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Discussion Starter #1
Hello,
I have a GPZ400 (ZX400C) from the year 1992 (63000km) with a shifting problem. When the engine is cold, it shifts fine, but after about 10 minutes, when the engine is at operating temperture, it is not possible to change gear.This occurs in any gear. (Is it normally not the other way around?)

A while ago, a (professional) workshop has changed the shifter (external) due to a fall a the parking lot (not running engine). This workshop does not seem to be able to assess this newly arised problem. As it was my brothers bike, and he was fed up with the problem which could not be sorted, he gave his bike to me so I could have some tinkering fun.

Before I begin to get the engine out and to split the carters and such, maybe someone of you already had the problem?
I had some comments from people I know (such as worn dogs, clutch problem, engine oil), but they never had such a problem, or have know anyone with the problem.

Thanks for your helping me out.
 

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Does the motorcycle continue to move in whatever gear it is stuck in?
Does the shift lever move thru it's full arc when it will not shift or is the lever jammed?
As long as the clutch is transmitting power it does not matter if it will not disengage, trans. would still work as long as the throttle is rolled off to break torque.
Damaged gear dogs would not prevent gear selection and you would not have bad dogs on every gear set. Damaged dogs usually show up as popping out of gear.
More info would help but at this point I suspect the tip over that damaged the lever also damaged the mechanism that transmits motion from the shift shaft to the shift drum. This is generally accessible without splitting the cases. Or you have a bent shift fork which is a split the case job. A more detailed description of what is happening would help.
 

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Discussion Starter #3
Does the motorcycle continue to move in whatever gear it is stuck in?

Yes, once in gear, it continues to move without a problem. But the gear remains stuck


Does the shift lever move thru its full arc when it will not shift or is the lever jammed?

The lever moves thru its full arc and is not jammed when trying to shift. The lever mechanism was revised by the workshop (because it was thought to be the root cause of the problem, hence it is not).


A more detailed description of what is happening would help.
  1. When cold, the motorcycle starts fine without any problem. I am able to shift from neutral, to first, second, third, fourth gear.
  2. After a while (about 5 to 10 minutes), when I want to shift backwards (e.g. from fourth to third), I am not able to do so. I am able to move the shift lever, but the gear does not shift and remains.
  3. When shutting down the engine, the motorcycle remains in its last gear. After a while (about 10 minutes), I am able to manually shift the gear back to neutral.
  4. When starting up the motorcycle again, the problem reoccurs (but now faster than the first time: after about 2 to 5 minutes)
Thanks for the answer.
 

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Interesting problem. Can you measure the stroke distance of the shift lever and compare the stroke distance when it is shifting correctly and when it is not?
Also what oil are you running? I presume you have checked the oil level and that the oil has been changed on a regular basis?
 

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It appears that your problem is related to heat expansion. In addition to what I said earlier, there is also the possibility of a bent shift shaft. Many times when someone asks for help they do not understand the importance of specifics. When the motorcycle fails to shift does the lever move freely thru it's full arc? Does it fail to shift both up and down? Is it consistent regardless of what gear it was in when the problem occurred. For instance does it act the same if the problem appears in fifth as it does in third? Though the transmission is from an engineering point of view quite clever, from a functional point of view it is rather simple.
 
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