bike back firing and mid range rattle in the engine

[jakesussex]

help!! : got this k 75 running the other week with the help of a nice man in norway a few weeks ago. been running around on it getting use to it . and sorter mid range it has  a rattle if you gun it a bit , sounds a bit like the valves rattling about , i can ride around this noise changing gear and different speeds and what not , the engine is smooth otherwise , also when i shut off the throttle, the exhaust backfires and rumbles (which i quite like but i think may get bored with by the end of the year) . i did a ground wire bypass to the start relay to get the bike going>> maybe this needs fixing properly ,

    any help much appreciated

[Motorhobo]

What do you consider midrange? Throttle under 3k makes them groan -- that's the way they are. They like to accelerate at higher rpms.

[TimTyler]

Tune it up!

Check the timing, valve gaps, air leaks, fresh plugs, clean fuel, air filter...

[jakesussex]

thanks for response !
 i haven't looked at the reves its not labouring its more say top gear 60mph ish that sort of reves .
I've replaced the plugs,air filter,just put some fresh fuel in as it came with a full tank that was around 10 months old,but i only did about 5 miles with the new fuel, will have to look into how to check the timing.

 thanks !!

[jakesussex]

hello!!
 quick update the fresh fuel seems to have done the trick, its a lot smoother now but still a bit of a rattle , going to check the rocker gaps when i have some time , and I've found a bit of an exhaust leek at the header so think this is the back firing problem , other than that I'm a k convert even tho its done 85000 miles its a nice ride. although a bit hot with the rt fairing .roll on winter

[Motorhobo]

I have one that backfires and one that doesn't. The one that does has no exhaust gasket. The one that doesn't has a makeshift exhaust gasket made of 2 wraps of beer can aluminum with a layer of high temp copper RTV sandwiched in between. The OEM gasket is made of mica but it tends to deteriorate and won't survive multiple exhaust removals...that's why I did the beer can and RTV thing -- with 2 Kbikes to service it was just too expensive. If you have no gasket, use the mica one or at least something...an exhaust leak there results in backfiring...or at least that's my experience.

Btw, congrats on your conversion...once you've tasted brick you can never go bick...uh, I mean back.

[richarddacat]

When I first bought a K-bike in 94 my dealer told me the back-firing was all in my wrist, that I kept the throttle cracked just enough causing this back-firing.

Maybe a little of this but more so a TB adjustment after a valve check would fix it. Also as mentioned, vacuum leaks.

[roninvt]

My K75 will backfire if I chop off the throttle..I have learned to finesse it a bit and now it rarely backfires.  My K1100 has fever backfired

[gladstone]

My K1100LT used to backfire occasionally. I'd noticed it always ran a bit cool so changed the thermostat and the backfire stopped. Fuel economy also improved so I figured the cool running was causing it to run rich which caused the backfire.

Cheers

[Grim]

I had intake leaks after the throttle body. Fixing that leak fixed almost all ove my backfire issues. Any air leak on a k75 after the AFM is going to cause issues. 

[jakesussex]

thanks everyone bike does have a bit of a leak on the manifold so will get new gasket or one of my beer cans and see if that does the tick,its not running cold as i sweating like a rap...st in the weather we've been having in the UK at the mo.
the only thing i cant get on with is the big screen that blocks my view , think i prefer to let the flies splat in me face and feel the wind, may ditch the rt fairing and have it more naked ...

[pdg]

The header/exhaust union leaking can cause backfiring, but it's not guaranteed to be the cause. I have no gasket in mine and there is a very slight leak, but no backfires.

Backfiring on over-run can also be the 'throttle closed position microswitch sensor oojimaflip' or whatever it's called because I can't remember on a sunday night...

With the engine off, move the throttle slowly and just after you start to feel resistance on the twistgrip there should be a very slight 'click' from the area around the throttle bodies - then when you close the throttle again there should be another 'click'. If there is no click look up (more than likely on here) how to adjust the microswitch as if it's not working it continues to feed fuel in on over-run with the throttle closed, causing backfires.

Of course, you might not be closing the throttle fully when you are slowing down...

[jakesussex]

thanks PDG i will try that...