My 85 K100 would only run when cold, which I just fixed by replacing the rubber on both sides of the throttle bodies (the old ones were disintegrated). (I also replaced all the fuel lines in and out of the tank, and replaced the fuel pump rubber boot that had just started to goo-ify).

Anyway I took it on a first ride and it ran great until the fuel got very low, then the bike backfired and ran rough. I pulled into a gas station and opened the fuel filler cap, and either vapor or smoke wafted out.

Anyone have any idea what that might be about? Could the fuel pump overheat if it's going bad? Or some kind of vapor lock if the tank isn't vented right?

I'm obviously concerned (and not riding it) until I make sure it's not something smoking in there.

If the fuel was very low then it will get very hot because that is the nature of the beast. Fuel vapor can look a bit like smoke so I would say that is the likely explanation.
However I would remove the pump and run it out of the tank for a few minutes but it needs the fuel to keep it from overheating. I will say that a bad pump that is getting hot could be a very dangerous thing to have in a fuel tank.
You can get pumps that do the job every bit as good as the original, the Mapco 22301 is a good choice and should be available for well under $100. I would be inclined to replace it as there aren't many auto clubs that will do a rescue from the moon.

shudder to think what it could be

failing pump ? drawing excessive currents through the..in tank wiring ?

investigate with due caution at least

and good luck

Is your K100 a naked model or an RS?  The fuel heats up in the rail and the pressure regulator and then goes back to the tank where it cools off a bit through the walls of the tank.

When the tank level gets low there is less surface area of the tank to allow heat transfer and less fuel to absorb the heat coming in so the temperature rises.  If it's a hot day the tank can get uncomfortably hot.  The fairing can cut down a lot on the airflow around the tank, especially around the lower parts where the gas is when the level is low.

The ethanol used in the fuel here in the states has a pretty low boiling point, and there have been reports of fuel boiling in tanks, and, on another forum,  even photos of the fuel bubbling in the tank.

First, do you have the tank insulation that is attached to the bottom of the tank?  This is critical to limit the heat coming into the tank from the engine. 

Second, what is the condition of the cooling system?  Bugs and dirt in the radiator fins cuts air flow, and the mineral deposits from using tap water coat the passages and insulate the hot coolant from giving up it's heat. 

Third, were you in traffic?  Does your fan work?  It doesn't take long for the engine to get very hot in traffic on a warm day, and if the fan is not working(common on older bikes) I wouldn't be surprised if your fuel got hot enough to boil.

Hope this helps.

Point-Seven-five wrote:Is your K100 a naked model or an RS? I wouldn't be surprised if your fuel got hot enough to boil.

Hope this helps.

 That was great info .75. As a K Bike newkid I'm trying to soak up as much info as I can.
Lots of good points that would take a life time to figure out on my own.

Thanks for the info everyone! It's a naked version with no tank insulation, and I haven't yet changed the coolant, so I'm pretty sure it's just getting hot in there. I'm going to put on some insulation and flush the coolant as a start. It also has the fan switch retrofit that lets you keep the fan on all the time, which I didn't have turned on.

Timz

Very important to have the under tank insulation in place.

There is also a plastic panel under the frame tubes on which the under tank mass of wiring sits, possible that may have been removed by a previous owner.

Make sure the foam strips for the tank to sit on are in place, they all act to keep an air space under the tank.

There is also some insulation that goes on the fuel rail and on the return pipe from the front of the fuel rail up to the tank.

Manual override fan switch is a great idea, had one on the LT and was especially nice in cold weather....!!!

Well there you go, the engine was designed to run hot with a fan that starts and stops automatically, but yours has been modified, I hope that you are more knowledgeable and less forgetful than the automatic system that BMW designed and fitted, otherwise you might end up doing some damage by not noticing the temperature. You could leave the fan on all the time and then suffer excessive engine wear because it never warms the oil enough.
I don't believe that the fuel gets too hot, I have seen very hot fuel in the aircraft that I have worked on most of my life and it is meant to be hot. It will never get hot enough to combust, it needs a spark or flame for that.

There is a product sold at Home Depot called Reflectix that works well for insulating the bottom of the tank. 

When you change your coolant disconnect one of the hoses and look inside.  It should be nice and clean.  Any mineral there means you need to do a flush with distilled water and cooling system flush.  I had to do one of my bikes twice to cure an overheating issue.

Check the fan for free turning by pushing the blades when the engine isn't running.  Then start the engine and let it idle for about 10 minutes.  At some point the fan should come on.  If not, the fan is probably bad.  The Bosch fans are notoriously bad with brush holders that melt and bearings that sieze, and many owners replace them with a unit made by Spal when they go bad.  The retrofit is pretty easy and uses the old fan bracket.

BobT, I suspect that the fan switch the poster refers to is the manual override that many brick owners install, especially when they ride in warmer areas.  It doesn't replace the thermo switch, but just allows the rider to turn on the fan when stuck in traffic before the temperature gets too high.  It also allows the owner to execise the fan motor from time to time to prevent bearing seizure.

BobT wrote:Well there you go, the engine was designed to run hot with a fan that starts and stops automatically, but yours has been modified, I hope that you are more knowledgeable and less forgetful than the automatic system that BMW designed and fitted, otherwise you might end up doing some damage by not noticing the temperature. You could leave the fan on all the time and then suffer excessive engine wear because it never warms the oil enough.
I don't believe that the fuel gets too hot, I have seen very hot fuel in the aircraft that I have worked on most of my life and it is meant to be hot. It will never get hot enough to combust, it needs a spark or flame for that.

BobT

You are right about that.

The fan override switch is not and should not be a replacement, just means you can turn on the fan when you want. It can't be one to replace the sensor and automatic switch on because most K riders are a little older and forgetful.....My LT retained the normal fan operation.

The override is also a good way of testing the fan is actually working and making sure it gets some exercise in cooler areas rather than being left to seize from lack of use.

BobT wrote:Well there you go, the engine was designed to run hot with a fan that starts and stops automatically, but yours has been modified, I hope that you are more knowledgeable and less forgetful than the automatic system that BMW designed and fitted, otherwise you might end up doing some damage by not noticing the temperature. You could leave the fan on all the time and then suffer excessive engine wear because it never warms the oil enough.
I don't believe that the fuel gets too hot, I have seen very hot fuel in the aircraft that I have worked on most of my life and it is meant to be hot. It will never get hot enough to combust, it needs a spark or flame for that.

The fan doesn't control the engine temperature, the thermostat does.

Avenger GT wrote:

BobT wrote:Well there you go, the engine was designed to run hot with a fan that starts and stops automatically, but yours has been modified, I hope that you are more knowledgeable and less forgetful than the automatic system that BMW designed and fitted, otherwise you might end up doing some damage by not noticing the temperature. You could leave the fan on all the time and then suffer excessive engine wear because it never warms the oil enough.
I don't believe that the fuel gets too hot, I have seen very hot fuel in the aircraft that I have worked on most of my life and it is meant to be hot. It will never get hot enough to combust, it needs a spark or flame for that.

The fan doesn't control the engine temperature, the thermostat does.

Mmmm, I wonder why they bothered fitting the fan then?

BobT wrote:

Avenger GT wrote:The fan doesn't control the engine temperature, the thermostat does.

Mmmm, I wonder why they bothered fitting the fan then?

Another tragic example of a lost memo.

The fan provides airflow through the radiator when the bike isn't moving.  While it promotes cooling, it doesn't control temperature. 

That is the function of the thermostat.  Hence the name: 

Thermo=heat/temperature

stat=static/unchanging.

Point-Seven-five wrote:There is a product sold at Home Depot called Reflectix that works well for insulating the bottom of the tank.  

Any tips on the best way to affix Reflectix to the bottom of the tank? I have reflective metal tape, could I tape it on? Or best to use an adhesive?

timz10000 wrote:

Point-Seven-five wrote:There is a product sold at Home Depot called Reflectix that works well for insulating the bottom of the tank.  

Any tips on the best way to affix Reflectix to the bottom of the tank? I have reflective metal tape, could I tape it on? Or best to use an adhesive?

Tape it on with what you've got. I used poly vinyl double-sided tape because I had some. Yours will work.

My thoughts are that adhesive tape will fail due to the heat and moisture. The insulation is factory fitted with contact adhesive. Comes in small tubes pretty cheap.

When I made my "Johnny Blanket" I just draped the Reflectix over the top of the frame starting at the steering head and running back to just past the battery.  I put the tank and seat on, cutting holes with a utility knife for mounts.  I had it go down behind the side covers.  Trimmed it so it didn't show.

When it was all cut out, the tank and seat held it all in place.

Ran like that for two years until I installed a fuel cooler and didn't need it anymore.

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