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  1. #1

    Smile oil pump check valves

    Is there really a need for the 2 small check valves on a yam.800 oil lines.NO other skis that I have worked on in last 25 yrs have them.The oil pumps are mostly if not,all made by same company.I mean are they there because yamaha really thinks the oil will drain uphill to the tank when engine is shut off.Or are they more there to cause a slight restriction in oil line to balance oil flow in both lines?Please could only a yamaha tech or dealer answer this for me.I would really like to know the answer behind using the oil line check valves.I think by removing them it would eliminate the one rare occasion that we have oil pump failure that is not related to oil line coming off at manifold,etc.I have been told by other shops mechanics to always leave them on ,but could give me no valid reason for doing so.I know most people who modify or work on their own skis think that its a necessity to remove the pumps and go pre-mix.I have lots of customers who do not wish to go that route.That said I am currently rebuilding a 800 that broke off the ears of the cylinders where they go in crankcase, knocking a hole in cases,I am rebuilding it using diff. cases,rebuilt crankshaft,new cylinders etc.BUt am curious about what caused it?oil pump was disconnected but left tie wrapped in ski,and premix was being used when current owner bought it,I believe it seized before it was bought and thats reason oil pump was disconnected,the kid said nothing of that so within 30 minutes of riding,it grenaded,Hence I am considering replacing oil pump also and using no check valves in oil linesTHanks>Marvin


  2. #2
    steve45's Avatar
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    Marvin, I wasn't aware that any of the PWCs used these, but they are in all the 2-stroke boats regardless of engine.

    These valves are referred to as 'check' valves, but they are not check valves. They are 'relief' or 'pop-off' valves. A check valve prevents backflow, while a relief valve allows flow when a predetermined pressure is reached.

    In a Yamaha jet boat, the oil reservoir is mounted above the engines. When the reservoir is full, there is about 2 feet of head (hydrostatic pressure) in the oil system. When a boat is stored for weeks or months, that pressure would cause oil to seep through the oil pump and flood the carbs. This causes heavy smoke and fouled plugs after starting. The relief valves are designed to prevent this seepage. Oil should only flow when the pump increases the pressure above the popoff pressure, opening the valve.

    Also, when the relief valves are in place, a very flexible hose should not be used (i.e. silicone hose or 'surgical rubber' hose). If you were to do this, the hose would flex rather than opening the valve.

  3. #3

    Smile

    on the skis the btm of oil tank is level with carbs or slightly above,but its only about max say 6 to 8" of hydrostatic head,which is less than a 1/4 psi (I am ex driller and hydrostatic of fresh h2o is .465psi per foot.I understand yamahas reasoning for these ,seems if a oil pump gets worn it would not get enough oil to engine,as in case of XL800 twins that use a 4 outlet oil pump with two outlets jumpered and returned to tank.None of the other skis (two strokes that use mikuni oil pumps have these valves except yamaha,not kawasaki ,or seadoo,or polaris,tigershark ,etc.Yet every yam. twin two stroke has them,now I do really like the thick wall yamaha oil lines on 1200/1300/800 much more so than the mickey mouse seadoo oil lines that for years broke or weeped at the curves where seadoo painted them at the factory.To this day I still get at least one or two 94 or 95s that had the oil lines painted and they crack slowly on the 90deg. rad.They dont just break and lock up top end they go for months allowing only 1/2 or less of oil to reach engine causing crank and piston failure.but hey thats like yamaha not admiting they had a problem with the cat. converters on the two strokes causing the oil lines to shrink and pull off#3 and #2 cylinders.Thanks for your answer I appreciate it very much>Marvin
    ps. as long as pop off is set on carbs the oil cant go anywhere so the most extra you might get is a few drops in carb on needle side where oil goes in.unless its old style that oil goes to intake manifold

  4. #4
    steve45's Avatar
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    Quote Originally Posted by mrjetskey View Post
    as long as pop off is set on carbs the oil cant go anywhere so the most extra you might get is a few drops in carb on needle side where oil goes in.unless its old style that oil goes to intake manifold
    Huh? Maybe the setup on the Waverunners is different, but on the boats, the oil lines don't even go into the carbs. There is a tray on top of the carbs that holds the flame arrestors, and the oil lines dribble oil into the air intake there.

    Also, the oil lines are the weakest link in the Yamaha 2-strokes. On the Yamaha jet boat board, we've had hundreds of engine failures caused by oil lines shrinking and falling off, most were not equipped with catalytic converters. The oil lines are made of crappy material and secured with nylon tie-wraps. Not good!

  5. #5

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    Quote Originally Posted by steve45 View Post
    Also, the oil lines are the weakest link in the Yamaha 2-strokes. On the Yamaha jet boat board, we've had hundreds of engine failures caused by oil lines shrinking and falling off, most were not equipped with catalytic converters. The oil lines are made of crappy material and secured with nylon tie-wraps. Not good!
    Hey folks,sorry newb here, not to hijack the thread, but, so is it safe to say its a good idea to replace all the hoses in an old yamaha fx1 with new hoses and clamps instead of the oem stuff that in there?....some of it does look pretty flimsy.

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