Reading spark plugs on R100

Hey everyone,

I have my R100t 81' and I was curious if anyone could explain or if this is a common thing with the airheads, the right side of my r100 not only seems to run a little richer but I pulled the plugs this afternoon after a ride of really using all positions on the throttle. ( was initially trying to see if my main jets were too large ) and noticed half was black soot and the other half was clean as a whistle. Then left plug looks steller all around but I've always had a hard time with the right one. it also taps a little louder too. I wonder if my intake valve is slightly loose?

Any insight is much appreciated.

IIRC the crankcase breather exits in the right air tube to the carb. What you see is the result of oil mist and not carb running rich.

Certainly on my '78 the oil mist tube exits out the right side, but at some point, a tee was put in the line and it was routed to both carbs...I figured by 1981 that would have been the case.

If there's only one mist exit, that could certainly be part of the problem. But it could also be:

- is the choke actually off on that carb. Check to be sure the chock lever is up against the stops.
- hole in carb diaphragm?
- weak spark on the right side? You can try swapping one thing at a time to see if the problem changes sides. First spark plugs, then high tension leads, then switch the high tension leads at the coil towers.

I don't think the shiny/sooty parts on the spark plug are anything to consider...I think the shiny part just had the soot flake off for some reason.

Hello

Hello, I recognize you from previous posts. I just can't remember the details about your bike's mileage or any major work you have done recently. Can you refresh us, it would help to know?

To be fair, the picture shows more than the normal oil mist form the breather system. Yes, it does vent into the right side carb.

There are a bunch of things which could cause this. Most likely suspects come about due to mileage.

Simplest is a booger in the carb.

Depending on mileage, the next simplest item is worn needles and jets. With this you will get a drop in gas mileage and black plugs.

Worst case is bad valve guides or bad piston rings. Despite some people's high regards for BMW's Nikasil cylinder coatings, rings and things still wear out and sometime faster than we would like them to.

Anyway, I am blathering here, I should keep out until you let us know more about the bike's history. Cheers, St.

Before I drew any firm conclusions I would do a throttle chop test. Find a good pulloff you can ride to and coast into from highway speed. Fully warm the bike up and then proceed to ride to your chosen pulloff. Riding at speed kill the engine and coast in to you location. Allow things to cool a bit and then remove and examine the plugs.

Had you recently topped off crankcase oil to dead full? Recent oil change? Most engines have a happy point on oil fill. As long as it is between min and max all is well. Many airheads are a little OCD and insist on topping to dead full - the engine then disgorges this overage back to its happy point.

For those of you who mentioned that the right side is where the crankcase breather empties, please remember that this is a 1981 model where the breather empties to both sides and uses the flat air filter.

Good catch

Good catch on the flat filter system for 81. I forget what year they started the set up. Indeed the breather vents to both carbs. St.

STEVENRANKIN said:

Indeed the breather vents to both carbs. St.

Click to expand...

... if both hoses are still hooked up and routed correctly. I have seen an awful lot of stuff get rearranged in a period of 39 or so years.

STEVENRANKIN said:

Hello, I recognize you from previous posts. I just can't remember the details about your bike's mileage or any major work you have done recently. Can you refresh us, it would help to know?

To be fair, the picture shows more than the normal oil mist form the breather system. Yes, it does vent into the right side carb.

There are a bunch of things which could cause this. Most likely suspects come about due to mileage.

Simplest is a booger in the carb.

Depending on mileage, the next simplest item is worn needles and jets. With this you will get a drop in gas mileage and black plugs.

Worst case is bad valve guides or bad piston rings. Despite some people's high regards for BMW's Nikasil cylinder coatings, rings and things still wear out and sometime faster than we would like them to.

Anyway, I am blathering here, I should keep out until you let us know more about the bike's history. Cheers, St.

Click to expand...

Hey Steven,

Thanks for the reply.
I unfortunately don't know the mileage on the bike since when I bought it the guy had removed the instrument cluster and he had no idea.
I recently just finished rebuilding the bike with bunch of stuff on the engine as far as clutch timing gaskets. gear box etc. A year pryer to that I had done a full carb rebuild, push rod tube gasket replacement as well as connecting rod bearing and piston ring replacement. Checked ring end gap when installing the new ones and everything seemed within speck as well as the honing was looking great no scoring! lapped the valves. I was a newbie ( still am ) when I did the piston ring replacement so I could of overlooked the guides as far as what is or isn't appropriate.

I switched the breather box and starter cover to from an earlier model but the breather box still has the split in the breather hose.

ebeeby said:

Had you recently topped off crankcase oil to dead full? Recent oil change? Most engines have a happy point on oil fill. As long as it is between min and max all is well. Many airheads are a little OCD and insist on topping to dead full - the engine then disgorges this overage back to its happy point.

Click to expand...

I topped it off at max with some fresh oil about a month ago.

20774 said:

Certainly on my '78 the oil mist tube exits out the right side, but at some point, a tee was put in the line and it was routed to both carbs...I figured by 1981 that would have been the case.

If there's only one mist exit, that could certainly be part of the problem. But it could also be:

- is the choke actually off on that carb. Check to be sure the chock lever is up against the stops.
- hole in carb diaphragm?
- weak spark on the right side? You can try swapping one thing at a time to see if the problem changes sides. First spark plugs, then high tension leads, then switch the high tension leads at the coil towers.

I don't think the shiny/sooty parts on the spark plug are anything to consider...I think the shiny part just had the soot flake off for some reason.

Click to expand...

Thank you for the suggestions. Ill give them a go through.

It has the split breather hose. with an earlier breather box. I liked that style for the engine.

I don't own a post 1980 bike, but as indicated they have a special cylinder wall coating made of Nikasil. This is a hard substance and normally the surface doesn't wear and one only replaces rings as mileage goes by. That said, the walls can be ball honed by using a special material made of aluminum oxide. This is done lightly...doesn't take much.

Nikasial

It does wear, not only the rings but the coating. It is better than cast iron linings but not perfect. I know, I had my engine rebuilt at 100K miles or so. Of course I ran the bike hard for that 100K miles. St.

Steve -

OK, I was being a bit generous! Sure, they're going to wear, even flake off. But I had heard that under more normal circumstances that at say the first 100K miles, all that would be needed is a change of rings and a light hone. That's a long time in most people's books. But of course, YMMV!!

Sorry

Sorry to beat on you Kurt, I know lots of people love Nikasil. I would still have it if the cost to replace it had not been so expensive. I have to agree it is longer wearing than cast iron.

In my case of wear out, the pistons started slapping the walls at their skirts. So, no simple hone and ring. At the time, the cost for BMW original parts was outrageous and there were no OM units on the market. So, I had the cylinders bored out, sleeved with cast iron and have been happy.

Anyway, the point here is black plugs, and worn rings can be a cause of the problem.


Oh yes, I forgot in my list of things to check the choke system. Are the chokes/choke actually off? Could a cable be binding on one side?

Paul suggested a throttle chop test, how did that turn out? Did it turn up anything?

If you have a valve gettin loud, when was the last time they were adjusted?

Oh airhead BMW bikes I love them but, sometimes they can be a royal pain to keep running. St.