Hi, my recently purchased K75 backfires and quite loudly. I am concerned firstly for possible exhaust damage it may cause so I have stopped riding it till I can sort it.

I sourced a workshop CD manual which states adjustment needs a diagnostic scan tool which I don't have.

I researched the issue on forums etc and came up with solutions of TPS adjustment via multimeter use and listening for a click upon throttle movement down at the TPS.

I have checked plugs, valve clearances.

The bike is a 1992 model with 72000 kms with an excellent service history. The previous owner told me he learned to ride with throttle adjustments to avoid it. I imagine there is a way to eliminate these loud bangs. The occasional pop is not an issue.

I have simply adjusted the TPS so that now, as the throttle leaves the closed position I hear a click and as it returns it clicks too.

My question is can I cause any detrimental effect to the engine from adjusting this TPS without using the correct technique/tools?

Thanks Alf

I should add that the backfiring was as most have said, on deceleration. My ride home after purchasing the bike was 900 kms so as I came into towns etc from highway speeds I would simply slow with engine braking and it would backfire loudly usually once or twice at the most. Enough to let everyone know I had arrived.

Altorr wrote:
My question is can I cause any detrimental effect to the engine from adjusting this TPS without using the correct technique/tools?

Alf,

It should not be detrimental to the engine to adjust or mal-adjust the TPS.  The TPS tells the bike's computer when the throttle is fully closed and when it is fully opened, so that the computer can adjust the fueling.

Your backfiring is most probably caused by a leak in the exhaust, anywhere from the exhaust ports at the block to the collector pipe and possibly beyond.  Possible sources are:
- one or more leaking exhaust gaskets
- loose exhaust nuts
- leaking gasket at the collector
- crack in the header
- hole in the muffler

The first chore is to check the tightness of the exhaust nuts.  Careful not to overtorque them.
- if you feel so inclined, remove the header and replace all the gaskets in the exhaust ports.
Next chore is to remove the muffler from the header and replace the exhaust gasket.  You can use the BMW mica gasket (expensive and fragile) or use a muffler sealant on the joint.
Next chore while the muffler is off is to check it for damage or holes from rust, a very very unlikely possibility.

I imagine that you will have solutioned the problem if you follow these steps.

might pay to test that the switch you are hearing clicking.... is in fact working

I have been surprised by k75s a few times when decelerating (backfires )

do as rob suggests

but test the action of the switch as well

unfortunately I have not been able to play with the k75 to test if the tps in the closed throttle position and the switch inactive still produces the same back fire

I have a suspicion that like the k100 injection is turned off during decelaration and above 1300 revs and reintroduced as idle speed is attained when the switch is in the closed position

I wonder if someone has tried it ???

just some thoughts

Unplug the TPS and go for a typical ride, see what's changed, if anything. If there's a change it'll at least point to the TPS as a problem which perhaps can be remedied with adjustment. It doesn't hurt a thing to not have the TPS plugged in. Mine has been disconnected (on my K100RS 8V) for yonks.

Re TPS... read THIS

Technically, when the TPS is functioning you should get less (or maybe no) backfiring and farting out the exhaust as the fuel is cut with throttle off and revs above 2k.

If you consider your backfiring to be excessive and your TPS is set correctly then possibly the switch is not working. So as suggested by charlie above, it would be worth checking the switching with a meter.

But its still important to ensure there are no exhaust leaks. Exhaust manifold gaskets would be my guess as they don't last forever. They'll compress over time with expansion and contraction, so will need nipping up a few weeks after installation and again sometime down the track.

My 75S also has the backfiring as described on deceleration.  As it is a relatively minor thing I have not bothered to chase it down.  As per RobMac's suggestions, I would have expected a leak in there somewhere. 

Think of it as a safety device.  Something to wake up the dozey car drivers behind you.

I haven't come across a K75 that does not do it to some extent.

Mine burbbles but it doesn't backfire. It used to backfire terribly but I renewed the exhaust seals and applied a compound to the muffler/header joint, curing the problem.

I would say more a popping in the exhaust on overrun is what I refer to but not  a fully fledged gunfire type of thing.

Kaptain Holister wrote:Re TPS... read THIS

Technically, when the TPS is functioning you should get less (or maybe no) backfiring and farting out the exhaust as the fuel is cut with throttle off and revs above 2k.

If you consider your backfiring to be excessive and your TPS is set correctly then possibly the switch is not working. So as suggested by charlie above, it would be worth checking the switching with a meter.

But its still important to ensure there are no exhaust leaks. Exhaust manifold gaskets would be my guess as they don't last forever. They'll compress over time with expansion and contraction, so will need nipping up a few weeks after installation and again sometime down the track.

indeed nigel

but additionally consider that one doesn't leave the throttle closed at all times ...a small blip off the closed throttle (as is the case with normal cornering, speed correction  ) will introduce fuel again to ...relatively hot and fuel starved cylinders (remember the windmill effect on three cylinder 2 strokes used to melt the centre plugs literally)  the k75 uses a different firing technique ...but  I wonder if there is some residual wasted spark built into the ignition process ...would be interesting to see the waveforms from the ignition system in real time and the injection process ...a conundrum indeed and typical of a lot of k75s  with a higher compression ratio than all k100 8 valve units which also might be a contributing factor ...

forgotten when I heard my last backfire from gerty ...was so long ago ...as rob says burbles ..yes but backfires ..well almost non-existent since the refitting of all the exhaust gear up to new and tightened ...btw  my throttle position sensor has been disabled for almost this exact duration come to think of it ...