Rotax 2-Stroke; bypass oil injection = engine suicide?

[Riverguy42]

Hi, I'm a 50-year powerboat veteran and generally considered competent with inboard/outboard/diesel/gas/2-strokes/4-strokes etc., but I have never seen a Rotax before.

Like many 2-stroke Rotax noobs, I asked myself the question "why should I rely on a possibly failure prone injection pump system, when I can just put oil into the gas tank and be SURE the engine is getting enough oil?"

It took me a little while to understand what I THINK is the answer. Please forgive me if this has already been discussed, I looked but could not find anything.

If I am correct, then bypassing the oil-injection system in a Rotax would eventually destroy the engine.

Here is my assessment, please correct me if I am wrong:

1. Rotax engines are designed to inhale an OIL/AIR MIXTURE into the crankcase.
2. This ensures that the rotary valve gets lubricated (because that's where the oil/air mixture gets into the engine).
3. It also splash-lubes everything in the crankcase with pure oil instead of oil that's diluted with gasoline.
4. Bypassing the oil injection system and putting oil into the gasoline instead defeats this entire design.

So, with all the discussions here that I've read about people just deciding to bypass and put oil in their gas, I've not heard anyone warn people what will happen if the rotary valve is allowed to (effectively) run dry, without the pure coating of oil (not gas/oil mix) that is intended.

Looking forward to comments, and to learning more about this!

 

[mikidymac]

No.

First of off there is no good reason to remove the oil injection on a seadoo as it is the most reliable system on any personal watercraft, they just don't fail.

What does fail are the little 3/32" hoses that connect the pump to the intake manifold with age. So just replace them every 5-10 years. Second is using the wrong oil.

On a seadoo the oil injection pump injects oil into the intake manifold, this mixes with the fuel and air mixture this lubricates the rotary valve and then the crankshaft and finally the pistons.

There is a second line from the oil tank that floods the center cavity in the crankcase that is sealed and provides oil to the rotary shaft. It is just a flooded chamber with no oil movement.

Most people that don't know remove the oil tank and switch to premix which removes the oil from the rotary gear cavity.

 

[Riverguy42]

No.

First of off there is no good reason to remove the oil injection on a seadoo as it is the most reliable system on any personal watercraft, they just don't fail.

What does fail are the little 3/32" hoses that connect the pump to the intake manifold with age. So just replace them every 5-10 years. Second is using the wrong oil.

On a seadoo the oil injection pump injects oil into the intake manifold, this mixes with the fuel and air mixture this lubricates the rotary valve and then the crankshaft and finally the pistons.

There is a second line from the oil tank that floods the center cavity in the crankcase that is sealed and provides oil to the rotary shaft. It is just a flooded chamber with no oil movement.

Most people that don't know remove the oil tank and switch to premix which removes the oil from the rotary gear cavity.

You answered 'No', but then I think you agreed with my assessment -- one should >>not<< bypass oil injection in a Rotax 2-stroke. Is that correct?

 

[Tiggo]

You can bypass the oil pump injection on a carbureted sea-doo by mixing the oil/gas. But you still need to have the other oil line connected to the crankcase. As he said, the oil pump system never fails so why not use it if you still need the oil tank to feed the crankcase. Also, you cannot bypass the system on an RFI/DI 2 Stroke because oil in the gas would ruin the fuel injection system. My 2 cents...

 

[mikidymac]

You answered 'No', but then I think you agreed with my assessment -- one should >>not<< bypass oil injection in a Rotax 2-stroke. Is that correct?

Sorry,

I meant that no your assessment is incorrect.

On the carbed skis you can run premix but you have to also supply the rotary gear with a constant supply of oil and it will be perfectly fine.

But the injection systems do not fail if maintained properly, use less oil and are much more convenient.

 

[Riverguy42]

Thanks @Tiggo and @mikidymac

Maybe I will repay your patient kindness by writing up a 'Rotax 2-Stroke Lubrication for Dummies' post.

I had assumed that the rotary valve >>gear<< and the rotary valve itself were both lubricated by oil 'injected into the intake manifold'.

I was not aware of the second (crankcase) oil line that supplies the oil bath chamber to lubricate the rotary valve gear/shaft.

In my original post I suggested that bypassing oil injection would be engine suicide - by this I meant that removing/bypassing the oil injection system completely (including removing the tank) and just running pre-mix would ruin the engine.

Re: "but you have to also supply the rotary gear with a constant supply of oil" and "you still need to have the other oil line connected to the crankcase"

I misunderstood the system but concluded correctly -- there is no way to completely replace the oil tank/injection system and rely 100% on pre-mix.

So, why are people (like me) even thinking about doing this?

In my case I was distrustful of work done by the previous owner. I bought the boat in August of last year, ran it for 20-25 hours that season, and it didn't seem like the oil tank level was coming down enough relative to the gallons of gas I was burning. After the first half-tank I wasn't sure the injection pump was putting enough oil into the system, so I pre-mixed at 70:1 just for a margin of safety, while still keeping the tank full at least until I could find out what was going on.

A problem (IMO) with the 717 2-stroke jet boats is that it is darn near impossible to adjust (or check) the oil injection pump and pump-throttle linkage without either finding a lilliputian contortionist mechanic or removing the engine. I'm not only too big for that kind of crap, I'm too old for it. On top of that, of the three Sea-Doo authorized dealers within 50 miles of me, none of them will work on any of the 2-stroke models any more, and none of my usual mechanics will touch it either. I'd happily spend the $$$, but no one will do it.

That said, I appreciate your perspectives that the oil injection system is rock-solid-reliable when maintained correctly, and I believe that...but checking/maintaining/adjusting it is a nightmare for me and as long as I am not seeing oil consumed at what I think is a proper rate, I have to assume it is messed up.

At this point I think I'll just keep running 70:1 premix in the gas, leave the oil tank full and keep an eye on things. I'm not seeing excessive smoke in the exhaust so I think this is a reasonable compromise.

Other suggestions welcome, of course ;-)

 

[WhiskeyD]

Stock motors are best left with the oil injection. If you're running a built motor, premix is the way.

 

[Grnbeano]

My 96 XP still has the stock oil injection. My 97 was converted to Pre-Mix 15:1

 

[mikidymac]

My 96 XP still has the stock oil injection. My 97 was converted to Pre-Mix 15:1

15:1? No, not ever. 32:1 if racing WOT. 40:1 is fine for a rec ski.

 

[skrillac]

@Riverguy42

I just use a bore scope camera to line up the oil injection pump marks on my 717. That and a 10mm wrench on the carb side of the linkage that runs between the pto carb and the injection pump.

Sometimes I find myself wishing i had 3 hands.

Also, the only recommended way to test the pump is take the cover off and spin the gear on its back side with a drill. I did think about just cutting the 3/32 tygon lines and replacing them and just spinning the starter to fill them. You could visually see there that the pump works without pulling the engine out.

I probably would have done this, but I had to pull my 717 out of the hull for a variety of reasons.