When I first got my 1976 R75 /6 it smoked on start up, but not terrible. I seldom, if ever, use the side stand. When I purchased the bike from the previous owner, he had bought the "barn find" at an auction and told me it had 12K miles on it which was indicated on the speedo. Shortly after taking it home the speedo started spinning like a Vegas slot machine and I had it refurbished. He said he did replace all the fluids in the bike.
Now, a few months later when I start the bike, it fills the neighborhood with white smoke, and I am worried someone is going to call the fire department. After it warms up, there is considerably less white smoke. I can’t really tell if it is completely white or has a bluish tint. The smoke made me think I should do a compression test. I got these results:
Left Side compression, (carbs off) 135
Right Side compression, (carbs off) 110
Left Side compression, (carbs off) - added oil into the head 150
Right Side compression, (carbs off) - added oil into the head 135
Normal Ranges:
Good - 147psi
Normal - 125 to 147psi
Poor - 125psi
Today, I did a leak-down test on the right side, and so much air was escaping out of the oil fill hole that I couldn’t tell if either valve had an issue. I didn’t bother doing the left side because I figured I would replace the rings on both sides anyway. When running, the left exhaust is 100 degrees hotter than the right; I am not sure if that is significant or means anything.
Is it safe to assume that the bike has a LOT more miles on it than the previous owner told me?
Also, anything I should consider doing before I replace the rings?
I figure I should do an additional leak-down test after replacing the rings to see if the valves need work.
If it has enough miles for the rings to be bad, what other components need a serious look at?
Thank you
Hello Rodney, sorry you got such an issue. Yeah, the milage could be anything from sub 100K to 2 or 3 hundred K. Also, barn finds are sometimes stored in barns where moisture does a job on the cast iron cylinders.
So if it were my bike, I would pull the heads off, check the valves seats and guides. Check the jugs, to see if they are not oblong or pitted. Check ring gap. As for just installing new rings, I would only do that if tolerances are good enough in the piston size and actual bore size and shape. You may find you might have to go one over what ever pistons are in the jugs now.
Make sense? LOL, I have the same issue now with a bike I am restoring, unknown miles and perhaps in need of one over bore. Good luck, St.
Beware! I do not suffer fools gladly! St.
> Your compression numbers without carbs are the ones to believe.
> However, compression has nothing to do with oil control. On the pistons are compression rings and an oil control ring. Two different ring types; two different jobs.
> IMHO, due to the difference in piston ring job descriptions, the "leak down test" actually tells the mechanic very, very little. The compression stroke that the engine runs on happens in a thousandth of a second. So how is a pressurization test that lasts minutes supposed to represent any type of equivalence or hold any relevant meaning ?
> Physical inspection of the cylinder and valve seats will tell you if the speedo is lying about the bike's mileage. The cylinder needs to be mapped for taper AND ovality, after being honed with a bead-type glaze breaker hone in hot soapy water. Taking these multiple readings requires a right-angle, thousandth-reading, dial indicator, an instrument you are not likely to own.
> Actually, I believe what you will find is a ring of rust pitting circling 1 or both cylinders. The only cure for this is a bore job and 2 new pistons. This is the most common cause of extreme oil smoke on vehicles that have experienced long storage.
> If you don't find extreme cylinder wall pitting, then I would jump directly to replacing the sump breather flapper valve with the later model Airhead reed valve. This is located in the top of the engine, under the starter motor. Snowbum can tell you more about why this is a needed update. In fact, you may wish to make this worthwhile update regardless.
> Your write-up can be read in a way that makes it sound like you may be doing an extended warm-up ?? With multi-grade engine oils this can be more harmful than helpful. My suggested cranking sequence is: start the bike, put your helmet on, turn OFF the choke, and then take off. Stationary warm-ups longer than 1 minute are really not necessary. In the shop, you might find it helpful to run a bike for extended times. In those cases a box fan should be employed.
Hope this helps.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
I’m not an experienced airhead rebuilder, but I will offer my thoughts. If the bike truly has only 12k miles, logic would say the heads / valves would be ok. When the heads come off, you can pour some solvent (paint thinner) in each port and see if it leaks out the valve. Kind of a quick test, but if the valves are leaky the solvent will leak through if short order. Now, because of the barn storage, I agree with Steve that the cast iron liners and rings are likely messed up. Question is, how badly? The fact that the smoke situation has grown worse might mean that scoring of the cylinder wall has occurred. Just have to check it out. The decision “tipping point” is whether the existing liners can be refreshed with a ball hone and new rings installed, or is the damage too deep, and only a rebore and oversized pistons fix it? I think when these engines develop ring / cylinder issues, it compounds with time - not only are there oil control problems at the oil ring, which leaves oil behind for combustion, but then the combustion pressure blows by the the rings, increases pressures in the crankcase, which’s pushes more oil out the breather, into the carbs, and up in smoke. Oil is just going every which way.
So, I think it’s gonna boil down to a careful evaluation of the cylinder liners, and pistons/rings. What can stay, and what has to go.
All that said, I hope you are very successful getting her straightened out. I think an R75/6 is an awesome airhead. The /6 bikes, to me, were a bit nicer fit and finish wise than my /7.
I removed my right head and have several pictures. The cylinder walls are very smooth and I can not feel any pitting. Go here for the pictures.
http://rodspears.com/rightcylinder/
Well again, I’m no expert, but I would find your pictures encouraging. I see some scratches in the cylinders, but they look very light. Plenty of hone pattern left, so can’t be much wear.
Two thoughts - 1. My last “refreshing” of my (Nikasil cylinders) R100, I deglazed the cylinders with the proper ball hone for Nikasil, and installed new rings. That worked out very well. I wonder if that same approach would work for your situation. Need someone with direct experience to chime in here. Cast iron liners should freshen up nicely with the right hone, I would think. And 2. Have you visited your crankcase ventilation valve? It is under the engine top starter cover, on the right rear corner, under a small cover housing. These valves can deteriorate, which can cause increased oil consumption. Need to check it out. Good luck!
Got a picture of the whole bike?
Thoughts...
> Time to inspect the other side.
> Pop these rings off, place them in the cyl (about 10-12mm from the top of the bore) and measure their "end gap".
> Pull the air filter out and replace it if it's not pristine.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Attached picture of crankcase ventilation valve location. BMW calls it a Non-Return Valve. If yours is original, it is a spring loaded disk on a little post, on the left in the pic. The replacement is a simple reed valve, on the right. Should be readily available somewhere. The original disks crumbled away over time and miles, leaving a ready path for oil laden vapors to exit the engine and run into the carb(s).
@4949 If the disk has gone bad, does it mean oil could collect on the air filter?
I’m not familiar with the plumbing on your bike. On my ‘78 /7, the breather plumbing dumps into the intake elbows, avoiding the air filter element itself. I have heard of these systems leaking however, and contaminating the air filter.
> Your crankcase breather valve is a phenolic disc (think circuit board material) that is held down by a small spring. The plastic is known to wear badly and/or fracture into pieces. The newer replacement part fits directly into the old location. It’s a stainless steel reed valve design that is not known to have any issues. Both type valves are located under a 2-bolt cover, just to the right of the starter motor. You should seriously consider this upgrade.
> The breather works by venting the excess air in the crankcase. This pressurized air occurs because both pistons travel down the cyl bores at the same time, thereby decreasing the physical volume of the crankcase. This air has to go somewhere, or the engine will be fighting itself. Then, the valve closes and the pistons travel back up the cylinders thus creating a vacuum inside the crankcase, which helps fight oil leaks. (In the automotive world this is known as Positive Crankcase Ventilation, or “PCV”.)
When compression rings are no longer doing a good job of containing the high pressures of combustion, the HOT! gases bypass the rings and end up in the crankcase. This gives the breather valve a LOT more gas/air to control than it was ever designed to handle. Excess amounts of heated and oil filled air now escape the breather.
When the breather opens and expels all this oil mist on an Airhead, it travels down a rubber hose into the air filter compartment and finally ends up being routed into a carb intake (or both depending upon model)… where the EPA thinks it should be burned inside the combustion chamber during normal running.
Several problem areas exist for the routing of these gases on the Airhead…
> The hoses that route this hot gas are rubber and continually bathed in oil, so that after 40 years they are often cracked, split and leaking. This leak can cover the air filter in oil. The problem here is that BMW no longer offers new replacement hoses.
> As the air filter becomes loaded with oil, air can no longer be drawn through the filter. This leaves the intake searching for secondary sources of air. The only other source is the breather hose. So whether or not excess oil-laden air is being pushed down the breather hose, in this case excess oil-laden air is now being sucked down the breather hose from the other end.
If you happen to have bad compression rings AND a clogged air filter, then you have a “perfect storm” and copious amounts of oil are thus being burned in the engine… which shows up as white smoke.
This is what I think is going on in your engine. Bad rings, a worn out breather valve AND busted breather hose leading to a soaked air filter all doing their part. Thankfully these other sources are cheap to fix compared to rings and gaskets.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
@wobbly Thank you for such a detailed response, that helps a lot. The hose you were speaking of that might have gone bad, is it: "Crankcase Breather Hose for BMW Airhead; 11 15 1 338 215 / EnDuraLast" number 9 in the diagram?
The hose you were speaking of that might have gone bad, is it: "Crankcase Breather Hose for BMW Airhead; 11 15 1 338 215 / EnDuraLast" number 9 in the diagram?
Different model Airheads (with the cylindrical air filter behind the engine) have different shaped hoses. But "Yes", that's the hose. It's a molded/ specially shaped hose and a lot more complicated than the drawing.
If you cannot get a replacement, then consider patching up the one you have with large dia heat shrink tubing.
Hope this helps.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
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