Rings/valves?

[79beem]

I'm abit puzzled.

Bike has excellant compression 150/155 psi, warmed up,carbs off.

But under hard acceleration she's blowing blue.

If it is the rings or valves, then why the high compression readings?

If I'm heavy on the throttle she uses 400 - 600ml/1000 -1200 kms, cruising 300 - 400 tops. She is also weeping and making a right mess around the pushrod seals and cylinder bases.  

[Julio A.]

Same thing here. Good compression, no smoke except during hard acceleration and engine braking. The only thing is a audible pinging sound that goes away when the engine is warm. Friend suggested piston slap. Replaced rings and compared the old with the new. Old one is significantly thinner. Haven't reassembled it though.

[montmil]

Mileage on your 1979 R65?

Compression numbers look good. Plus, you were correct to pull the carbs for the test. Since the compression test results are registered with the valves in a 'closed' position, I'm wondering if the oil smoke under hard acceleration isn't pointing to worn valve guides or possibly loss of tension in worn piston rings.

High engine oil usage may be related to an over-filled crankcase. Many Airhead riders have found that maintaining the oil level at the "halfway" on the dipstick greatly reduces dino usage.

Those rubber pushrod tube seals will harden with age and start to weep. A slight bit of oil stain around the cylinder base is also common. Replacement could be done at the same time you pull the jugs, if necessary, to handle any repairs in the cylinder and/or head areas.

Try lowering the crankcase oil level before breaking into the engine.

[Julio A.]

What should be the ideal, marginal and bad figures for the cylinder compression?

[Barry]

Good   142 PSI or above
Normal 121 - 142 PSI
Poor     Below 121 PSI

Figures and terminology taken from a genuine BMW Repair Manual and they are not exactly right because they didn't use the correct conversion factor from Bar to PSI

All sources I've seen agree the break points are 8.5 Bar and 10 Bar so if we try that again we get:


Good   145 PSI or above
Normal 123 - 145 PSI
Poor     Below 123 PSI

[Armen]

I used three piece oil rings on mine and don't have a problem. Use a bead hone to hone the cylinders. Follow the well documented reassembly/break-in voodoo and see how it goes.
Of course, use a bore gauge and check the cylinders for out-of-round. The '79 and '80 models like to egg out near the bottom. I went through two new sets of barrels under warranty before I made a sleeve to fit over the unsupported part of the jug that hangs into the crankcase.
-Armen

[tvrla]

It's possible the oil rings are at fault. But before going to the trouble you might consider taking a good look at the breather. If it's not working properly or the drain is partially plugged crankcase fumes can be sent directly to the carb(s)

[79beem]

She's done 91,000.00km.

The actual oil useage is, heavy throttle 200 - 400ml/1000 -1200 kms, cruising 100 - 200.

Got my figures mixed up.

I don't fill her past halfway.

I don't believe the breather is faulty.

As I'll be doing the pushrod seals soonish, I will farm out the heads and check the jugs myself.

I figure I'll wait to get her apart before ordering new rings.

Cheers for the input chaps.  

[Armen]

With 90K clicks on the ODO I'd def throw in a set of new rings. Again, check for wonkiness at the base of the cylinder. The stock rings are harder than a priest's willy in a boys shower room, and have to be broken in right or they'll never seat.
The three piece oil rings are a bit more flexible and conform to the cylinder walls better than the one piece stockers.
Good luck!
love these bikes  

[tvrla]

The actual oil useage is, heavy throttle 200 - 400ml/1000 -1200 kms, cruising 100 - 200.  

You know, that doesn't sound all that bad really. I think in quarts and miles - so let's see...

That seems to equate to .25-.5 Quart in 1,000 miles under heavy throttle (1 Qt in 2-4K miles) and .1 to .2 quart in 800 miles while cruising (1Qt in 4K to 8K).

I'd say it's doing really well and leave it alone. Don't mess with the rings! BMW says to start thinking about rings if usage drops below 1 Qt in 1,000 miles.

If you really really want to mess with it - you could possibly improve usage by installing 3 piece oil rings. But leave the top two as they are.

But, frankly, I wouldn't change a thing!!! It definitely aint broke!!!

[79beem]

Yes, thats what was generating the confusion. Good compression, oil useage but blowing blue.

The pushrod seals are toast so I'll be going in anyway.

As far as the breather is concerned, it does the turkey impression intermittently.

How do you check if its working?
Wheres the drain?

The air filter,carbs and plugs are clean.

I have the newer none spring type waiting in the parts bin.
Just a tad nervous about the removal/slide hammer technique.

I do love the old girl  [smiley=bmw_smiley.gif]and she's a joy up and down the Mt I live on, I just wonder if its time to ease off her [smiley=whip.gif] a bit?

[tvrla]

I wonder if the blue smoke is oil from the push rods dripping on the exhaust?

If you don't want to pull the old breather, you can make a new 'washer' for it out of circuit board material. Oak seems to think the older style is superior - so you might give that a try.

But as for rings, I wouldn't touch them. It's really easy to cause problems and make things worse in that area. From your stats, aint nuthin wrong over there!