I’m going to have the Nikasil cylinders on this bike replated and honed to match the new 9.5:1 pistons I’m going to install in this build. I’ve learned from a well versed airhead mechanic, Tom Cutter, at Rubber Chicken Racing Garage, that the connecting rods are subject to deformation over time such that the distance between the hole centers of the big end and little end of the rod becomes a bit longer than when new. It’s worth having them checked and the flat of the connecting rod cap machined to achieve the design distance between the hole centers. This is not very expensive and ought to reduce wear and tear on the wrist pin and crank shaft throw.
It’s quite straight forward once you have the top end removed as I document in this write-up:
When BMW introduced the Nikasil cylinders in the US in the 1981 model year, they came with the 8.2:1 low compression pistons to help meet EPA requirements. So, my 1983 R100RS has low compression pistons. But, in Europe, BMW provided 9.5:1 pistons and rings for use in the Nikasil cylinders and they are still available.
The higher compression makes a useful difference in torque and horsepower:
The 8.2:1 compression pistons produce 53-Ft-Lb and the 9.5:1 compression produces 56 Ft-Lb and, or about +5% for the higher compression.
The 8.2:1 compression pistons produce 66 Hp and the 9.5:1 compression produces 70 Hp and or about +6% for the higher compression.
I intend to use this bike for two up touring and it has over 80,000 miles on it. So I am going to install 9.5:1 pistons sized for the “B” sized cylinders I have (part# 11 25 1 337 175) to get a useful boost in torque and horsepower.
New 9.5:1 “B” Size Piston Kit For “B” Size Cylinder
New Piston Kit Contents-“B” Size, 9.5:1 Piston (93.97 mm)
For the best fit and an oil tight motor, the recommendation from Tom Cutter at Rubber Chicken Racing Garage is to replate the Nikasil inside the cylinders and then hone it to get the proper clearance that matches the actual pistons. There is some variability in the pistons and this approach mates the cylinder dimensions to the piston dimensions for a tight seal.
Here is the link to the write-up of the procedure.
I think Truth is a slippery concept. Here are three statements commonly held to be true with little debate:
1 + 1 = 2.
The sky is blue.
What goes up must come down.
The first statement is true as long as there is agreement about the meaning of the symbols “1”, “+” and “2”, and you are talking about counting things. But, it isn’t true if you use it to predict what happens when you add two drops of water together: in that case you end up with only one drop of water.
The second statement is not always true, for example at night, on a cloudy day or if you are standing on the moon. The last statement is not true for the two Voyager space craft which will never return to earth.
These examples are my feeble attempt to show that “context” is very important when you are looking for the truth. Truthfulness exists within a context. You can fall into a “truth trap” if you blindly assume truth in one context is a universal truth for every context.
Since truth exists in a context, you have to look for and understand the context surrounding a statement about the truth and then evaluate if a different context is sufficiently the same before you can can conclude if the statement will be true in the new context. That’s not so easy. I think the ability to judge how similar two contexts are when looking for the truth is one hall mark of “critical thinking”.
Defining a context is not so easy. This is often due to unspoken, or unconscious, assumptions you have which do not apply in a different context. The three true statements I made above all had hidden assumptions about context which leads a casual reader to agree that they are true. It’s very hard to see your assumptions about common experience.
I point this out because when I work on motorcycles there are times when I can’t find the cause of a problem. Every test I try indicates nothing is wrong, but yet, the truth is, something is wrong.
I’ve learned that when I find myself in this situation, it’s time to write down all my assumptions. Often I have to do this more than once because the hard part about assumptions is you don’t recognize you have made them. Then I test every one of them. Every time I have used this meticulous listing of assumptions and testing, I’ve found one that isn’t a valid assumption in the context of the problem.
I hope this helps you solve problems that are too slippery to get a grip on.
I found the right cylinder was running rich, but not the left.
Right Spark Plug-Can You Say SOOTY
So I started from the beginning.
I checked head torque at 26 Ft/Lbs. Nothing was loose
I checked the valve lash. Spot on.
I checked the jet needle position. Correct at #3 just the same as on the left.
I checked the main jet and needle jet sizes. Correct sizes and matched the ones on the left. O-rings are in good shape and not torn or brittle.
I checked fuel depth in the bowls. They are the same on both float bowls; between 19 and 20 mm. The fuel level is correct on both sides.
I checked the timing. It’s spot on and there is no ghost image. The timing advances smoothly.
I switched the spark plugs and rode the bike for 50 miles. The original left plug, now in the right cylinder, is sooty and the original right plug, now in the left cylinder, is fine. So, there is no issue with the plug, plug cap or plug wire.
If it’s sooty, it’s rich. So, what’s going on that causes only the right side to run rich?
The, valves, timing, jets, needles, spark plugs, wires and caps are fine. There is no obstruction in the air tube feeding the right carburetor. The crank case breather isn’t showing any more oil than normal, so it’s not getting a lot of oil mist into the right carburetor. I mulled this problem over for several days with no new insight as to what to try next.
A couple days later I was at coffee with some friends and was chatting about this problem when an idea hit me. What if the slide isn’t going all the way up at full throttle? If that’s the case, the vacuum will be higher than normal at higher engine speeds. A higher vacuum means the fuel delivery is going to be higher than normal but the air volume will be lower than normal. That certainly would create a too rich condition and sooty plugs on one side, but not the other.
I take the air tubes off both carburetors and push the slide up on the right side. It won’t go all the way up.
The left one goes all the way up.
So, my hypothesis is correct. The rich condition is caused by the right slide sticking.
But what’s the cause of that?
I updated these carburetors with springs (not originally used on the /5 carburetor, but added to later versions). These help smooth out the transition from idle to mid-range.
As I look at the spring, it looks like it can get caught on a pin that protrudes inside the air well the spring fits inside of.
I can get the spring to hang up on that pin. When I do, the spring is cocked and then snags on the split washer under the screw that holds the plate onto the diaphragm. That stops the slide from moving.
I think this “cussedness of inanimate objects” is what stopped the slide from going all the way up.
I install the spring and make sure it isn’t hung up on the pin. I also loosen the screws holding the plate that secures the diaphragm and push the split washers outward so they aren’t protruding into the air well.
After I put the carburetor back together, the slide moves freely. I’ll take the bike for a ride when it warms up and see if I have fixed the problem. I’ve grown accustomed to not concluding I’ve fixed something until I take a longer ride and the problem doesn’t reappear. 🙂
This may be helpful to you if you are doing a frame off rebuild/restore, or you need to know how to remove various subsystems. The bike, with the exception of the body work, fits into a reasonably small pile of boxes … sort of a boxed, boxer if you will :-).
A Boxed “Boxer” 🙂
Onward. But first I’m giving “Grover”, the 1973 R75/5 some well deserved attention.