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norwaybike
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Hello dear bmw specialists.

My 78 R100S kidding with me. (Bing 40mm carbs)
It runs fine on idle, and when I twist the trottle everything is normal. The problem is when the engine has a small load, hold steady speed without the engine have to work something special. Then the engine picks, probably at one cylinder, because the spark plug at the left cylinder's get very dirty and black (the right one is normal). When I increase the load again everything is normal. It only happens when the load is small. Both carb is adjusted equally

I have cleaned the carb, changed the diaphragm, changed the sparkplugs, checked the contact breaker, changed the coil and the wires to the sparkplugs. Checked the ignintion timing, and the valve clearance. Now I do not know what else to do ....

Could some of you experts help me ..?

Regards from Norway

schrader7032
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VBMWMO #7032
San Antonio, TX
Joined: 10/27/2006
Posts: 6805
78 R100S problems.

By "both carb is adjusted equally" do you mean that you've gone through the full carb synching process? Done after a good warm-up run, like 20-30 minutes? This has a lot to do with how the carbs transition from one throttle setting to another.

As part of the carb work, did you remove the throttle butterfies? These must be installed perfectly symmetrical around the bore and with the beveled edge positioned correctly.

What are your carb numbers? Do you have the flat top carbs? These have been known to have problems.

Are the cable ends at the carb properly seated in their small ferrules? Maybe one of them is hanging up and finally seats.

Since you have a problem with one side, try swapping plugs, wires, coils, etc., one at a time to see if the problem follows a switch. That would rule out the electrical side of things. You say you changed things...not sure if you mean swap or put everything new in place.

Kurt in S.A.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

norwaybike
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78 R100S problems.

Yes, I have done a totally carb synk prosess.

I have done so much swapping in the ignition system, so I am 99.9% sure it have to be a carb problem. The carbs is not the one with flat tops, and I have controlled the top dome for leak.

The butterflies is only visually checked.
The cabel ends at the carbs is ok.

Regards,

schrader7032
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VBMWMO #7032
San Antonio, TX
Joined: 10/27/2006
Posts: 6805
78 R100S problems.

Well, I guess I'll just have to shrug my shoulders. My '78 R100/7 carbs end with the numbers 19/20...we probably have similar carbs. I can't imagine what it could be.

How about your floats? Are they working OK? Not sure how much this affects the situation you're talking about. But good floats will float about 1/3 above the fluid line in a beaker of gas. When set properly, there should be about 22-24mm of gas in the float bowl when measure in the center of the little well. Turn the gas on for a few seconds, then turn the petcocks off. Remove the bowl...measure. A little will run out of the line as you remove the bowl, but I don't worry about that too much.

Beyond that, you might have stiction of the slides in the carb. Or maybe your diaphragms are not sealed around the edges completely.

Did you have the enricheners off? Possibly something's not correct there.

Are all your settings - jet sizes, needle notch position, etc. - all stock?

If it were me, I'd probably end up going through the "offending" carb at least one more time. Something is obviously wrong, but the carbs are both simple and complex items.

Sorry...not much help... Kurt in S.A.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

norwaybike
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78 R100S problems.

I am sure my floats is ok. I have checked everything around the floats several times.
Enricheners are off. The jet sizes and needle position is controlled.

Today I have tryed to sync the carbs with a vacummeter. The right one is pretty easy to adjust, and works ok. But the left one is strange... To get as close sync as possible I need to almost need to close the idle/mixture complete... We all know thisis not correct. The engine is still good when i twist the trottle. But it pinks with small loads. When I decrease the trottle it "pops" in the exhaust.

I wonder if I need to swap the carbs left/right...

If the valves on the left sylinder don't open enough (because of the cam), could I have these problems...??

Regards

schrader7032
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VBMWMO #7032
San Antonio, TX
Joined: 10/27/2006
Posts: 6805
78 R100S problems.

What kind of vacuum device do you have for the synching? I have a TwinMax which is a differential pressure gage, telling me the difference between the two cylinders. Other devices, using either water or mercury in tubes, tries to balance the heights of the liquid in the tubes.

Synching can be done without a device, but with a couple of spark extenders which allow you to temporarily short the spark for a cylinder to ground. Shorting is done by using a plastic or wood handled screwdriver and touching the extender and the cylinder fins in a quick, firm motion. This lets you see how much RPM the non-shorted cylinder runs at. The idea is to get each cylinder to pull the same load when the other is shorted.

The synching process is three steps:

- idle speed
- idle mixture
- cable tension just off idle

Idle mixture doesn't need to use the shorting process, but can actually be "listened" for. The mixture screw is turned CW or CCW until the maximum speed is attained. Then the screw is turned a slight amount CCW to make it slightly rich...better for the engine.

The mixture screw controls a gas circuit. CW makes it leaner...CCW makes it richer. If you can turn the mixture screw on the left one almost closed before you notice any changes, then gas is somehow getting through the idle circuit besides this screw. This needs to be figured out. Is the o-ring on the left idle mixture screw OK? A little silicone grease or equivalent on the o-ring on installation helps to prevent it from cutting. BTW...how many turns out from seated is the right mixture screw? It should be pretty near 1 turn, plus or minus. Much different than that, might indicate an underlying problem there.

Pinking suggests either a lean condition or advanced timing. At what RPM does the full advance mark come into the timing window? For the '78, it should be around 3000-3200 RPM. It should stay in the window once that RPM is reached. IIRC, if the advance mark is in the top of the window or even higher, that would indicated the timing is too far advanced.

How about your advance unit? Is it thoroughly cleaned and lubed? What if it's hanging up and not advancing as quickly as needed when you just apply power. Maybe the springs on your advance unit are sacked, thus advancing much too quickly.

If it's pinking, then it might be lean or be leaned out by air leaking into the exhaust...into the exhaust separately or possibly in the rubber boots that connect the carb to the head.

I have read that valves, if burned, might also creating some popping noises. Basically, the valves aren't seating properly.

Swapping carbs would definitely confirm that it's a carb situation. Might be worth a try.

Kurt in S.A.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

norwaybike
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78 R100S problems.

Today I swapped my carbs, and I also swapped the problem. It is a carb problem...!
After 1o kilometers driving it was my right sparkplug who was black and dirty. Once more I have to pick my carburetors in bits. Let us hope I can find something this time.

Regards,

norwaybike
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78 R100S problems.

Think the issue is resolved, I had overlooked a small channel from the air intake to the carburetor, and to the idle yet. I opened this with a needle and carb cleaner, so it worked very well. My bike is now running with the factorysettings in the carbs, and everything works perfect. Thanks for all the good suggestions Kurt!

My only problem now is a voltage drop of almost 3-4 volts when I turn on the light. The charging is ok. Have measured that there is no problem in the headlight, or the switch. Could be the backlight?

Regards, Frode

schrader7032
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VBMWMO #7032
San Antonio, TX
Joined: 10/27/2006
Posts: 6805
78 R100S problems.

Frode -

The voltage drop (3-4v) is seen where? What were you measuring to see that? But you say the charging is OK. Does this mean that when the bike is running at say 3K RPM, the voltage across the battery terminals is ~14v but when you turn on the light, the battery terminal voltage drops to 10-11v?

I would try and isolate specific elements. For example, remove the connector at the back of the headlight...repeat experiment...see what happens...and so on.

Kurt in S.A.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

Bruce Frey
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Texas Hill Country, USA
Joined: 10/27/2006
Posts: 574
78 R100S problems.

I had a similar experience last week. The voltmeter on my R90S usually flutters between 13 and 14v when running. I looked down and noticed that it was between 11 ad 12v. I turned off the headight and it went back up to 13-14. Headlight on again, back to 11-12. The generator light did not come on. I rode home with the light off.

When I went to check the situation a few days later, it behaved normally. I think voltage drop problems like this are usually a result of corroded connections. I think I need to dedicate a day in the future to checking conections.

Good luck,

Bruce

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