Heavy breathing (Update)

Found the oil level was the culprit. If you run with the level about 25mm (1inch) below the max mark all is well. FYI the max to min distance is approx 63mm.
Thanks for all your replies

Hopefully the title of the post will get all to read!
I have V50 3 with original air box. When the bike is driven on the motorway at 70 I can feel the engine going off, what I would call "fluffy ". I then get a lot of oil from the air box breather. I think it is excessive crankcase pressure. If I ease off and run at 60 it’s fine.
Anyone had anything similar?

Have you over filled the sump?

have you done a compression test
could be worn rings
or perhaps a blocked vent and its pulling oil into the air inlet, that would slug the engine

The oil level is just under the max mark with the dipstick screwed in.
The rings are new but I haven’t done a compression check since the engine was rebuilt, about 2000miles ago. I was concerned about piston wear, the bores are good.
I will investigate the vents, and the return pipe from the filter to sump.

My Veefer got real bad - to the extent that I used a LM oil breather box for a couple of years before it overwhelmed even that. I’d get oil on the rear tyre in all that. It always seemed to run fine though. Fixed by putting in new rings. I was worried about them bedding in - given that I wasn’t going to scrub up the plated bore, but it worked out fine. I think that was all at about 55k miles. Bores looked like new at that point, pistons seemed OK. Now fine again.

You have new rings, so what I’m saying is, from my experience that shouldn’t be the issue.

My Veefer will not tolerate having the oil level above half way between low and high on the stick. Any more and it gets spat out. It never affects running though.

Yes, it can get bad. Oil over the rear wheel etc. Starts fine, runs well but it starts throwing oil if I hold a steady 70 for about 5mins on the motorway. What’s the LM breather box?
I will play with the breather vents this weekend. If the weather is good may do it Monday!
Thanks for the ideas.

Just an odd thought, compression should be ok if you have fitted new rings, those Chrome bores never seem to wear out
have you changed to a different type of oil ?

The barrels are Nikasil plated, never seem to wear out. I will borrow my friend’s compression gauge to check. And no, the oil type has not changed.

I’ve now checked the breathers, all OK. I think the oil is coming from drain pipe from the air filter to the sump and from the message that cyclobutch posted a lower oil level will (should) cure the problem; it will stop the oil coming back up the pipe.
I’m going to lower the oil level and keep you posted.
Thanks to all.

I’m not that familiar with the small blocks, but on the big block motors, if the non return valve gets blocked in the main vent outlet, then oil is pushed up the return pipe that should return any oil back to the sump from the air box or collector tank depending on your system. The return feed is located rear of the engine, bottom left.

Not sure if a non return valve is fitted to a small block. Wonder if the pressure relief valve is stuck? Oil pressure is OK but could be stuck closed?

It’s the up and down action of the pistons that blow air and oil out via the breather system, nothing to do with the circulation pump or pressure regulator valve.

On the older big block motors the non return valve mentioned is a caged ball bearing that closes the big pipe between the crankcase and the “breather box” aka oil catcher, when the pistons are rising - opens when the pistons are descending. The idea is to create a partial vacuum in the crankcase so oil doesn’t come out past the seals and so on, also to offset blow-by.

Neat, did it work?

Alright if you don’t rev it high for too long.

Err, this is the small block section Mike, it’s the only way to go.

Yes, at 5000rpm it’s 60mph. 70-75 it’s 6000+. You’re making it work hard to maintain a steady speed on the motorway.