I have tried holding in the clutch, but it didnt have any effect.
Some more background information:
I washed it with a pressure washer after I got it and used three plastic bags to cover the surge tank and the sylinder head exhaust ports, because I had removed the mufflers and exhaust system. After it had dried, we put in the battery and tried to start, but it didnt start, however, the starter seemed to be working well. Then we pulled the plugs, cleaned them and checked the spark; everything was working there. We also checked the fuses and the resistance of the coils and ignitor (its readings were somewhat different from the manual, but it was from a working 86 bike). One of the coils showed 0.2 ohms more resistance, however, I was getting a spark at every plug (blue and strong). Then we recharged the battery, and the engine started on the first try. I went on to warm it up for a little while and put in the first gear and drove for about 10 meters when the engine died. After that it wouldn't start anymore.
We assumed it was a carb related problem and drained them; some gas came out. Then my brother and I started to disassemble the carbs and he found he had left one of the plastic bags in (the one covering the surge tank) and it was sucked into the air ducts. We pulled it out, and assembled everything and tried to start it, but it then the starter motor didn't seem to be working anymore. This is where we are now. So, we actually got it to run well for a while, with the plastic bag in the air ducts, but now it seems that the starter motor is not working proprly. In addition, I cleaned the petcock and found out it was missing the vacuum valve spring, but i replaced it with another spring (not original, will try to get one though; it worked previously with this spring, so I don't believe it is a petcock/fuel supply problem).
I am now going to start taking apart the starter motor to check it, but other than that, and going through the carbs, I don't know what else to try.
I appriciate the advice from everybody.