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Z400 spitts back through carbs 26 Feb 2017 01:33 #755238

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Hi, I just got myself a Z400-R1
It had a bad valve, so I have now replaced it with a brand new. I have just got the engine together and back in the bike. Now it starts and runs.
But it spitts and coughs back through the carbs causing it to almost stall.

What could it be?

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Z400 spitts back through carbs 26 Feb 2017 01:47 #755240

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Do you have a FSM?

Check the camshaft timing, valve clearance and ignition timing.

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Z400 spitts back through carbs 26 Feb 2017 03:30 #755241

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Do a compression test.

A new valve on an used valve seat can be leaking if not lap to a good valve seating surface.

This can leaking gas (spits back through carbs).

www.kz400.com/WorkshopManFrameset-5.html

www.kz400.com/Workshop%20manual%20bilder...0man.%20p156-170.pdf



This is the tool i use for resurface valve seats.

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Z400 spitts back through carbs 26 Feb 2017 08:15 #755251

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Does the spitting back through the carbs only happen at or near idle speed? If so you need to check the carb holders for leaks. Leaking carbs holders will definitely cause that. Ed
1977 KZ650-C1 Original Owner - Stock (with additional invisible FIAMM horn)
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Z400 spitts back through carbs 26 Feb 2017 08:45 #755254

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Is the airbox installed w/filter, or is the bike set up for pods. And if it's pods are the carbs jetting properly? Also, how long has the bike sat with the carbs off and are they properly clean?

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Z400 spitts back through carbs 26 Feb 2017 09:09 #755256

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An imperfectly sealed intake valve that leaks when it's supposedly fully closed will allow some of the combustion within the cylinder to escape through the leak and go out backward through the carb.

The leak could be due to various causes, such as too tight valve clearance or a defective valve head (burnt, cracked, or needs re-facing) or a defective seat (burnt, cracked or needs re-facing) or a bent valve or other reasons.

If the valve leakage isn't due to a defective component, the usual remedy is a re-grind and setting correct clearance.

Good Fortune! :)
1973 Z1
KZ900 LTD
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Z400 spitts back through carbs 28 Feb 2017 01:10 #755411

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Alright.

I used lapping paste when fitting the valve and after that i filled the chambers with fuel to se if there where any leaks. None that I could see.

I Have standard filter, I want pods later but first I need to get the bike ready for the road standard style.

But I think I have solved the coughing bit :P
The spark plugs where totaly wrong! One whas a B6S and the other where of another make but similar lenght. The right one is B7ES witch are a little colder and double the thread lenght!
With those in place it ran fine!

But only with the choke! If i turn the choke of, it revs uncontrollably by it self! What can that be?!

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Z400 spitts back through carbs 28 Feb 2017 05:10 #755422

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Sounds like you have an air leak. Commonly caused by leaking rubber carb boots which attach to the head or maybe at the back of the carbs feeding air from the airbox.
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