85' K100 Whirring. Also popping

[david32knyte]

Dear Forum

My '85 K100 every now & then develops this quite loud whirring sound - seems to be coming from the petrol tanks area.

Doesn't effect 'Performance' - then disapears by itself.
Seems to increase / decrease with revs, sometime disapears if i blip the throttle in neutral.
Been like this for a few years.

Thought it was the fan chaffing against something at first - but presume the fan is constant speed

Thinking perhaps it's something to do with fuel pump???

Any tips or suggestions gratefully received

David

[Laitch]

It sounds to me like the fan is being activated by increasing operating temperature then shuts down when temperature stabilizes. It's working like it should, but if it does this often when not in slow traffic and warm temperature, the coolant level could be too low in the system. If you don't know when the coolant was last replaced, or if it has been a few years since it was replaced, replace it. The BMW K100/K75 2-Valve manual downloadable from this site has instructions for doing that.

[Laitch]

What's poppin'? 

[david32knyte]

What's poppin'? 

Hi - Sorry forgot to describe that - loud pop or backfire from the exhaust on engine braking.
Usually just the one.

Usually happens about 5-10 mins after setting off - which makes me think fuel.

Thanks D

[david32knyte]

It sounds to me like the fan is being activated by increasing operating temperature then shuts down when temperature stabilizes. It's working like it should, but if it does this often when not in slow traffic and warm temperature, the coolant level could be too low in the system. If you don't know when the coolant was last replaced, or if it has been a few years since it was replaced, replace it. The BMW K100/K75 2-Valve manual downloadable from this site has instructions for doing that.

Thanks for your comments.
Whirring noise does increase as revs increase,  would this still be the fan? 
But i haven'y changed the coolant for er...a few years. It's true

[frankenduck]

What's the fuel level when the whirring noise occurs?  When the fuel gets low the fuel tank becomes a larger resonating chamber for the fuel pump.

Back when I started riding and did not wear ear plugs the whirring noise when low on fuel used to drive me batty until I understood what caused it.

[david32knyte]

Hi There

Fuel level is various.
This whirring is def a 'New' noise when it occurs, rather than a louder version of what was before.
Almost like the bearings in the fuel pump are intermittently loud? Or somthing similar.

[Laitch]

You seem to be stuck on the fuel pump; however, we both know a coolant change in the radiator and coolant reservoir is long overdue. That's because coolant doesn't change itself. Once that's done, the fuel cap assembly can be removed, the fuel pump's wiring checked for flaws, the pump's terminal connections can be checked for tightness and the pump's sound can be analyzed for changes while the engine is running.

You could do a check of the throttle switch to be sure it's set correctly. If it is set correctly, try to get in the habit of closing the throttle completely when decelerating to lessen backfire, but only one pop per engine brake event isn't really a big deal, unless it's rattling windows in the neighborhood.

[david32knyte]

Hi there
Thanks for your reply.
Yes coolant change is long overdue, it's true.

Also - The throttle switch  (Same thing as TPS? presume so) - I have never really looked at that or done anything with.
But reading comments on the forum I can see overrun backfires can be related to this.

So my plan, if I understand, is to change the coolant - then check the fuel pump wiring.
Then see if the noise goes away.

As for the solitary backfire - seem to remember that I changed the z pipe about 5 years ago and this did make things better.

(K100 is such an amazing bike it does tend to get neglected as a result.
I have other old classic bikes that need to be permanently pampered otherwise they have problems)

[Past-my-Prime]

(K100 is such an amazing bike it does tend to get neglected as a result.

Absolutely - a problem with a very well engineered machine is it tolerates neglect until all hell breaks loose! You take it for granted. Not a good idea with bikes or one’s spouse!!

Whirring and popping are the least of your problems.