Oil circulation in 720 engine

[greginmadtown]

So I understand that the two small oil inlet and outlet on the bottom of the crankcase go back to the oil reservoir tank. But the oil reservoir tank is actually higher in the crankcase like quite a bit. What stops the center crankcase from filling entirely with oil?

Or does the center crankcase where the rotary timing gear is supposed to fill up to the level of the crankshaft?

I ask this because I have a leaking front bearing seal. And I'm wondering if the reason it's leaking is that what oil is getting into the front crankcase under the piston and leaking out the front seal.

If this is the case then the oil would be impeding the volume and the movement of the piston.

I know I should take out the engine to do a total replace all the bearing but I'm trying to put that off.

 

[AKnarrowback]

The two large oil lines going into the motor case are for the crank shaft and rotary valve shaft cavity in the case where the two mesh together. There isn't a pump feeding this area, it relys on gravity which is why the oil tank is above the level of the engine.

The seals on the crank shaft for containing the oil in the case chamber can fail and let oil seep into the area of the motor bellow the pistons. If these seals fail you will can have a motor that smokes a bunch for the first minute on start up after the motor has sat overnight or longer. If the seals really leak bad you can have enough oil getting into a case to make starting near impossible without first pulling a spark plug and blasting the excess oil out using the starter.

When you say "front" bearing seal are you refering to the seal in back of the stator and stator flywheel? If so that "end" of the motor is more commonly called the "mag" cylinder and the other is "pto" just to clear up any confusion.

If you suspect a leak is bad enough to need attention you can do a pressure test on the motor to see how "air tight" the seals are. This is a bit involved and requires some special tools and patience your first time through.

 

[greginmadison]

The two large oil lines going into the motor case are for the crank shaft and rotary valve shaft cavity in the case where the two mesh together. There isn't a pump feeding this area, it relys on gravity which is why the oil tank is above the level of the engine.

The seals on the crank shaft for containing the oil in the case chamber can fail and let oil seep into the area of the motor bellow the pistons. If these seals fail you will can have a motor that smokes a bunch for the first minute on start up after the motor has sat overnight or longer. If the seals really leak bad you can have enough oil getting into a case to make starting near impossible without first pulling a spark plug and blasting the excess oil out using the starter.

When you say "front" bearing seal are you refering to the seal in back of the stator and stator flywheel? If so that "end" of the motor is more commonly called the "mag" cylinder and the other is "pto" just to clear up any confusion.

If you suspect a leak is bad enough to need attention you can do a pressure test on the motor to see how "air tight" the seals are. This is a bit involved and requires some special tools and patience your first time through.

Thanks for the info.

Seems like then my mag side oil seals both on the inside and outside are leaking, due to oil showing up inside my flywheel/mag cover. Seems like I need to pull the engine and get a new crank (not what I was looking to take on). From what I'v read this could cause the (the pto side) cylinder to run lean and burn up.

Any suggestions? I could clamp off the oil lines to the rotary valve shaft cavity when not in use.

Anybody know of a good engine rebuild I can buy for a 720? I've heard not so good things about SBT rebuilds (the price is nice $800 but now worth it if they fail).

 

[AKnarrowback]

People do put valves on the lines feeding the shafts. How much oil are you seeing leaking past the seal?

 

[etemplet]

the 720 is a pretty darn easy engine to rebuild. Just the crank in there. You can find WSM rebuild kits for about $550 (pistons and related parts,and crankshaft along with the gaskets needed) Add cylinder boring and all the work you have to do it's a toss up unless you really like doing your own work. Then if you have to pay shipping that needs to be added as well. What scares me about SBT is the core value. If they say parts are not acceptable you'll pay even more. I use SES (Seadoo Engine Service) but you'll hear negatives about them here. Best to be patient and work with whatever shop you feel is the best value and easiest to work with. I find the best method is to do the engine myself. I can control costs to an extent and timeliness of the repair. After you disassemble the engine external parts and remove what needs to be removed... there ain't a whole lot left to do in building the engine.. Good Luck.

 

[greginmadtown]

People do put valves on the lines feeding the shafts. How much oil are you seeing leaking past the seal?

When are pulled off the mag cover, wipe it out, the next day it would probably be about a teaspoon at the bottom of the mag cover wherl area.

 

[etemplet]

People do put valves on the lines feeding the shafts. How much oil are you seeing leaking past the seal?

I'm doing a repair on my cousin's ski. He has a valve for his oil line. I don't know how bad the seals are leaking on this one but he says it will fill up the engine. I'm rebuilding the engine. On my wife's ski, I did my first engine and put in a used crank with supposedly low hours. I didn't use any sealant on the seals for the rotary chamber and it leaks into the engine. If the ski sits for 10 days the engine will hydro-lock. I have to pull the plugs, get the oil out and it works fine. I put a valve in there. I'm not building another engine and spending $400 on a crankshaft when it runs so dang good. Her ski is still faster than mine.

 

[etemplet]

When are pulled off the mag cover, wipe it out, the next day it would probably be about a teaspoon at the bottom of the mag cover wherl area.

Question... do you know for certain the rotary valve area is leaking? It could be the lip seal on the mag side that is bad and NOT the rotary valve seals. There is a hole where oil drips into the bearing chamber while the engine is running that is how those bearings get oil. In order for oil to get to the level inside the engine that would cause that problem....
number one, you'd have to have two leaks
number two, the level would be so high the engine would not turn over.
NOTE: it only takes a couple of ounces to lock up the engine so I don't think you are getting enough oil in the chamber.

 

[etemplet]

I have a 687x in pieces right now if you need any pictures. I am pretty sure you could remove that front oil seal and replace it without splitting the cases or even pulling the engine. I just did a 717 a couple weeks ago. I look for things like that when I am assembling parts. I like being sneaky.

 

[greginmadtown]

Question... do you know for certain the rotary valve area is leaking? It could be the lip seal on the mag side that is bad and NOT the rotary valve seals. There is a hole where oil drips into the bearing chamber while the engine is running that is how those bearings get oil. In order for oil to get to the level inside the engine that would cause that problem....
number one, you'd have to have two leaks
number two, the level would be so high the engine would not turn over.
NOTE: it only takes a couple of ounces to lock up the engine so I don't think you are getting enough oil in the chamber.

well I was assuming that since the engine is not running and I'm getting a leak through the Mag side oil seal but the only route for the oil to come from would be from the chamber under the piston. But maybe that's not true. I'm not sure how the oil reaches the bearing in the front crankshaft area. what I did start up the engine last fall when I was a glitch rising it did blow a lot of smoke for the first minute. But it did turn over easy and start.

When I bought the Sea-Doo couple years ago I'd no idea that I'd basically have to pull the whole thing apart replace every piece! Lol

I spent way more hours messing with it then actually riding it!

 

[etemplet]

Welcome to the club. After you get them right though.... just hit the button and ride with a reasonable amount of confidence.

I can tell you that 3 oz of oil in the casing will lock that engine in a heartbeat. I'm not sure 6 ounces would get the oil level close to that chamber. So...... leave the cover off the Mag. Wipe it out and monitor it. If the leak is not from the rotary, the leak will stop when the small amount of oil in the bearings has drained. If it is leaking now, you need to change the seal anyway so.... pull the mag cover and pull the seal. Good Luck !!

 

[mikidymac]

If the front crank seal is leaking oil it will also leak air when running and has a good chance of seizing the front cylinder. Air leaks on a 2-stroke are fatal.

 

[motoring]

If the front crank seal is leaking oil it will also leak air when running and has a good chance of seizing the front cylinder. Air leaks on a 2-stroke are fatal.

amen to that. i just opened up one of my 717 's took bottom off and what i though a simple seal change is not . the story...( its coupler side). the two bearing are messed up the cage is destroyed on both bearings. all 9 balls stayed in races saving the cases etc i got the end bearing off not sure yet how to get the inner out yet.
and the cylinder related is low . 150 psi to 170 the good side , this is sbt motor dont know why compression is high (170 flywheel side just means a bit more hp to me ) i guess sbt puts out higher compression when they build ? BUT IT SEEMS EVERYTHING IS RELATED. WHICH EVER WENT FIRST IS THE QUESTION IM GUESSING A CHEAP SEAL WENT FIRST. I BOUGHT SKI THIS WAY I DIDNT DO IT , LOL it could have been a better ending but it not to bad to fix ....... also found the rotary plate is warped ......and every 717 i open the surfaces all have some fairly deep scratches on the aluminum intake pieces

 

[mikidymac]

You will need a new rebuilt crank and cases if they are damaged. You can't have grooved rotary valve surfaces.

There is no way that one cylinder is 170 psi especially on a SBT engine as they are notoriously low. My guess is a bad gauge or there was oil in that cylinder.

 

[motoring]

i certainly have a lot of oil in cylinder. that must be it i believe my gauge is good i check many outboards with it . and four ski motors all ranging from 135 to 150. except this engine but yes i flooded engine with oil because i bought ski 2 years ago it was locked i freed it up . and decided to build some motors now. whyb a new crank the bearing that cant be changed look good so far . im guessing the next bearing should come off... i see you said may need a crank lol as it stands as long as i change the two end bearing ill have a good crank seems all the other bearing and seals are good
basically all my motor are shot lol because all have grooves. WHAT IS THE QUALITY OF THE ENGINE RUNNING WITH GROOVES ??????? I WOULD THINK IT WILL RUN BUT MAYBE ROUGH AT IDLE.??????

 

[mikidymac]

So your crank actually has 6 bearings, two at the front two at the back and two in the middle. If any water or debris took out the outer bearings they took out the inner bearings also on the bad cylinder.
You can't do the inner bearings yourself as the crank webs have to be split and pressed back together and trued so not a DIY project. Much better off buying a reman crank.

If the rotary surfaces has grooves not only will your rotary clearance be excessive but the cylinder pulses will interfere with each other so no, it probably won't even start let alone rung with any performance especially in the water. It is that important. Basically like running a car with the valves broken and stuck open.

 

[motoring]

So your crank actually has 6 bearings, two at the front two at the back and two in the middle. If any water or debris took out the outer bearings they took out the inner bearings also on the bad cylinder.
You can't do the inner bearings yourself as the crank webs have to be split and pressed back together and trued so not a DIY project. Much better off buying a reman crank.

If the rotary surfaces has grooves not only will your rotary clearance be excessive but the cylinder pulses will interfere with each other so no, it probably won't even start let alone rung with any performance especially in the water. It is that important. Basically like running a car with the valves broken and stuck open.

i have three motors it was looking good for awhile that id have two running out the three but now i may have to get parts off ebay to even make one good one maybe even two but ill need at least rotary valve stuff.
as far as the crank were discussing the waste from the cages stayed contained in the seal area and didnt go past the bearing cavity . the previous owner shut it down probably as the bearings went bad. theres isnt any debri in cylinder (not now at least) but the seal probably make cylinder run lean causing low compression . this crank has a slot in the connecting rod so i can magnify glass inspect the bearings. looking at the crank fast it looks good . but the engine is really only good for parts .i agree with your above post , thanks for the rotary info , , im probably better off slaping it back together and send it in as a core. but with this virus in NYC and having a new ski and my 96 gti you helped me with ....im just doing this to stay busy. out of three motors i expect enough good parts for at least one good one. its really a nice little stock engine and easy to work on, so small but 85 hp is very good

 

[mikidymac]

Be very careful buying used parts on eBay as a lot are junk or posted wrong.
Much safer buying from Nick or Jess at Westside Powersports Seadoo.

 

[motoring]

Be very careful buying used parts on eBay as a lot are junk or posted wrong.
Much safer buying from Nick or Jess at Westside Powersports Seadoo.

ill look into them i certainly need rotary plate and the carb side of the valve , i know ebay is a mess but some good stuff comes out of ebay as long as they have good pictures i usually do okay. thanks