TOPIC:

Quck backstory on the bike, it's new to me early this spring, I don't get to ride as much as I like, so I'm still getting use to the bike to some degree. It's an '80 KZ550A, pretty much stock, stock TK carbs and airbox intact.

When I first got the bike it had been sitting quite awhile, I got it fully serviced, including carb sync, was running great. This is my first bike with a push/pull throttle, so there has been a little learning curve there, getting use to that set up.

What's happening now is...every couple rides or so, as I'm riding, the idle slowly starts to increase. It becomes noticeable and I know it's happening primarily during shifting. When I pull in the clutch to shift the idle doesn't die down as it's suppose to, it sort of revs up and makes shifting pretty clumsy.

So here is a typical several ride scenario. First ride, I start bike with the choke on, can usually turn choke off within a few minutes. Idle is a hair high until the bike warms up, but after 10-15 mins it settles down and seems fine. During all this time it's shifting fine, without the idle surge when clutch is pulled, including during those first 15 min before it's totally warmed up. I stop for an ice cream, the bike sits for 15 min. I start it up (no choke necessary), start riding, I notice during shifting I'm starting to get that little surge. In neutral at a red light the idle is sticking a little high, I push (or pull?) to close the throttle, once or twice, the idle drops back down after a couple pushes. After a bit more riding it's increasing. I pull over and adjust the idle adjust on the carbs, set it nice and low (my tach is not working so I can't give you a number, but it's low, almost sounds like a chugging twin, this is the idle where this bike performs best), now with the idle nice and low the bike runs and shifts beautifully.
A few days later I go for a ride and repeat all the above steps. So I'm constantly having to lower the idle. At first I thought the throttle cables were getting hung up, maybe the idle was set fine, but the cables weren't allowing it to return properly? But that doesn't jive with the continually turning down the idle, if it was the cables eventually they would return fully and the idle would be way to low, that has never happened.
Now I'm wondering, is it possible that the idle adjust knob is vibrating during riding and actually turning itself up? Maybe I need to put a second nut on there so I can lock it in place?
Anyhow, I"m stumped.
I do have two new throttle cables on order from MotionPro (had to be custom made), but not sure that's going to make a difference.

thanks for listening!

Sounds like a vacuum leak. You should check the carb holders for leaks. They will eventually develop tiny cracks in the mating surfaces between the holders and the cylinder head that can cause them to leak at idle. These cracks cannot be seen with the carb holders in place. I suggest you try the following. Get the engine warmed up and temporarily set the idle as low as possible without killing the engine (lower than 900 rpm if possible). The lower the better because with the idle very low the slides are nearly shut and the vacuum inside the carb holders is at its highest. Then spray carb cleaner around the carb holders, especially where they mate with the cylinder head and where the carbs attach to them. Carb cleaner works best for this test as it doesn't leave an oily mess with WD40 or evaporate so quickly that it is not drawn into the leaks as can happen with propane or starting fluid (ether). If the engine dies or reacts to the carb cleaner, you need to replace the carb holders. Carb holder leak related problems are noticeable at idle but generally don't have much impact when riding because the throttle is open. If you find you need to replace the carb holders soak the screws with Kroil for a couple days before attempting to remove them, and use JIS standard, NOT Phillips, screwdriver bits, and this will greatly improve the odds that you will not break or strip a carb holder fastener as so many have done. Let me know if you need info on JIS bits, I have a good link for making your own at no cost. Ed

Sounds like a air leak at the head. New carb holders?
Steve

Sounds like replacing the carb holders when I replace the throttle cables is probably a good Idea. I also want to remove the Air Suction system.

You won't regret it.
Steve

Agree with replacing the carb boots. They are relatively inexpensive so it's a good preemptive measure regardless on a 35 year old bike.

BTW, the "spray such and such on the boots and listen to idle changes" method of determining leaks is ineffective unless the leaks are dramatic. Typically the type of hunting and/or increasing idle when warm air leaks will not respond to such tests. It's best to just replace the boots unless they are soft and pliable and you can verify they are not cracked by flexing them back and forth with your hands while inspecting them under a magnifier light.

I got new boots installed, carbs cleaned and synched, air/emission system removed.
Good lord....what a difference. Bike is running on a whole other level.
I was happy with the bikes performance before (with the exception of the idle issue), I had no idea it could be this quick.

I have only had one bike. What is the difference to learn about two cable throttles ?

Hawghunter wrote: I have only had one bike. What is the difference to learn about two cable throttles ?

One cable opens the throttle and the other closes it.
Both work in conjunction with each other and are there to improve and provide reliable throttle action and response .
The close cable pulls the throttle shut rather than just relying on the return spring as any weakness in that area or stiction of the slides could prevent or delay the throttle closing.

The two throttle cable system is typically a denoted push pull system, which frankly strikes me as odd since there is no pushing involved. It should be called a pull pull system shouldn't it?