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Compression significantly different between throttle open and closed…why? 16 Jun 2022 20:04 #868943

  • jekyllman
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‘80 KZ650-F1. Pods. 

I’ve been struggling with cylinder #2 not firing for months now. I’ve been completely unsuccessful diagnosing the issue. I feel like I’ve tried everything. However, today, while testing compression once again, I noticed that the three working cylinders will read around 135-145 psi cold and throttle closed. I panicked when I saw only 75 on cylinder #2 but that could explain it. However, I did it one more time with throttle open and….jumped quickly up to 145psi. 

Can somebody explain to me why this would happen?! I’m pretty confused.
 
1980 KZ650F1 - 10k miles (see photo)
1981 KZ305 CSR - 10k miles (didn't have the heart to customize this one, will stay original)

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Last edit: by jekyllman.

Compression significantly different between throttle open and closed…why? 16 Jun 2022 20:19 #868944

  • Nessism
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Have you adjusted valve clearance?  And if so, did you find any valves with zero clearance?

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Compression significantly different between throttle open and closed…why? 16 Jun 2022 20:43 #868945

  • jekyllman
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I just finished up a valve seal job and verified all the clearances were in spec as well. So I think we're good there.
1980 KZ650F1 - 10k miles (see photo)
1981 KZ305 CSR - 10k miles (didn't have the heart to customize this one, will stay original)

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Compression significantly different between throttle open and closed…why? 16 Jun 2022 20:45 #868946

  • TexasKZ
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With the throttles closed, the piston going down makes negative pressure in the intake tract, thus not fully filling the cylinder giving the low reading. The compression test must be done with the throttle wide open to give an accurate reading.
Edit-Oops, did not see your reply about the valve job.
1982 KZ1000 LTD parts donor
1981 KZ1000 LTD awaiting resurrection
2000 ZRX1100 not ridden enough

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Last edit: by TexasKZ.

Compression significantly different between throttle open and closed…why? 16 Jun 2022 20:55 #868948

  • jekyllman
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I know throttle open is ideal but usually we're talking 10-20psi difference. This was on the order of 70psi whereas the other 3 pistons were maybe 10 psi different when measured with throttle open. 
1980 KZ650F1 - 10k miles (see photo)
1981 KZ305 CSR - 10k miles (didn't have the heart to customize this one, will stay original)

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Compression significantly different between throttle open and closed…why? 17 Jun 2022 00:56 #868952

  • howardhb
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A definitive "answer" can be attained by squirting a tablespoon or two of engine oil into that plug hole and measuring again...
An increase in pressure will indicate an issue with the bore and / or rings, being either worn and / or rings seized in the ring grooves.

My 2c

H.

 
'81 GPz 550 D1 Click this link: www.kzrider.com/gallery/kz-550/1981-gpz550d1-9094#joomimg
'81 GPz 1100 B1 ELR "Tribute" www.kzrider.com/forum/11-projects/617040-1981-gpz1100-b1-rst
'82 Yamaha YB100 Fizzie
'79 Suzuki GT200 X5 TWO STROKE TWIN - SMOKER!

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Compression significantly different between throttle open and closed…why? 17 Jun 2022 01:20 #868953

  • hugo
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......or/and maybe the synchronization is not great, and the butterfly of #2 is almost closed...with not throttle, related to the others.

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Last edit: by hugo.

Compression significantly different between throttle open and closed…why? 17 Jun 2022 04:22 #868960

  • hardrockminer
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Throttle closed means the slides are blocking air from flowing through the carbs and into the cylinders.  
I have several restored bikes along with a 2006 Goldwing with a sidecar. My wife has a 2019 Suzuki DR 650 for on and off road.

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Compression significantly different between throttle open and closed…why? 17 Jun 2022 04:57 #868963

  • TexasKZ
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I know throttle open is ideal but usually we're talking 10-20psi difference. This was on the order of 70psi whereas the other 3 pistons were maybe 10 psi different when measured with throttle open. 
You are right, I got nothin. I do know that the oil in the cylinder is a controversial method and that a leakdown test is more informative. 
1982 KZ1000 LTD parts donor
1981 KZ1000 LTD awaiting resurrection
2000 ZRX1100 not ridden enough

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Compression significantly different between throttle open and closed…why? 17 Jun 2022 04:59 #868965

  • TexasKZ
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As long as I am pissin in the wind, maybe there is a sticking valve in that cylinder.
1982 KZ1000 LTD parts donor
1981 KZ1000 LTD awaiting resurrection
2000 ZRX1100 not ridden enough

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Compression significantly different between throttle open and closed…why? 17 Jun 2022 05:03 #868967

  • Wookie58
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I know throttle open is ideal but usually we're talking 10-20psi difference. This was on the order of 70psi whereas the other 3 pistons were maybe 10 psi different when measured with throttle open. 
You are right, I got nothin. I do know that the oil in the cylinder is a controversial method and that a leakdown test is more informative. 

 
The "wet test" method does work but you need to be sparing with the oil to avoid filling your tester with it 

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Compression significantly different between throttle open and closed…why? 17 Jun 2022 08:17 #868975

  • Mikaw
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Shop air dialed down to 80 psi pumped it the cylinder is a poor mans leak down test. Make sure piston is at bottom dead center on power stroke. Listen at crankcase, exhaust, and carbs fo air leakage. 
1976 KZ 900 A4 kzrider.com/forum/11-projects/613548-1976-kz-900-a4
1976 KZ 900 B1 LTD
1978 KZ 1000 B2 LTD
1980 KZ 750 E1
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