kz750 twin pistons/rings

  • Schmeck
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kz750 twin pistons/rings

03 Sep 2013 11:41
#604591
Okay so After I started having a spark plug problem in one of my other topics, I started to think it wasn't the plug. I did some research and I am thinking my piston rings are wearing out to the point that oil is getting onto my spark plugs. I started the bike, ran it around the block a couple times till it was hot and gently removed plugs for a compression test. These are the results sitting on the bike. Left chamber is a hair line above 90 and right chamber is a hair line below 90 . Can anyone tell me if these readings are an indication that I need new pistons and rings? Whats the compression suppose to be at? -Tyler
1979 kz750 twin -Soon to be roadworthy
1988 El250 engine in custom hard tail frame - collecting dust

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Re: kz750 twin pistons/rings

03 Sep 2013 12:33
#604600
It should be above 100 cold.
Have you checked valve clearance?
Valve guide seals could have dried out.
Letting oil pass thru to the valve guide to the combustion chamber.
When you checked compression did you pull both plugs,throttle wide open?
Check your compression cold and add a tea spoon of oil to each cylinder.
If the reading is higher it's rings that are bad.

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2002 ZRX1200R
81 GPz1100
79 KZ1000st daily ride
79 KZ1000mk2 prodject
78 KZ650sr
78 KZ650b
81 KZ750e
80 KZ750ltd
77 KZ400/440 cafe project
76 KZ400/440 Fuel Injected

www.dotheton.com/forum/index.php?topic=39120.0


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Re: kz750 twin pistons/rings

03 Sep 2013 12:53
#604603
I suggest you also perform a leak down test. This may help identify if compression is leaking past valves or rings. Ed
1977 KZ650-C1 Original Owner - Stock (with additional invisible FIAMM horn)

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Re: kz750 twin pistons/rings

03 Sep 2013 14:40
#604630
Ill try the leak test . im hoping its the valves
1979 kz750 twin -Soon to be roadworthy
1988 El250 engine in custom hard tail frame - collecting dust

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Re: kz750 twin pistons/rings

03 Sep 2013 16:33 - 03 Sep 2013 16:34
#604652
Schmeck wrote: Okay so After I started having a spark plug problem in one of my other topics, I started to think it wasn't the plug. I did some research and I am thinking my piston rings are wearing out to the point that oil is getting onto my spark plugs. I started the bike, ran it around the block a couple times till it was hot and gently removed plugs for a compression test. These are the results sitting on the bike. Left chamber is a hair line above 90 and right chamber is a hair line below 90 . Can anyone tell me if these readings are an indication that I need new pistons and rings? Whats the compression suppose to be at? -Tyler
Service limit for 750 twin is 115 psi HOT. Low compression can be rings but it can also be valves.

Oil on the plugs is almost always worn out valve guide seals.
1979 KZ-750 Twin
Last edit: 03 Sep 2013 16:34 by bountyhunter.

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Re: kz750 twin pistons/rings

03 Sep 2013 19:33
#604692
Okay so I sprayed alittle bit of oil in the chamber, turned it a couple times to get it around and then checked compression, Hasn't changed at all. Actually when the bike is warm or cold the compression doesn't change much. Cold Im at 90 and hot im just a hair line above. The compression would have went up if it was the rings. Sooo I take it my valves need to be serviced? Maybe that valve guide you mentioned is worn out?
1979 kz750 twin -Soon to be roadworthy
1988 El250 engine in custom hard tail frame - collecting dust

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Re: kz750 twin pistons/rings

04 Sep 2013 15:52
#604812
Valve guides or seals probably don't affect compression. The valves need to be lapped (at least) so they seat properly.
1979 KZ-750 Twin

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Re: kz750 twin pistons/rings

04 Sep 2013 16:09
#604815
If the valves clearances arent within spec that could lower compression right?
1979 kz750 twin -Soon to be roadworthy
1988 El250 engine in custom hard tail frame - collecting dust

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Re: kz750 twin pistons/rings

04 Sep 2013 17:00
#604821
Yes. If there is not enough clearance the valves may not be closing all the way or may not be closing until late and opening early. Either of those conditions can reduce compression. Ed
1977 KZ650-C1 Original Owner - Stock (with additional invisible FIAMM horn)

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Re: kz750 twin pistons/rings

04 Sep 2013 19:14
#604833
650ed wrote: Yes. If there is not enough clearance the valves may not be closing all the way or may not be closing until late and opening early. Either of those conditions can reduce compression. Ed

+1 , a little loose is better than a little tight


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2002 ZRX1200R
81 GPz1100
79 KZ1000st daily ride
79 KZ1000mk2 prodject
78 KZ650sr
78 KZ650b
81 KZ750e
80 KZ750ltd
77 KZ400/440 cafe project
76 KZ400/440 Fuel Injected

www.dotheton.com/forum/index.php?topic=39120.0


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Re: kz750 twin pistons/rings

04 Sep 2013 20:52
#604848
what feeler guage size do i need ti measure my clearance ? smalled i have is .008 then .010 and neither of them fit . the lobes are up and the shim is touching.
1979 kz750 twin -Soon to be roadworthy
1988 El250 engine in custom hard tail frame - collecting dust

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Re: kz750 twin pistons/rings

04 Sep 2013 21:44
#604860
The acceptable clearance range on my K650 is .00315 -.0071 inches or .08 - .18 mm. Yours may be somewhat different, but it sounds like you need a different set of feeler gauges if the smallest you have is .008 inch. Ed
1977 KZ650-C1 Original Owner - Stock (with additional invisible FIAMM horn)

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