My father was notorious for improving - getting more speed- out of his Motorcycles.  He passed his 1985 K100RS on to me.  At the time he de-tuned it to slow it down. I rode it a total of ten times before parking it - The problem was as i was on the way home from work i saw a puff of smoke - behind the wind screen. I leaned the bike over just in time to see a drop of fuel come out of the coupling to the air box and hit the hot engine igniting - I decided not to be that flame on the road-  Seven children and 16 years later i am cleaning her up. has any one had this problem?  No, it did not come from the tank i checked it first.

Check the timing (the position of the HES). If it's too far advanced it might spit fuel backwards, but what you say also suggests you are going to need to replace the z-tube, all three vacuum covers (cyls 1- 3), the vacuum tube (cyl 4), inlet rubbers and manifold rubbers.

Dai wrote:Check the timing (the position of the HES). If it's too far advanced it might spit fuel backwards, but what you say also suggests you are going to need to replace the z-tube, all three vacuum covers (cyls 1- 3), the vacuum tube (cyl 4), inlet rubbers and manifold rubbers.

If it is 85 you may have a vacuum sensor, another vacuum tube goes from No 1 around the front of the airbox, the sensor is mounted beside the fuel pressure regulator. Replace it with a cap [vacuum cover] same as 1-3.

But I am a devil and going to say replace the fuel lines because it could have been coming from a cracked line as they are all under pressure and bouncing off the air box.

Does the bike have the CA tank venting system? ........ I am not familiar with it but I have an idea the gas tank vents into the air box....possible the pipe inside the tank is set damage and too low and its acting as an 'overflow'

Moved to a more relevant section
Rick G

A puff of smoke? A drop of coolant, grease or oil will smoke if heated enough. A drop of gasoline evaporates; gasoline vapors flash and flame when ignited, not smoke. Always taste whatever leaks to make a positive identification. If it tastes like chicken, check for broth leaks.