Anybody want to take a look at these spark plugs and tell me

[Banzaibob]

Brand new NGK B6's. The same '91 R100RT that I've been posting quite a bit about lately (stud repair, de-carbonizing piston and heads)

It's been a couple of days since reassembly, re-torqued heads, etc. I've put probably 100 or so miles on it in the last few days, interstate speeds, 2 lane country roads, suburban, etc. The plug on the left with what appears to be oil glazing was on the left cylinder. The right plug appears a little lean but ok by my standards (hopefully by yours, too). The left plug would not necessarily be an issue except the the Bosch plug that I just replaced had a cracked insulator, which caused me to de-carbonize the head and piston.

My first and second guess are: 1) there is oil glazing and I should keep and eye out for oil usage. There is no smoke on start up or otherwise and new exhaust valve guides 10k miles ago. or 2) there is an acute overheating problem in the left cylinder causing "flash glazing" of some sort.

There are NO performance issues with this motor, no pinging or knocking, it is VERY strong on power, fuel mileage is 40mpg, no unusual noises.

Thoughts?

Merry Christmas!

[Bamboo812]

Do you know how to adjust carbs by ear? If so, I'd richen the lean cylinder and lean the rich cylinder a bit and see what happens. You do know you have to chop the throttle at speed to get a clear example of what it's doing most of the time. If you idled or rode slowly to your stop the results will differ. Also, at that low miles since rebuild, I'd want to retorque the heads and check valve clearances. They can close up rather quickly due to the head gasket compressing through heat cycles.

[Banzaibob]

Yes. The heads were re-torqued just before I took the last ride. Valve adjust too. Carb sync is spot on through the entire rpm range. When riding home yesterday there was a 15º temperature drop and it was in a solid rain storm. There was no effect to the performance, I rode pretty sedately though.

Both fuel screws are set similarly, no radical difference between the two. The color on the header of the right cylinder leads me to believe that the right one is operating at a higher temp. However, with it sitting on the bench during carb syncing and tuning, the temperatures (infrared thermometer) of both headers were pretty similar at around 400º.

The black substance on the left plug appears somewhat sooty and oily.

I am planning on doing a plug chop as soon as it is feasible. Maybe I am not as bright as I think I am but quite often the results of plug chops bring up more questions than they answer.

[Bamboo812]

Try swapping the plugs and see if the problem persists on that side. When turning the mixture screw, the difference between too rich and too lean stumble can vary, between 1/4 and 1/2 turn in my experience. I try to find where each cylinder runs the fastest, and turn the screw out/richer a tiny bit. Too lean can do more damage than too rich, and quicker.

[Major Softie]

I don't think that has anything to do with fuel. It looks like oil to me.

My first thoughts are too much oil in crankcase (don't fill over 1/2-way between the Fill and Full marks), or bad breather valve. If those are not causes, then I'd do a leak down test.

[melville]

Do you park it on the sidestand or the centerstand usually?

The oil might be just from the left lean of a sidestand park.

[Banzaibob]

Only briefly on the side stand, mostly on the center stand. No smoke on start up.

[barryh]

Left plug is oil contaminated and you can't judge the mixture from that.
Judging the insulator colour never seems to work for me these days with unleaded fuel. I go off the amount of carbon build up on the base ring of the plug body. If there is any appreciable carbon build up that can be scrapped off then it's running too rich. When the mixture is spot on there is only a light discolouration. Even this method doesn't work when a lot of choke has been used as that carbons up the plugs in very short order.

[Banzaibob]

The oiling issues are uppermost in my mind. I literally pounded this bike last summer with many rough trips. A lot of 85 mph hot, Ozark, 2 lane hill climbs. It came through like a champ. So far, it has yet to burn it's first drop of oil (I hope that isn't in the process of changing). It has had charging system issues but they were all easy to diagnose and fix.

The full story: In November of 2014 I rescued the bike from some hick who was just about to go down that "chopper, bobber" route with it. The fairing was destroyed when the barn it was stored in collapsed in a tornado. The previous owner could not get the star nuts off and wound up stripping them. Ted Porter fixed the spigots and my local machine shop, Noland Cylinder head installed new valve guides and re-ground the surfaces. I rebuilt the carbs with new parts. In June the alternator rotor crapped out on me. I bought a Euro-Motoelectrics from my local dealership and 3 weeks ago IT failed. My local dealership warrantied it and switched it out with me.

There was always a persistent leak from around the base of the right cylinder so while the bike was on the lift waiting for a new rotor, I decided to switch out the cylinder o-rings. All went well until I had to torque it down. The top right stud pulled out, dammit! I had my local dealership do the thread install ( viewtopic.php?f=1&t=9473 ) I torqued everything down and was about to install my old spark plugs (Bosch) when I noticed that the one on the left side had a piece of the insulator broken off. Rather than take a chance on a piece of ceramic bouncing around in my combustion chamber I decided to have a look.

There was quite a bit of carbon on the head and piston. It was chunky but not especially oily. It was more than I was comfortable with simply bolting everything back together so I de-carbonized it. Also as many of you know, heavy carbon deposits COULD account for hotspots in the combustion chamber causing dramatic heat differentials which can crack a spark plug insulator. The right side was also pretty carboned up but not quite as bad. Everything was bolted back together. I re-torqued the heads and re-set the valves after several heat cycles. I also made sure that carb sync was still on the money.

Both right and left cylinders are virtually identical with readings at 145psi. I do not have an accurate leakdown tester in my shop other than turning the psi way down on my air supply and sticking my ear into the exhaust, carb, etc. Besides, I would think that a leakdown test would not necessarily give me an accurate assessment of my exhaust valve guide's condition. I always removed the springs and evaluated the general side to side play. If you know of another way I'm interested in hearing about it.

I cannot underscore how well it appears to run. It has always been a high stepper and after all of these repairs it is still on the ball. If I was to evaluate everything from the perspective of how well it runs, I would have never removed the plugs and taken a look.

[Ray from Indiana]

Leak down test could tell you something about the condition of the valves/seats. A common cause of valve failure and valve seating problems is guide wear.....on any engine. No, it won't directly tell you the condition of the intake or exhaust valve guide but it might tell you a valve isn't seating right which could be caused by a bad guide....and if valves are not seating right it could lead you to pulling a head and checking the guides via measurement or wobble test.

I had a slide where the spring ring ( or whatever the BMW name is ) for holding the needle in position down in the slide was faulty. It made the needle projection shorter on one side. Bike ran well but had to do a lot of fiddling to sync and tune. Was rich on one side but my fiddling compensated for it. Till I saw that the needle length differing I wouldn't have causght it. Might want to check that too. How's the float bowl levels-the same?