TOPIC:

Hey.
I just bought a Kawasaki Z750 L4 from 1986.
The engine works nicely. Unfortunately, I have to check why the idling speed suddenly accelerates.
Through the winter, I want to give her the original paint and add the missing elements: reflective lights, side badge, etc.
I will be showing progress on the forum

The engine is sucking in false air through cracks in the rubber carb boots that attach to the head. You can source those boots through OEM Kawasaki part dealers.

Fantastic looking bike by the way. Nice pickup!

Nessism is correct.

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 sometimes 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 like 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

650ed wrote: Nessism is correct.

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 sometimes 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 like 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

Ed,
I've seen you promote the "spray stuff on the boots" trick to find air leaks, but I can tell you for a FACT that this technique does not always work. Yes, it will find the huge air leaks but not small ones which can cause idle drift.

Anytime you see cracks in the boots or if the boots are hard and unpliable they should just be replaced straight away. We are talking about 35+ year old rubber parts that are critical to the function of the bike. I say just replace them and be done with it.

I don't disagree that the spray test may not always work, but it definitely will expose large leaks.

On my KZ650 the outer surfaces of the original (28 year old) carb holders looked like they were perfect, but the spray test actually caused the engine to stop running. That was prove enough for me that the carb holders were leaking. When I removed them I found many very tiny cracks in the mating surfaces, and that's where the air had been leaking in causing erratic and high idle. After replacing the carb holders my bike idled like it did when it was new.

If the OP's carb holders are original I do agree that they need to be replaced since age and the heat of the engine will take a toll on the rubber. However, I didn't see any indication that the carb holders were original, so if they have been replaced at some point they may or may not be the source of the problem. The test I described may not be perfect, but it can show whether or not they are leaking badly. Ed

650ed wrote: If the OP's carb holders are original I do agree that they need to be replaced since age and the heat of the engine will take a toll on the rubber. However, I didn't see any indication that the carb holders were original, so if they have been replaced at some point they may or may not be the source of the problem. The test I described may not be perfect, but it can show whether or not they are leaking badly. Ed

The photo I posted showing cracks was taken directly from the OP's photos. The boots are cracked and should be replaced straight away.

Wow, I didn't realize the photo was of the OP's carb holders! I agree wholeheartedly that those carb holders are totally shot and should be replaced. Ed

Thanks for all the comments.
I did not notice this crack. I'm blind
It's still warm outside, so I'll check what you've written.
Thanks again.

Curiosity:
Carb boots in Polish is "Króćce gaźnikowe" very rare word

PS. I bought some good, for very little money