My KZ750 nearly got me killed today. Need Older Guru Help

Im 27. I got a1977 KZ750 Twin in great condition a few years ago. I ride it every so often and never let it sit for over a month.

Last month the coil failed because it wasnt properly attached, which caused it to overheat -  and the bike shut off while riding. I have since changed the coil.

After sitting for 3 weeks , today this happened:

1. Bike started shuddering while Im riding
2. RPMs climb to 2-3k when im holding the clutch or its in neutral
3. The clutch is hard to pull in. Not terrible but noticeably hard to pull in
4. At one point, the bike REALLY started bucking, like a horse, Im kidding, trying to throw me off while doing about 30mph
5. Stopped in city traffic and nearly had an accident. Was able to kickstart and get it home
6. Seems like turning the petcock on for a half mile, then off for a quarter mile and on again seemed to help. If I left on on "ON" it would stammer and shut off.

Does this sound like debris in the tank or carbs? Or electrical? or clutch? Battery is fine, charging while running (tested with volt meter) if fine.

Sounds like a float needle stuck open. 

ah. thanks. ive heard the carbs are easy to take off. I will have a go at them

Maybe first tap both bowls with the handle of a large screwdriver see if you can jimmy it free

Thanks. I will do that first thing tomorrow. But i think, if something is floating in there, might be best to take the carbs out and clean them?
 

Do you have an inline filter in the fuel line? If not it’s a good time to put on in. Make sure it for a gravity system. Automotive pressure type won’t work. Might get away with just pulling the bowls and cleaning them out. It will show you if you have debris. 

yeah i have no filter. need one

I agree with using an inline filter, it's cheap insurance. My KZ250 I had many years ago started having fuel problems after maybe 500km since I got it - turns out the fuel filter I added was already full with crud that would have otherwise made it into the carbs/engine.

Climbing RPMs in neutral is often a sign of an air leak. The rpm of an engine is really limited to how much air can get in more so than fuel. First thing I'd check is that your cables are all working fine, and that the throttle is snapping back when released. Then check your carb boots, they're the main cause for air leaks and do crack and leak over time. If spraying an aerosol over the boots (you could use propane gas or anything else flammable, just be careful) changes the rpms then that confirms a leak. A vacuum gauge is also useful for checking for leaks (and to make sure carbs are balanced).

I agree with all that has been said and all needs to be checked out.  But first I would make sure the vent on the fuel tank is not blocked.

never heard about this. whats involved?

i'll use carb cleaner as Ive heard about this approach to checking leaks. Let the bike run, spray carb cleaner on the boots. If RPMs decrease, its a carb boot leak. Propane would be safe, but thanks so much for all your advice

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But first I would make sure the vent on the fuel tank is not blocked.

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never heard about this. whats involved?

The cap on the gas tank has a vent to allow fresh air in. The idea is to allow air in to take the place of the fuel that's leaving. If the vent is plugged, a vacuum builds up in the tank and the gas stops flowing. But the tank isn't perfectly sealed, so turning the motor off and letting it sit for a few minutes equalizes the pressure and the bike would then start and run normally for another little while.