Starter clutch

My bike is a 1978 KZ650B, when I try to use the electric starter, it tries to turn the engine over but won’t, if I pull the spark plugs out, it turns it over fine. Is the clutch the problem?

Can you hear the starter motor turning only the engine won't spin?  If so, that sounds like the starter clutch.  When that happens a funny noise usually occurs at the same time.

Just in case. make sure the battery is good & fully charged. you never know sometimes. 
I check the battery w a digital multimeter & do a high beam load test. Ctek US 0.8 smart charger & VC97 meter.

ps: my scorpion AGM battery tested 12.6+ VDC after charging the other day but wouldn't start the motorcycle. it was bad. voltage dropped below 10 VDC when pressing the start button.

If it is a case of the starter turning very slowly, or not turning the engine at all, it could be a weak battery or poor electrical connections. Much less likely, but still possible, is the starter motor may need repairing.

If as Nessism said, the starter motor is running and there is a grinding or rocks rattling in a can sound, the starter clutch is the likely culprit.

To test the battery, connect your volt meter to the battery terminals, not the cables. With the key off, note the voltage. Ideally, the reading should be 13.2. If it is less than 12, the battery is undercharged, or bad.
Turn the ignition key to the on position. If the reading falls more than a few tenths, there is a bd connection or the battery is bad.
Now press the starter button and note the reading. If the reading drops into the single digits, the battery is bad.

Definitely make sure the battery is fully charged before testing.

Yes, the starter will turn the engine over if I pull the plugs out, but won’t with the plugs in. It tries to, but makes a funny noise and just doesn’t turn the engine over. The battery is fully charged and has 13.4 v. Doesn’t drop much when turning engine over without plugs. So, I too believe it is the starter clutch. I don’t know much about motorcycles, but I am an auto mechanic, so I do have mechanical skills, but motorcycles are new to me. I bought the bike and it wasn’t running. There is some wiring problems and burned out coils, also I think the ignition modules are bad. Not sure, don’t know how to test them. They are Dyna S. I got most of the wiring figured out with a wiring diagram. There is no turn signals, no fuse box, no relays anywhere on the bike. It has been “chopped” into a bobber style. I need help to get it all back to street legal.

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Yes, the starter makes a funny sound when I try to start it and won’t turn the engine over. It will when the spark plugs are out just fine.