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  1. #1

    polaris 780 fuel pump

    This is my first post but I have been reading numerous posts for 2 years to learn as much as I can about the 2 slt 780 we purchased about that time. These forums have already been so helpful to us. Thank you. Both have been a real pleasure but we have had some problems. I'll designate the skis as #1 and #2. Ski #1 when first purchased started easily, idled great but would bog and then die at 1/4 to 1/3 throttle unless we pulled the choke slightly for a split second then it would take off and go 48 to 50 mph with no problems until you idled back down again. Then pull the choke slightly and away you go. Toward the end of the first summer my daughter was running WOT for an extended period and burnt a hole through the center piston and pieces off of the PTO piston. compression M 115 C 0 P 100. I did a complete top end rebuild to one oversize but never did a compression check. Last summer the rule was not over 40 to 42 MPH and I would try to view the pistons through the spark plug hole to see if problems would repeat. This year I bought a compression guage and found readings of M 115 C 115 P 75. Ski #1 has 150 hours on it. Ski #2 compression readings are M 115 C 115 P115 but late last summer it started to require a periodic slight pull of the choke to get past a fast idle but would then go just fine. This ski has about 160 hours on it. The difference because ski #1 was down for 10 hours for the top end rebuild. Question: Would the fuel pumps be tiring with this many hours? Both skis with first startup of the day have to crank with full choke and throttle for 8 to 10 seconds but from then on pop right off with a hit of the start button. Can I rebuild the fuel pumps or is there a recommended replacement? The skis haven't been out this year yet because of time and I would like to make any easy adustments before I try them out and do more damage. Sorry about the long post. Thank you for any advice.

  2. #2
    Moderator beerdart's Avatar
    Join Date
    Aug 2006
    I would clean/rebuild the carbs. Inspect/replace all fuel lines. Inspect/clean the fuel pump.

  3. #3
    If I would include rebuilding the fuel pump do you know if the rebuild kit for a Mikuni 5 sided dual high volume pump would be the same as that for a stock 780 5 sided fuel pump. The pictures look much alike but the outlet nipples are in different locations. What do you think?

  4. #4
    BBCaprice's Avatar
    Join Date
    Apr 2007
    Titusville FL
    If it burned a hole in the piston its to lean, period. Oil injected or pre-mix? Check your mixture screw settings and maybe go 1/8 turn richer. Once adjusted correctly the ski should holeshot w/no hesitation or blubber.

  5. #5
    Moderator beerdart's Avatar
    Join Date
    Aug 2006
    Yes the five side pumps are the same some have two outlets and yours has three same internals.

  6. #6
    Thanks. I'll work with the fuel pump and carbs. The fuel hose is a very stiff USCG type BI 1527 SAE Marine fuel line. Is this type of hose notorious for deterioration. When I remove it from the nipples it appears very well intact yet and I see no signs of deterioration.

  7. #7
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
    Join Date
    Oct 2006
    Cleveland OH
    If the hose is stiff, it may be cracking and leaking air into the fuel. Replace it, for $20 why chance it?

    You can buy a rebuild kit for the fuel pump for around $15.

    Do not start turning your adj screws out to compensate for the lean condition. Find the problem and solve it.

    Anytime you need to use the choke to make the ski perform, you've got problems and will hole a piston unless you remedy it.

  8. #8
    thanks everyone

  9. #9

    more questions

    so I've had the stock 780 fuel pump apart and everything looks clean. I plan on rebuilding it with a Mikuni rebuild kit but how do I know how much of the problem the fuel pump may be contributing to the lean problem. Should the diaphrams be flat and soft? The ones in my pump seem to be stiff and dished in on both halves of the pump. The pump seems to hold suction and pressure. I am ready to go into the carbs. What do I need to remove to get the carb rack out? Am I just removing the 6 bolts holding the carb rack (do these bolts go straight up from under the carburators, how difficult is reinstalling these?) or all of the bolts and removing the cage from the crankcase also? What new gaskets will I need if I just clean and don't rebuild for now or is that a stupid questions and should I absolutely rebuild? The machine does really run nice once I get past fast idle speed so I'm hoping the fuel pump and lines are the problem because I have little time to work on it and the season is getting away.

  10. #10
    Not sure if you're still working on this but you need to remove the carb intake cover, filter/grill/flame arrester/whatever, choke and throttle cables, then underneath the carbs there are 2 nuts for each carb which attach them to the intake piece. You'll probably end up using stubby wrenches to get them off. Then all 3 carbs come out as a unit.

    Once you've done it before, it'll be a 20 minute job. Might be an hour or more the first time.

    If the problem is around 1/3 throttle, I'd check the pop off valve carefully. There's a tool to measure the pressure required. Your carbs use the pop-off valve setting as part of the transition from low speed to high speed flows.

    As others mentioned, there's no doubt you have a lean condition. There's also an internal filter in the carb that's worth checking. Maybe just needs a good cleaning.

    I feel your pain -- After 18 hours on a new engine, I just put a hole in my PTO piston. The ski had been used literally 4-5 hours each of the last 2 weekends without seemingly any issues at all.

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