I'm new to both motorcycles and carbed engines but I've read about this a bit.
I've got an issue with my bike where it will start with no choke and idle for a few minutes. After that, the idle will increase to gradually to 4000. I can get it to return to 1100-1200 by putting the choke on and then taking it off. The idle will increase shortly after the choke is taken off until it gets back up to 4000. The bike is mostly stock, but has pod filters installed in place of the stock airbox.
edit: I went out and inspected the petcock vacuum line. The line is in good condition. The material is pliable w/ no cracks or other damage to it. The petcock pulls vacuum off of two carbs. 1 and 3. #2 has a line on it but its clamped off with a bolt in the end. I guess the PO did this when he installed pod filters.
I've read about this sort of issue on the forum and tried most of the diagnostic steps.
1-The throttle snaps back and the racing idle doesn't go away no matter the position of the handlebars. I don't think the issue is cable binding.
2- Vacuum leak at the carb boots. I've sprayed WD-40 around the boots and detected no drop in RPM.
3-Advancer. I disassembled the advancer and checked that the weighted arms move freely. They do. I lubed the arms and reinstalled. Started bike and issue remained.
4-I recently replaced the petcock diaphragm because it was weeping fuel from the side when on prime. The oil also smelled like fuel when I changed it. I attached the vacuum and fuel lines to the correct spot as near as I can tell. Big line to big fitting and small line to small fitting. When i was checking the carb boots, I also sprayed WD40 on the vacuum line fitting but no change in RPM occurred. The clamp looks like its just bent heavy gauge wire so that could be the issue. Could it also be a torn diaphragm or a bad connection?
5-I have not cleaned the carbs but the PO used it as a daily ride until he upgraded to a big yamaha(v max something or other). In another thread, someone mentioned that racing idle is related to a dirty pilot circuit. Because there was oil in the fuel, I also suspect that the float needle was not seating itself properly. Could that be the cause of my issue? I tried the 'put the bike on prime and tap on the float bowl housing' thing a few times and that hopefully flushed the debris out.
I'm hoping that its 4 or related to it. If the lines were hooked up backwards, the bike wouldn't start, correct? The vacuum line only comes off of one carb if I understand what I've read. But, a bad connection, either at the carb or the petcock OR a torn diaphragm would lead to a racing idle condition if I understand what I've read.
for what its worth: there is only about a gallon of gas in the tank.