, and spark plugs. The jet pump is in good condition, compression is good, no water in gas, starts every time, and idles good, accelerates good, but after you give it full throttle and you get up on plane for about 30 to 40 sec it stalls out and does a nose dive, if I start feathering the throttle when it stalls it will usually pick back up and will not die, then I can do go through this cycle all over again (I was able to get it to run around the lake about 5 times but that is the longest I have able to go without it stalling). Even when you give it about ˝ throttle it still stalls but takes a little longer but not a as harsh as when at full throttle. Kind of like I am running the carbs out of gas and then feathering the throttle until the gas catches back up (but same results with 3 different fuel pumps).

    I am running mixed gas not sure what the mix ratio is, but to be safe I have been running about 32:1 to 40:1. I am thinking this is my problem. It smokes at idle and the spark plugs are usually gas/oil fouled. Also I do have a little hotter spark plug than comes stock.

    Any suggestions


  2. #2
    Connecticut CrazyA's Avatar
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    Quote Originally Posted by polaris780 View Post

    I am running mixed gas not sure what the mix ratio is, but to be safe I have been running about 32:1 to 40:1. I am thinking this is my problem. It smokes at idle and the spark plugs are usually gas/oil fouled. Also I do have a little hotter spark plug than comes stock.

    Any suggestions
    The fuel/oil mix isn't the problem.

    I would suggest removing the hotter spark plugs immediately and going through the carbs again even though you already have.

    Have you ever noticed any water mist in the engine compartment or taken a very careful ride with the seat off to see if any water leaks are apparent under wide open throttle?

    Also, get a piston wash light and look down your spark plug holes to inspect the condition of your piston tops.

    I can't think of any good outcome when a two stroke dies under wide open throttle. With the exception of electrical, it almost always isn't good. I said almost....

  3. #3
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Welcome to the Hulk 780.

    If you truly have a 95 780, it's a SLX 780.

    Those came with a high ouput triple outlet fuel pump alkready. I hope you didn't throw it out. It's better than most pumps.

    When you rebuilt the carbs, how did the internal filters look?

    Which rebuild kit did you use? O.E. Mikuni I hope.

    Did you check the water/seperator filter for obstruction?

    Did you replace or clean/inspect the fuel petcock?

    What do you mean "fixed" the fuel lines?

    What was the number on the CDI you used to replace yours with?

    What was the number on the spark plugs you put in it?

    Have you ever checked the temperature of the engine after it stalls? Can you place your hand on it for a few secs?

    You mention you're running premixed gas/oil. Did you remove the oil pump?

    You should be running 40:1 ratio, unless you're breaking in the engine. Then 32:1

    Check to make sure your battery cables are nice and tight with clean connections.

    Like CrazyA said, you need to get a good look at the piston tops. That will tell us what's going on inside the engine.

    Get a tiny flashlight that fits through the sparkplug hole ($8 at Home Depot) to see them. All black tops are not good. (lean) Clean tops would be rich. You want a black top with some clean spots on the outer edge.

    Search for piston wash pics, yu'll see examples of a good wash pattern.

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