K75s won't rev, sputters, then is suddenly perfect again

[keloyd]

Howdy,

I've had this problem a few times now that I've had the bike 3 months.  It starts fine, revs fine, everything is fine, then all of a sudden, maybe after hitting a bump in the road, it refuses to rev at all.  It idles.  If I turn it off, it starts normally, but it refuses to go above ~1500 rpm no matter how much I twist the throttle.  In fact, giving it lots of gas makes it sputter and backfire a bit.  It is as though it had a governor that let it run perfectly in that narrow 900-1500 range and nothing else - just enough to creep home 10-15 mph in 1st or 2nd.  THEN, after 1 to 15 minutes of this nonsense, it instantly is back to normal. 

After some poking around, the only thing that looks wrong is a fuel pump that the previous owner claims he replaced - it is loose in the tank.  I can push it back into its little hole with a long screw driver, or lift it up again.  At home, I cannot get it to make the symptom by playing with the fuel pump, but maybe it just takes a while for air bubbles to get where they need to go.

What else could it be?  Do I have any hope of fixing, or paying a mechanic to fix something that goes wrong once every 10th time I go ride?

Regards,
Keith

[keloyd]

a few other details - it's a '94, 58k miles, starts the first time, every time, even when it's in this 'limp home mode', and this has happened on both a hot day and cold morning, 4 times total now that I've done more remembering.  Also, there's a bit of a clattering noise when it's refusing to rev, but that just may be from being in first gear and getting little blasts of propulsion, then it cuts out, the pushes again, then cuts out again, every few seconds.  Surely it's not a slipping timing chain/belt or something expensive like that.  If so, could it slip back into its right place?  As irritating as this symptom is, it has always returned to normal again.

[wmax351]

sounds like its your throttle position switch. Thinks the throttle is closed, and cuts fuel above 2k rpm or so. Try unplugging it. The connector is aft of the throttle bodies.

[Chaos]

A sticky tps would do that, you should be able to hear it click with the engine off when you just start to open the throttle.  Unplugging it won't hurt anything but the engine will will backfire more on de-accelleration.

[keloyd]

Wow, that is (1) effortless, (2) costs nothing, and (3) the bike is running right again, though the problem was intermittent, so I can't be certain from one quick trip.

I was bracing myself for 'your timing belt/chain is skipping' or 'something internal that costs $1000 and will take 3 weekends to get at'.  Thanks for letting me pick your brain and I will update in a few weeks if it acts up again or remains in good order.