violent flat spot and missfire

[Mr andy spanners]

hello all, maybe some clever person can shed light on my problem

86 k100, bike has 72k miles and was running fine.

I haven't owned the bike for long but it has been running fine.

last week I took it out for a quick spin and noticed that it has developed a very bad flat spot around 3k rpm, basically under full load the power drops like a stone the snaps back very violently, in fact so violently that in first gear it lifts the wheel.

the bike revs cleanly in neutral and ticks over fine but this flat spot happens in every gear at the same rpm

I have recently replaced the plugs with ngk versions and I have ordered new leads.

I have also noticed that the airbox has a significant amount of oil in it, so much so that when I remove the afm the oil spills out! id say there is a good 1/4 of a cupful. the oil level seems ok, is this normal

my next port of call ref the misfire is the hall sensor, are these still available new?

any help would be welcome

[mark_r]

Sounds similar to a problem I had. I found some cracks in the rear crank case breather pipe so decided to change all the breather pipers and inlet rubbers while I was there.
Cured.

[K75RT Keith]

Have to agree with Mark R.  Check for cracks in the rubber inlet tubes, vacuum port caps by the fuel rail and the crankcase vent hose at the rear left (injector/spark plug side) by the airbox.  If the Z hose used for crankcase ventilation is cracked or split even by the clamps it really messes performance.  To learn how to check for air leaks on the Inlet tube or the vacuum port caps check the Liberry section of this site.   

[Elipten]

The bike is 29 years old, need to replace all rubber parts.