I just bought a 1991 k100rs. The bike is having some hesitation (or surging) issue. It seems to happen in all gears. It's as if the engine is loosing a bit of power, then kicks back in. The duration of the hesitation then surge varies from feeling like being buffeted by the wind, to more like hitting the brakes, or running out of gas.
I just replaced the spark plugs, which seemed to help a bit. The old plugs did not have the ferrules on the end, and I don't think they were the right kind. The threaded exposed end appeared corroded and the inside of the cable connector also looked like there was a bit of corrosion and 'gunk'. I have not replaced the cables. I will post photos of the old plugs.
There also sounds like there is a puttering sound that you can hear at idle. Potentially an air leak or vacuum leak? I can post a video with sound later too.
I'm new to motorcycles so I'm not sure what is normal as far as sounds, but I'm pretty sure the hesitation is not normal.

The correct plugs are NGK DR7EA or equivilant Bosch but I recommend the NGK
Probably pay to get plug leads as well I use EME leads which are from NGK http://www.ebay.com/itm/BMW-K75-K100-K1100-K1200-Ign-Spark-Plug-Wires-Copper-Core-SILICONE-Jacket-/251263624189?hash=item3a807aa3fd&item=251263624189&pt=Motorcycles_Parts_Accessories&vxp=mtr
The puttering could be a leaking exhaust gasget or a cracked exhaust pipe, if the exhaust is cracked then it can be TIG welded and if you do that also weld a steel strap to the outer side of the pipes about 40mm from the head this will or should prevent it cracking again.

Thanks Rick. Do the NGK leads also have the resistor built in similar to the OEM?
Here a picture of the old spark plug (a NGK D8EA)
The new plugs are NGK DR7EA (with the screw on ferrule or bobbin or terminal.) I thought I read somewhere that the "R" in the spark plug type indicates that it has built in resistence. Is this true for the NGK?

Any other thoughts about what might be causing the hesitation, even with new plugs? Would cleaning the fuel injectors help? The fuel lines look like they could be replaced, but would that be a poential cause? Do you have good sources for fuel lines? I have noticed a pretty strong fuel vapor smell from the bike, but the fuel lines don't seem to be leaking.

I will check out the exhaust gasket and pipe, thanks for your thoughts!

-Just looked at the specs on the cables...they do have built in resistence.

The D8EA is wrong all round the K100 has D7EA with a restive lead where the 16v have a resistive plug, that is what the R in the plug type is for. The K1100 and K100 16v have a different end on the lead at the coil than the 8v engines so if you get them from EME specify that is what you need.
There are some fuel lines that will fuel to permeat through without actually leaking. You need the lines that are for submersion in fuel which is the same as in the tank I know NAPA has the correct lines but I don't know part numbers etc. Being an Aussie we often have different standards and so have different numbers.

more information about the surge would be helpful, providing you still have it after the plugs & leads are correct.
I for one can not tell if you are experiencing just a gentle slow down, starve for fuel or are having total intermittent loss of complete ignition which would be a lot more "stacotto like" in nature.
As you study & read more through the forum, you'll find many problems asked & addressed over & over & you'll get a feel for what is more/less likely.
Probably want to read this: http://www.verrill.com/moto/newkbike.shtml
first.

I had an issue with the external fuel lines and ordered new ones from BMW. They actually gave me a 1m length of the correct line and it was very reasonable. Just cut to length. But its a good idea to make them longer so you can raise the rear of the tank without removing them, means you can access electrics more easily, also coolant top up and earth connections under the petrol tank, and all with less risk of spilt fuel, fuel vapour and dirt getting in fuel lines.

Those fuel lines should really be replaced after no more than 10 years. They do let vapour escape when they fail. At 80mph you would be well fried if it ignited by the time you get to stop. It is really a safety issue as much as anything else. My new fuel line had the manufacture date stamped on it, as did the old ones which were seriously old!!

As far as I can see we seem to use the NGK D8EA plugs here for the 8 valve K100. I have been using them for 3 years with no problems. The difference between 7 and 8 is temperature only, but I take mine out occasionally to check and they are always the right colour. However I have not found anywhere that the D8EA is recommended, seems to be D7EA. I did try to change them but could not get the Bosch ones so decided to leave it alone.

Sounds a little like fuel starvation, also check fuel filter [inside petrol tank] and the in tank fuel lines. A failure in the supply side means you will lose fuel pressure as the fuel would simply leak from the supply pipe back inside the petrol tank. This might show in a visual inspection. A leak in the supply pipe outside the tank would most likely [but not always, as I know] be visible.

92KK