TOPIC:

Hello! I just bought a 1982 KZ750 Twin (CSR). When purchased I was told that there was a bad starter motor. The bike was able to start with the kick start. I pulled the old started motor off and replaced with another starter motor I bought off Ebay. The old starter motor was extremely damaged (pic below). I put everything back together and tried to start the bike with the starter however it doesn't seem to be turning over the engine. It sounds like the starter motor is only turning the chain and there is no movement elsewhere. Does this mean that the starter clutch needs to be replaced? Is there any way to diagnose that issue without pulling it apart. I am fairly new to motorcycles and have read that the starter motor clutch is a very involved repair. Any advice is greatly appreciated.

I attached a video of what it sounds like when the starter button is pressed. 

Did the missing commutator bars fall out when you took the cover off or had somebody been there before you? It looks to me like the commutator has centrifuged, this happens when the starter doesn't disengage and becomes driven by the engine (starter clutch fault) as the starter is not made to spin at several thousand RPM. I am guessing the previous owner knew what what is involved to rectify, hope you got a good deal 

Based on the info you provide and Wookie58’s assessment, I’d say there is a chance that somebody installed the starter clutch backwards, though I am not sure that is possible. However, that would explain the situation. With the clutch reversed, it would disengage when the starter motor is activated, then it would engage when the engine is running, causing damage the starter motor.

The factory service manual will be a huge help in what is to come.

www.kzrider.com/modules/ServiceManuals/K...l%20KZ750%20Twin.pdf

I need to spend some time and take the starter clutch out. I have a replacement - one came with the new starter motor. Will report back. 

Please remember that the flywheel / dynamo bolt has left hand thread.

Starter clutch replacement on the 4 cylinder kz750 is a bear but it should be relatively easy on the 750 twin. Youtube has a decent video.  Search youtube for " 1978 kz750 ebay bike rebuild part 1" which shows how and also confirms the left hand thread which Tex mentioned.

Also , I seem to remember hearing that on the 750 twin the crank rotates counter clockwise (backwards) like the 400 and 440 twins. Just something to check and watch for when testing.

Also wondering if possible to incorrectly wire a startor motor. Would it run the wrong way ?

good luck

JR wrote:

Starter clutch replacement on the 4 cylinder kz750 is a bear but it should be relatively easy on the 750 twin. Youtube has a decent video.  Search youtube for " 1978 kz750 ebay bike rebuild part 1" which shows how and also confirms the left hand thread which Tex mentioned.

Also , I seem to remember hearing that on the 750 twin the crank rotates counter clockwise (backwards) like the 400 and 440 twins. Just something to check and watch for when testing.

Also wondering if possible to incorrectly wire a startor motor. Would it run the wrong way ?

good luck


 

The only way to reverse a motor is to swop the polarity (electric windows, sunroofs, seats etc) but in these cases the motor isn't chassis earthed and the "positive" and "ground" run through the switch/ relay
In this case with such extensive damage it's a miracle that it didn't blow the battery up or catch fire