Small back fire at strat up

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Small back fire at strat up

10 Jan 2025 12:46
#907705
 My KZ900 cylinder # 4 has small backfire at startup and disapere when warm up. 
I just cleaned the carburetors, ajust the floats level and check the vacum with a carbtune pro. Note: I had the same issue prior)

What you guys think ?



 
Kawasaki:
S1 250 triple 1973
H1, 500 triple 1975
LTD 1000, 1982
Ninja 1000R 1987
Ninja ZX12R 2006
KZ900 1976.
Yamaha RD 350 1973
Ducati 1098 2007
Harley Softail 2009 For sale !

Mechanic at:
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Kawasaki Center.
Pont Viau Motorcycle.
Chabot Marine & Sport.
Honbec Mtl.

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Re: Small back fire at strat up

10 Jan 2025 13:04
#907706
With all of the machines you list and the experience as a wrench turner I am not sure I will be able to tell you more than you currently know. But I do have a suggestion or 2 which may or may not make any sense. That is just me.  But, when you say backfire what do you mean? Is there a flame coming out the air intake of carb or is it making a noise that you do not think it should. I may not be correct, but as I understand it a backfire means an intake valve is part open when a spark or heat make the air.fuel mix fire. If after cleaning and setting up the carbs did not solve the issue I would look at the valve settings and the ignition as a place to look for solutions.
1977 KZ650 B1
Pods and Denco header

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Re: Small back fire at strat up

10 Jan 2025 13:16 - 10 Jan 2025 13:18
#907708
gd4now wrote:
With all of the machines you list and the experience as a wrench turner I am not sure I will be able to tell you more than you currently know. But I do have a suggestion or 2 which may or may not make any sense. That is just me.  But, when you say backfire what do you mean? Is there a flame coming out the air intake of carb or is it making a noise that you do not think it should. I may not be correct, but as I understand it a backfire means an intake valve is part open when a spark or heat make the air.fuel mix fire. If after cleaning and setting up the carbs did not solve the issue I would look at the valve settings and the ignition as a place to look for solutions.
I would tend to agree that it sounds like a sticking valve or bucket (had this with a V6 Audi that an "oil treatment" additive rectified)
1982 KZ1000 Ltd
www.kzrider.com/forum/11-projects/617631...-82-begins?start=192
kzrider.com/filebase-alias?view=download...d-fault-diagnosis&ca
Last edit: 10 Jan 2025 13:18 by Wookie58.
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Re: Small back fire at strat up

10 Jan 2025 13:39
#907713
I'm not even sure if I can call that a back fire, it's small detonation that I can ear and feel at the muffler. I had the valve clearance in mind too but wondering why it is only before it's warm up.

I was also thinking that the starter plunger could be the cause but I will check the valves first.

Thanks for your reply !
Kawasaki:
S1 250 triple 1973
H1, 500 triple 1975
LTD 1000, 1982
Ninja 1000R 1987
Ninja ZX12R 2006
KZ900 1976.
Yamaha RD 350 1973
Ducati 1098 2007
Harley Softail 2009 For sale !

Mechanic at:
Daytona Motor Montreal.
Kawasaki Center.
Pont Viau Motorcycle.
Chabot Marine & Sport.
Honbec Mtl.

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Re: Small back fire at strat up

10 Jan 2025 13:55
#907714
I must admit that my experience as a mechanic was a few decades ago. I liked these jobs but the salary convinced me to do something else.
I then worked in industrial radiography and as a welding inspector.
But now, I have the chance and the pleasure to reconnect with my passion and get my hands dirty LOL
Kawasaki:
S1 250 triple 1973
H1, 500 triple 1975
LTD 1000, 1982
Ninja 1000R 1987
Ninja ZX12R 2006
KZ900 1976.
Yamaha RD 350 1973
Ducati 1098 2007
Harley Softail 2009 For sale !

Mechanic at:
Daytona Motor Montreal.
Kawasaki Center.
Pont Viau Motorcycle.
Chabot Marine & Sport.
Honbec Mtl.

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Re: Small back fire at strat up

10 Jan 2025 16:40
#907720
Kaw-boy 900 wrote:
I'm not even sure if I can call that a back fire, it's small detonation that I can ear and feel at the muffler. I had the valve clearance in mind too but wondering why it is only before it's warm up.

I was also thinking that the starter plunger could be the cause but I will check the valves first.

Thanks for your reply !
The symptoms I had with the Audi was a "pop" in the airbox when cranking on cold start !
1982 KZ1000 Ltd
www.kzrider.com/forum/11-projects/617631...-82-begins?start=192
kzrider.com/filebase-alias?view=download...d-fault-diagnosis&ca

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Re: Small back fire at strat up

10 Jan 2025 22:07
#907723
Sounds to me it's a minor exhaust leak.
Z1b1000 1975 Z1b
kzrider.com/forum/11-projects/598262-kz-...-will-it-live#672882
kzrider.com/forum/2-engine/597654-poser?start=240#704229
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  • Rolf1976_KZ900
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Re: Small back fire at strat up

11 Jan 2025 06:00 - 11 Jan 2025 06:04
#907736
Sounds like you got a misfire like minor pop in exhaust ? .Not backfire. It could be that it runs lean on cyl4 when cold and when engine warms up and runs more rich it goes away. Have you tried playing with the mixturescrews? or it can be like Steve says
 
Last edit: 11 Jan 2025 06:04 by Rolf1976_KZ900. Reason: text
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Re: Small back fire at strat up

11 Jan 2025 12:47 - 11 Jan 2025 16:07
#907759
It look like this post is in the wrong section ! It' now belong to the engine section :(

You guy's were right. I have # 3 & 4 exhaust valve that are out of spec at less that .0015'' / .038 mm and # 2 intake at .008'' / .203 mm. 
The others are withing spec (as per Kawa spec revision) .002'' to .006 '' / .051 mm to .152 mm.

I am surprised ! The engine was recently overhauled according to the $10,000 invoice provided by the previous owner.
So, I just ordered the valve tool and a set of shims. Hopefully this will fix it !

Anyway ... It allowed me to update my photo....48 years later :)






 
Kawasaki:
S1 250 triple 1973
H1, 500 triple 1975
LTD 1000, 1982
Ninja 1000R 1987
Ninja ZX12R 2006
KZ900 1976.
Yamaha RD 350 1973
Ducati 1098 2007
Harley Softail 2009 For sale !

Mechanic at:
Daytona Motor Montreal.
Kawasaki Center.
Pont Viau Motorcycle.
Chabot Marine & Sport.
Honbec Mtl.
Last edit: 11 Jan 2025 16:07 by Wookie58.
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Re: Small back fire at strat up

11 Jan 2025 14:53 - 11 Jan 2025 14:55
#907766
Many here feel the Kawasaki FSM valve cleararances (.002 to .004) are too tight. The revised clearances are .10mm to .15mm (.004 to .006 inches)  and depending on shim selection, you should favor the larger end of the range.    (Thanks Zed1015 !)

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Last edit: 11 Jan 2025 14:55 by sf4t7.
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Re: Small back fire at strat up

11 Jan 2025 16:08
#907768
I fixed your pics, next time click "insert all" before posting and don't set as private :)
1982 KZ1000 Ltd
www.kzrider.com/forum/11-projects/617631...-82-begins?start=192
kzrider.com/filebase-alias?view=download...d-fault-diagnosis&ca
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