I have an 85 K100RT.  Sat for six years before I bought it so I cleaned everything the fuel system interacted with including professional injector cleaning.  Seems like #2 cylinder does not fire all the time.  At idle, I can pull the injector connector off and no noticable drop in RPM.  Pull any other injector connector and RPMs drop.  Swapped injectors (2 and 3) with same result.  Checked, cleaned, and swapped plugs.  Same result.  If I go out on hard ride, the #2 pipe at the head is as hot as the others.  Go on an easy ride and it seems like #2 pipe is cooler than the other three pipes at the head.  I test this b wetting a paper towel and pressing it against all four pipes.  I do get a bit of backfiring occasionally particularly on downshift.  Seems a bit rougher between 2500 - 3000 rpm.  Above 4000 rpm, I roll on the throttle and it runs quick.  I have run three tanks of gas through including seeking out non-ethanol gas.  LOTS of new fuel parts given it sat.  Fuel pump, filter, lines, etc.  Fuel rail is clean and clear.  About the only fuel part I did not replace is the pressure regulator but, if that is the problem, it seems like it would effect all cylinders and not just #2.

Any ideas?  A bit stumped.

Plug lead or spark plug?

Would guess a intake leak at #2.

Stupid question but some have been here before.
As Inge says, could be an air leak.
Is the small, rubber blanking cap on the throttle body, where you would attach a vacuum pipe to synchronise the TB's?
I had a similar problem and discovered that I had put the rubber cap on one of the threaded TB linkage adjusters.
Took me a while to suss it. Was blaming duff plugs.
Not the only one here to do it either!

Inge K. wrote:Would guess a intake leak at #2.

+1 I would be looking for a leak in that area.

Three rubber caps are in place on 1,2, and 3.  Hose from 4 goes to pressure regulator.
Swapped spark plug wires for 1 and 2.  Same result.  Seals from throttle body to 
all cylinders looks good.  Rotated injectors 1 and 2 in their seats to make sure no leak there.  
There are new seals on all injectors.  Took it for another high rpm ride and #2 pipe is hot 
meaning I am getting firing in #2.  
But again, at 1000 rpm idle, I can pull injector wire off #2 without drop in rpm.  Pull any
other and rpms drop. 

Where are common culprits for intake leaks if throttle body hoses are not cracked and
appear in good shape?

There are a few places you can get a leak but if its just one zylinder then it'll be just the injector o-rings, vaccuum take-offs and TB rubbers. Spray some brake cleaner or similar around the TBs while its running. Revs will pick up if there's a leak.

I would've thought swapping SP leads #1 and #2 would make a noticeable difference because they fire alternately off different coils. Neither would have been firing at the start of their combustion stroke. Are you sure you have the HT leads connected correctly?

Have you done a compression test? Could be a valve guide.

Completely swapped the SP cables plug to connector.  I didn't just pull them from plugs and swap.
Trying to see if the plug wires had a short.  Also "massaged" the injector wires to make sure I didn't
have a broken wire.

I'll replace injector o-ring (again) and try brake fluid on TB rubbers.

Not brake fluid. Brake cleaner.... alchohol type stuff that will combust. Starter spray we used to spray down our carbies to get her going etc etc. In Oz we have a product called 'Start Ya Bastard'

the common culprits are the rubbers between the head and the throttle bodies ....

many have refitted them down after finding a leak in the rubbers at the head interface

sometimes a good sealant (petrol resistant ) can aid in this task ...as the rubbers distort slightly with age ....also the rubbers above the throttle bodies have been known to deteriorate ...many not being found until a removal process ...they tend to crack around the circumference of the rubber as the rubber levels change ie at the clamps locations

this can let unmetered air bypass the afm unit ....and additionally run very lean on those particular cylinders affected

but to me sounds like you have a couple of issues

a tight shim possibly on the outlet of number two ...and possibly a rubber leak

just my thoughts

it isn't hard to inspect the gaps on the shims

and usually if you remove the airbox you can see the reverse sides of the throttle body linkages and rubbers ...commonly a stain is evident on the head where the leakage has - is occurring ....the upper rubbers is harder to detect but the brake cleaner method (or propane gass ) of spraying at the rubbers whilst engine is running and noticing if you get an increase in revs ,...caution this stuff will catch on fire if you get a backfire ...so all the usual precautions apply .... have an extinguisher handy if you try that method .

good luck

Thanks, guys.  Used some electrical connector cleaner (ethanol based).  Some quick shots at the
TB and head and revs picked up for 1, 2, and 3 cylinders.  Looks like I have some top end work to do.

BTW...I read brake cleaner and typed brake fluid.  I knew what you guys were talking about.

There's nothing like a definitive result! It is amazing what a new set of intake rubbers will do for an old K100. The tingly vibes at motorway speeds can be all but eliminated when this task is completed, along with a proper TB sync, valve clearance set to spec, tyres perfectly balanced, wheel bearing replacement, fork oil level correct, steering head bearings at the right tension, thick hand grips and massive bar end weights, a stouter seat cushion, keeping your knees off the tank while riding, and, gritting your teeth at 3500 to 4200 rpm.
:silent:

But we love 'em anyway!