Dipstick sucking air

[79beem]

Whilst turning engine over with allen key to find tdc for valve adjustment, I noticed a sucking and then hissing sound. Turns out its the dipstick. I've ordered a new washer, but is this a symptom of something not right?

[Bob_Roller]

That's a naormal occurance, when the pistons both go go the bottom of the cylinder, the engine case is pressurized and air will escape from the dipstick if the seal isn't good, when the pistons go out towards the cylinder head, a low pressure is created and air will be sucked past the dipstick .

I replaced the metal washer on the dipstick with an o-ring after I kept losing the metal washer .

[nhmaf]

There is a crankcase ventilation valve up under the top engine cover, aft of the starter motor.  your model year had a disc with spring valve, I believem while the later models had a reed type valve - I mention this because it is also possible for those early valves to "stick", and instead of opening and closing, if stuck closed it could tend to exacerbate the sucking/pumping effect through the dipstick and seals, such as the rear main seal.   IF you start to hear something that sounds like a turkey gobble when starting up or shutting down your engine - you'll want to look to that valve..

[79beem]

Thanks Bob I'll try an O-ring.  nhmaf, she does irregularly gobble like a turkey at idle. I have a new crankcase breather on the way. How do I remove the old and install the new valves???

[nhmaf]

Snowbum writes quite a bit about this topic.   Are you planning to install the later style reed-valve?   I haven't' personally converted one myself.    This page from Snowbum's online 'tome'  should get you well along, if needed - especially farther down the page:
http://bmwmotorcycletech.info/oilsketch.htm

[Barry]

The sucking / hissing sounds when turning the engine over by hand are fairly normal but the dipstick certainly shouldn't leak. That will be giving the crankshaft seals and the breather a hard time.  The 79 has an alloy dipstick handle and is supposed to have some sort of fibre washer as a seal.  I just use an O ring of suitable size and it works fine.  Later dipsticks with the plastic handle had an O ring seal so I can't see why it shouldn't work for both types.

To remove the old disc breather assembly you need to contrive some sort of slide hammer. Some say they come out reasonably easy and some say it can be a pig of a job.  I think the spider which supports the disc post is steel in which case I would try heating the crankcase to take advantage of the differential expansion rates.

[Barry]

79beemer  - did you manage to resolve the air leak problem ?


Your post got me thinking about the various ways that air could leak into the crankcase. Piston blowby aside basically anywhere oil can come out air can get in.


1. Crankshaft rear main seal

2. Crankshaft  front seal (alternator).

3. Dip stick washer/O ring

4. The breather valve disc or reed.

5. Camshaft seal -  Pre-bean can airheads had a camshaft seal proper - simple and obvious.  Bean can models I presume it's a combination of the large O ring seal on the back of the can and also the bean can shaft seal - not a seal proper as far as I can see but a mechanical seal of sorts.  Could it be the bean can is responsible for more air leaks than it's credited for ?


Did I miss anything ?

[79beem]

Barry, thanks for thinking it through, o-ring seems to have worked. If oil leaks means air leaks then there's plenty of air getting into my bike. What exactly do you mean by piston blowback? Nhmaf , thanks for the link. I still have turkey gobbles. I have the newer  reed type breather already installed. Snowbum cautions mistaking noises from rear crankshaft seal as the breather but doesn't say how to tell the difference. Do you know?

[Barry]

Hope your oil leaks are not too bad. When I was thinking about anywhere oil can get out I meant anywhere above the oil level in the sump as in misting out of the dipstick seal.

Piston blow by is when the piston rings are worn or not properly bedded in so that they don't form an effective seal against the combustion gases resulting in pressurisation of the crankcase. Worn valve guides could also pressurise the crankcase via the valve covers.
 
Some have suggested that the turkey gobble noise is not necessarily made only by the breather itself but also by air passing through the crankshaft seal. This is a result of the an ineffective breather or piston blow either of which can prevent the crankcase being pumped down. The idea is that instead of the air in the crankcase being pumped in and out on every stroke the breather's one way valve operation causes a partial vacuum to be achieved when the pistons are on the way out so that when they are on the way back in again air movement out of the crankcase is eliminated or at least minimised. If this isn't happening efficiently enough air (and oil) can be continuously expelled by the breather and maybe the crankshaft seal resulting in the turkey gobble noise.  

Another theory about the seal is that a defective rear main seal is allowing air into the crankcase which then makes a noise when it is expelled by the breather.

I don't know how you tell the difference but my best guess would be that noise from the seal occurs when you have severe turkey gobble noise and any mild gobbling noise at idle is from the breather operation.  Running the oil level a little lower might help with the noise by increasing the crancase air volume.