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  1. #1

    rear cylinder on 1996gtx

    I had a new customer bring me a 1996 seadoo bored both cylinders .5mm over using proper clearance SBT pistons .The pto piston was originally bad when he brought it in hence top end rebuild,I always check and or rebuild carbs,drain fuel add premix,check oil lines and pump,carbs were not good,rebuilt same,lake tested it ran fine for 30 min,brought it in called owner and he picked it up,this was last year he didnt use it,supposedly until his grandson came in in march,it ran an hr and fried pto piston all way around.seized therings evenly,no hole ,it was oily and had premix,so I thought ok old fuel no problem I'll bore it chamfer ports and check rave clearance added fresh fuel,and adjusted carbs for no hesitation .ran it 30 min he picked it up and rode it he said across lake from his home and it died.rear pto cylinder gone again plenty of oil seized uniformly all around .any ideas I'm thinking rear seal leak but the 787 usually hold up well its only 100+hrs has the lead and dummy carb ,if it was lean wouldnt it hole piston???its either overheating or something any pros run into this problem,many thanks ps its first 787 I have run into this problem with,and I have rebuilt maybe 35 of them

  2. #2
    you gotta do a pressure test before pulling it apart again..

  3. #3
    SURFnTURF's Avatar
    Join Date
    May 2007
    I had 2 similar situations but with a 947 engine it would be OK for 20-30 minutes then lock up.I went through 5 pistons before I found the problem,I changed clearances right out to 9 thou and still it happened.

    1) it was the cylinder liner .It had water between the aluminum and the steel liner and rust had formed dramatically.When it heated it expanded at a different rate and deformed the liner and bang a lock up.
    it was random any rpm and any time.Solution was to change cylinders as I didn't trust the other side now. Still going strong today

    2) Another time another 947 with the same thing it would lock up only at WOT or near there.

    Went through3 pistons/rings, turned out I had swapped the O rings on the low and high speed adjuster screws on the carbs and it was leaking air at WOT. Different sizes!!!
    On the trailer it was fine it wouldn't free rev at all it behaved as normal but when in the water under stress the O ring would deform from suction and let air in and then bang lock up.
    So maybe this is not your problem but it was worth mentioning.

    I would do a leak down test on it ,that is the only way to see if you have a seal leaking.
    Can get costly re boring and gaskets and piston/rings real quick.

  4. #4
    canuck's Avatar
    Join Date
    May 2006
    The wet left coast.
    I'm curious about this as well.

    This is my thread from a few years ago

    Last ride this year and it did the same thing seized the rear piston with a nice second hole in the piston and ring pieces stuck in the head. I have Prok flame arresters, 75 pilot jet, 2.0 N/S, 80 gram spring, 1 1/2 on lows, 1/4 on highs and I think around 20 psi pop off.

  5. #5
    ok here goes found out my gtxs problem from number one post.Testing pressure wouldnt have helped.After I first rebuilt it with front piston bad .he stored it over winter well guess what the t cooling line coming from between cylinders under exhaust was plugged back at exhaust pipe where it exits 3" inside exhaust outlet .I had just checked all lines for clear flow but did not ther second time .A dirt dobber (red wasp) plugged the outlet hose for 3" and 120psi of air wouldnt clear line .I had to make a spade bit from stainless wire and grind it out,so hardly any cooling water was getting out of rear cylinder.Guess what even if I checked a cooling line a few days ago I check again before installing and he puts scotch tape over outlet!!!!

  6. #6
    Myself's Avatar
    Join Date
    May 2007
    North Central Arkansas
    I always check the lines on any service. The wasps are bad around here to plug them up. I see it all the time.

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