Correct Oil Pressure '78 GS 750

[Forden]

Hi All,

I've got smoke (not a ton, but noticeable) coming from the crankcase breather, and I want to test oil pressure first and make sure it's ok. Original service manual says minimum 1.42 psi at 3000 rpm (oil temp warmed to 60 Celsius). That seems surprisingly low, and when I do a search people cite numbers that are all over the place. What is a normal range?

thanks in advance. cheers

[Datsa Noydb]

Have not consulted the manual recently, but that seems about right.

On cold startup, you might see as high as 10 psi.

[Grimly]

Roller bearing crank engines don't need pressure, just flow.
However, the cam journals do need a bit of pressure, but how little can be seen from the spec sheet. They're not under the same loadings as the plain crankshaft bearings on a conventional engine, so live quite happily with the few psi of oil pressure in these engines.

[Forden]

thanks guys. I was wondering if something might be causing higher pressure than normal (and/or temp), but maybe on the wrong track. Some of the posts I've tracked down about smoky breathers have said just ride it and it will go away... I'm a little distrustful of do-nothing advice, though.

I'm not new to engines, but new to the bike, so looking forward to learning the ins and outs.

[GregT]

Depends what you know of the bike's history. If it's recently had a top end rebuild then, yes, ride it and bed it in.
If it's been laid up for years, ride it long enough to show up what needs doing.
9/10 in my experience it's valve stem seals that have hardened with age.
Have a look at the plugs - are the colours even or is there one black one ?

[bwringer]

Is the bike using an excessive amount of oil?

Some blowby and oil/water vapors (water is a byproduct of combustion) are perfectly normal and natural; normally the breather tube is plumbed into the airbox so you don't really see them. That's one reason the airbox has a drain tube; the vapors condense into an oil/water sludge.

So assuming you're not running an airbox (otherwise, why would you even see any vapor?) or maybe the breather tube became disconnected, it's impossible to say what a "normal" amount of vapor looks like. I mean, it shouldn't be a fog machine, but you will get some in normal operation. If you're not running an airbox, you will need a catch can for the breather tube.




The other consideration is that excessive oil pressure really just isn't possible in this engine; there's no pressure regulator widget in there that can get stuck as there is with plain bearing engines. It just pumps oil around, but there's no real pressure, just volume.

You can get excess air pressure inside the crankcase causing seal leaks if the breather is blocked, but your breather obviously isn't blocked. (The fitting at the top of the airbox is metal on some models, and can rust inside and get blocked, or you might have a kinked hose, etc.)

One thought is that there are a couple of pieces of folded up steel mesh inside the breather cover on top of the valve cover. These are intended to give crankcase vapors a place to condense and run back into the engine before exiting. If these are missing, you'll get more vapors than usual.


As others have said, if it's running well, run it for a while and see what happens. Maybe you'll find that it's pretty normal for this engine as you get used to it. Maybe you'll find that one or more bores got rusty during storage and they're not wearing in as they should.