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  1. #1

    GP1200 engine failure questions

    Hi all

    I am new to this site and skis in general, but i am also a mechanic so experienced in that department lol

    Now to my problem , I have a GP1200 (65U) ski that has run flawlessly until a week ago when it lost all power. story short the rear cylinder has blown a whole in it on the exhaust side. I have disassembled the motor and i am trying to figure out why it has failed.
    I know of the common oil injection lines blocking but i can blow through all three of my lines, do these injection pumps fail at all?
    I have also disassembled the carbs and there is no cracks etc in the diaphragms and all the jets and ports appear to be clean and clear. I did notice on the failed cylinder carby that there was a slight amount of corrosion build up on one of the diaphragms?
    I have attatched a few photos to show the different parts
    Does anyone have any ideas as to why this piston may have failed? I am guessing its a fuel lean out problem the way it has blown the hole on the exhaust side.
    Unfortunately I have no idea as to the hours this motor has done as when i purchased the ski it had another motor installed but the owner in his ignorance wasnt sure how many hours it had done !!

    Also a couple of other quick questions, Do the pistons look like standard size pistons?(65U printed on tom with F and arrow etc) Obiously i will measure them to find out but at the moment i am just trying to get an idea as to cost etc of the rebuild. And if i get the rear cylinder re-bored with a new over sized piston - do you think i would be safe just honing and re using the other two pistons? (i will measure piston to bore clearence to make sure its still in spec still).

    the piston side picture below are of the good pistons - there seem to be slight scoring on the pistons, is this fairly expectant as to what you would see in used pistons?

    Thanks greatly to anyone that can provide any help


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  2. #2
    Yamaha artisan Cutlass's Avatar
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    The most common rear PTO piston failure comes from a leaky rear crank seal. That'll let extra air into that cylinder and lean it out to the point of failure. You could have pressure tested the engine before dissassembly to comfirm this but its probably too late now.
    I'd recommend cleaning up those carbs and rebuilding them with genuine Mikuni carb rebuild kits (all the aftermarket kits are junk, don't waste your money). Also, its best to bore/hone all three cylinder to the exact same oversize. Mismatching piston size is not desirable, but many people do it without much issue.
    To answer your question about about oversized pistons, the oversized oem and Pro-x pistons have a 25, 50, 100, etc stamped into the little circle on the top dome of the piston. if yours are blank then that means they are standard size...which is 84.00 mm.
    What are you looking for out of this ski? Are you keeping it? there are a few reliability mods that I consider are must do's if you wanna a worry free ski to last a while. Search around and you'll find them or ask away and we'll help point you in the right direction

  3. #3
    Huge thanks for the response Cutlass. You have answered my questions perfectly Unfortunately you are correct i didnt pressure test it before disassembly but from what you have described from the leaking crank seal it sounds like that may be the root cause.

    I do want to keep the ski for a while but in saying that i am also trying to keep the costs down as well. I might measure the piston to bore clearence on the other two cylinders and see if they are in spec. Do the minor vertical scores on the piston photo there look normal to you for a used piston?

    Is there any chance you could quickly list those reliability mods if you have time ?

    Again a huge thanks for your time and knowledge mate

  4. #4
    one more quick question mate , Is the rear coupling right or left hand threaded? and do you have any tricks of the trade as to removing it and the flywheel?

    Thanks

    Scarp this question started using the search function lol
    Last edited by hornet; 09-21-2012 at 04:00 AM.

  5. #5
    Yamaha artisan Cutlass's Avatar
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    Here's some reliability mods you can do the carbs:
    1) You can do the drilled returns mod described in this thread --->http://www.greenhulk.net/forums/showthread.php?t=177671
    2) Double up the long skinny clear plastic anti siphon check valve. Put the old one on top of the new one from the rebuild kit.


    3) These ski can be a pig to start when they are cold and have been sitting a while. A good solution is to install a primer kit. The most common way to install the kit has you removing the choke plates from the carbs so you'll get an additional benefit of increase air flow through the carbs...ie more horsepower.

    4) If you want to install aftermarket flame arrestors, we have a carb jetting/pop-off/hi and low speed adjuster recipe that works great and it reliable.

    Lubrication mods:
    1) the ultimate in oil lubrication is premix (if you can remember to mix it correctly ). You'll need to remove the oil pump/hose and buy a block off kit
    2) If oil injection is sounds easier and more convenient to you, there are some things you should do to make it as reliable as possible. First thing is to buy some new, good quality oil injection hose. Old hoses get hard, brittle, and shrink a little bit. The shrinking can cause them to pull tight as they weave around from the pump to the flame arrestor. If they get too shrink too short, they fall off and leave you without oil to your engine. Next you'll need some metal crimp style clamps to replace the zip ties that don't always work to great. The metal clamps hold tighter and virtually eliminate the possibility of the hose falling off. The next thing to do is frequent inspections of the hoses. Make sure they are still soft and flexible. Also make sure they aren't starting to slide off. Here's a thread to read --> http://www.greenhulk.net/forums/show...853&highlight=

  6. #6
    Yamaha artisan Cutlass's Avatar
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    Other things you can do to make the engine as reliable as possible is to use quality parts. Only use OEM crank seals. OEM gaskets tend to be better but some aftermarket gaskets are ok. Use Threebond 1211 to seal the crankcase halves. Its good stuff.
    Pressure test the engine after you assemble it to make sure it holds pressure. Read these threads for more info:

    Pressure Testing Two Stroke Engines
    GP1200 65U pressure test
    pressure Testing a 65U SUV
    Pressure Testing

    This info should give you a good start. I'm sure I'm forgetting something. If I think of anything else I'll post back here.

  7. #7
    WOOOOW Cutlass i can not thank you enough mate that information is excellent
    One last question i know you are in the US , I am in Australia and there seems to be a small range of supplier over here, also most seem to be opting for buying from the US. Do you have any suggestions as to the best place to purchase parts from over there? I will get OEM crank seals over here but the rest of the parts i might order from the states

    Cheers mate

  8. #8
    Yamaha artisan Cutlass's Avatar
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    Hey, its no problem. A lot of people around here helped me out when I was rebuilding my engine so I'm just trying to give back.
    Depending on what you need to buy there are several places to buy stuff. I usually buy all my OEM gaskets and seals from Boats.net, my primer and Mikuni carb kits from Parker Yamaha, and my Pro-x Pistons from Pro Marine.
    While I'm here, I did think of one more important piece of info. Measure, bore, and hone your cylinders with the exhaust manifold torqued onto the cylinder block. The bore slightly distorts with the manifold on or off. If you measure and machine with the manifold off, your cylinders will be egg-shaped when you torque it back on. The goal is to have perfectly round, straight cylinders with the exhaust manifold bolted on.

  9. #9

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    The round check valves in the square metering body are the ones that fail and need doubling up.The above valve only feeds low speed circuit.Look for creases in the plastic.When you keep the throttle on jumping chop,the motor revs up,when the pump loads up at landing the excess pressure creases the valve that will cause a lean-out.The rear seal issue is correct due to the large crankcase pressure in Yamahas.I've seen the seal totally blown out! Use threebond around the rear seal circumference and it will be good.The third main issue is the square exhaust port shape.A loose piston ring when revved high can snag the port and break it off.I was lucky,mine went through the port smoothly and amazingly still gave 100psi and ran good.Give the port roof a rounded shape and a nice chamfer.A GP is very strong and reliable after fixing these 3 issues.Low speed tuning on an older ski can be challenging as the throttle shaft on number 1 carb will be worn and suck abit of air because all the cable torque is applied to the one carb pulling it sideways.It might be possible to adjust the synchronicity and get the front carb butterfly to open slightly later than the other 2.

  10. #10
    Yamaha artisan Cutlass's Avatar
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    Quote Originally Posted by ozwiz View Post
    The round check valves in the square metering body are the ones that fail and need doubling up.The above valve only feeds low speed circuit.Look for creases in the plastic.
    As far as I know, nobody doubles up the round fuel pump check valves. But doubling up the oval shaped check valve is a mod that comes directly from the Mikuni carb tuning manual to fix a hesitation and engine stumble after a high speed deceleration.
    Some members here have found the cure for creased fuel pump check valves is the "drilled returns/fuel pressure balance mod". Another guy found his creased check valves came from really high pulse spikes causing the pump diaphragm to pump harder than it should. High pulse pressure spikes could be caused by high crankcase pressure/compression due to lots of piston ring blowby.
    See this post: http://www.greenhulk.net/forums/show...=1#post1478973

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