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Author Topic: good compression but burning oil?  (Read 959 times)

Offline doc_rot

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good compression but burning oil?
« on: Nov 24, 2017, 18:08:42 »
so I'm a bit confused by this, hoping for some insight. 1978 KZ650, head recently serviced by APE, with a valve job, guides, springs, and seals. kept the original pistons and rings because they looked good. It smokes pretty good on accel, smells like oil. I did a compression test and its was 150,145,150,155. I also did a leakdown test and all cylinders are less than 1%. Is it possible the rings are leaking with good compression? Its either that or APE did not install new valve seals or installed them incorrectly.

Offline teazer

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Re: good compression but burning oil?
« Reply #1 on: Nov 24, 2017, 21:04:01 »
How many miles since the rebuild?  What I'm alluding to is that perhaps it's assembly oil that's burning off. 

If it has done enough miles for that to be eliminated as a possibility, things get more serious. First place to check is the head and specifically valve guide clearance and oil seals. Perhaps they fitted new guides that don't take seals or forgot to fit new seals.  Seems unlikely with APE but anything is possible.

It's also possible that oil rings are upside down or missing the expander or worn allowing oil up past the rings.

Offline doc_rot

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Re: good compression but burning oil?
« Reply #2 on: Nov 24, 2017, 22:46:37 »
Its been about 200 miles since I put the head on there. The rings were the originals, there was still cross hatching  in the cylinders, and the pistons looked good so i just reused them. When I passed it off to the owner it was running great. About 200 miles later he said he noticed the oil was a little low so he topped it off, rode it a bit and then realized he over filled it and drained the excess oil. at this point it started smoking.

I just found out he let it sit for a couple months, and that when he started it up it was overflowing gas from the carbs, I'm now wondering if the carbs puked gas into the engine he ran it like that and did some damage to the guides/rings. He has since changed the oil so I cant tell if there is gas in it. It seems to run pretty good right now, no knocking, just smokes. Looks like i'm tearing the top end off when I go back in a few weeks.

Offline teazer

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Re: good compression but burning oil?
« Reply #3 on: Nov 25, 2017, 00:44:28 »
I think I would retest compression and leakdown and then check oil pressure.  That whole oil was low story and then it was too full, sounds highly suspect.  If the oil was badly diluted it may have been enough to ruin the crank bearings and or the big ends - it is plain bearings isn't it? 

If oil pressure is OK, tear off the head and inspect one set of valves for wear and if they look OK, pull the barrels off and inspect the rings, bores and pistons for signs of fuel in the oil and wear.

Offline Nebr_Rex

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Re: good compression but burning oil?
« Reply #4 on: Nov 25, 2017, 08:40:37 »
I would put some miles on the bike first. Make sure the owner checks the oil before starting the bike. And correct the gas leak.


.

Offline doc_rot

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Re: good compression but burning oil?
« Reply #5 on: Nov 25, 2017, 20:07:21 »
I put some new float needle assemblies in when I was there so hopefully it will not overflow. How do I check the oil pressure?

Offline teazer

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Re: good compression but burning oil?
« Reply #6 on: Nov 25, 2017, 22:48:54 »
I am not familiar enough with that particular bike, but typically on older bikes there tends to be a cross drilled passageway with an end plug that can be removed and replaced temporarily with a pressure gauge and adapter.

Offline Nebr_Rex

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Re: good compression but burning oil?
« Reply #7 on: Nov 26, 2017, 10:48:38 »
I put some new float needle assemblies in when I was there so hopefully it will not overflow. How do I check the oil pressure?

As Teazer said there is a main oil passage under the crank with a plug on both sides of the engine.

Offline doc_rot

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Re: good compression but burning oil?
« Reply #8 on: Nov 26, 2017, 19:46:39 »
cool. I just got a cheap tester to try when I head back in a couple weeks.

Offline datadavid

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Re: good compression but burning oil?
« Reply #9 on: Dec 02, 2017, 10:41:29 »


so I'm a bit confused by this, hoping for some insight. 1978 KZ650, head recently serviced by APE, with a valve job, guides, springs, and seals. kept the original pistons and rings because they looked good. It smokes pretty good on accel, smells like oil. I did a compression test and its was 150,145,150,155. I also did a leakdown test and all cylinders are less than 1%. Is it possible the rings are leaking with good compression? Its either that or APE did not install new valve seals or installed them incorrectly.

All that work and you didnt change rings? Not even a hone?
Did you measure bores and ring gaps?
Looking good is quite different from within tolerances..
Although tired rings should be smoking on decel its not always like that.
Changed the cylinder head on my old triumph with all new everything last year, still smoked, still had great compression, but bores were barreled.. was enough with a hone and new rings, no more smoke now.!