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Discussion Starter #1
Please bare with me, I had a thread on here before but I suppose it was so old it got deleted. So I'll post what I've got and whats happening again, in sincere hopes someone will know how to help me. I've got a 1975 KZ400S (twin) that's blowing smoke out of the right cylinder. It was suggested to remove the valves and inspect valve guides for dryness and cracking. In the process, I bent a valve. So, I got a parts bike off ebay for real cheap and threw the parts bikes head on to see what would happen, which leads me to where I am now. Compression is low but useable in both cylinders ~115psi...but god only knows if my gauge is calibrated decently. Anyway, I got a leakdown tester for more accuracy. When I plug it in the left cylinder I hear air coming thru the oil fill (past the rings) but only reads a 6% loss, and a little past the rings is fine I'm told. WHen I plug it in the right cylinder, same amount of air past rings, but 68% loss because the exhaust valve isn't sealing properly. I've had the head and cylinders off a few times. Have replaced o-rings in the head and the thick little ones at the cylinder base gasket, and the cylinder base gasket. Haven't replaced head gasket because I can never see oil on it and it looks beautiful. Cylinders aren't gouged up. Anyone have a suggestion? Been three years of restoration, and man I want to get her on the road this summer. Thanks.
 

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Just a couple of thoughts:

Head gaskets are not reusable. They crush when installed and will not seal properly a second time.
Are your valves adjusted properly? A too tight valve will not seal.
 

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Discussion Starter #4
It just still looks so pretty! haha. yes, the valves are are well adjusted, not too tight or too much slop...right to manuals specs. And perhaps the one exhaust valve I mentioned is bent...as like I said when I did the leakdown test there was leakage out of that one valve. If any other valves weren't sealed, I would've caught it with the leakdown. Now here's my thought on that, I don't think even a badly leaking exhaust valve could cause whats happening. I say that because with our decent compression there really isn't the capability of enough oil getting in there to cause this. I forgot to mention one symptom. It will run fine for a few seconds then as the oil starts coming a little heavier and a little heavier the idle will also climb. Well, climbing idle when warms up usually means intake vaccuum leak on cars...but I don't know here and I especially don't know when you pair it with oil. Should the head gasket be my next starting point? Really don't want to spend another 150 bucks on it and have it not be the problem, then have to buy another one just because I have to pull the head off AGAIN. lol. Thanks for the help guys, I am not tying to argue as I really do need the help, just makes me think.
 

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Argue away if you want, it helps the thought processes :)

I'd definitely replace the head-gasket, as Bill said this is a real necessity. The o-rings you mention sit around the oil-feed and seal this, the old gasket may be allowing some oil from the right-hand feed to enter the cylinder.

Oil itself wouldn't cause the tickover to rise, the opposite more likely so it's possible there is an air-leak from the manifolds.

I'd swap a valve from the original head and lap it to the new head too.

I'm probably telling you nowt you didn't already know but worth a shot.
 

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Discussion Starter #6
yeah, I suppose you're right. Just was crossing my fingers for a different suggestion, haha. I suppose I'll order the blasted head gasket off of ebay tonight. If it doesn't fix my problem will I have to buy another one? I really don't want to have to pull this thing apart again any time soon, so I think I'd like to suck it up and put up the money for some new rings and just hone the cylinders. My old mans a toolmaker and his shop can lap the valves, might as well do them all. And a friend of his said he'd do the cylinder work for me and remove the valves for cheap, as he has the right compressor. Maybe that'll fix the low compression problem. Anyones opinion on that or know where I can get rings? Thanks folks, this young dumb kid appreciates your help learning.
 

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I think with the time and effort you're putting into the bike, you'll be happy you put the rings in and do the head work. With that and new valve seals/gaskets, it should put an end to the smoke and low compression.
 

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Discussion Starter #8
Hey Stargate! you helped me the last time I was on here begging for help! haha. I suppose this time I should listen to the suggestions given, and get those valve seals too, as long as the valves are out. They're just so blasted expensive, but you're right, do it right. I bought a new head gasket and that's on its way. I'm going to re-measure piston to wall clearance before I order rings, just in case I find the cylinders need to be bored over. I gave up on my original cylinders as well, before I posted this, because the originals had a nice dig in the right cylinder. So I honed these out a little and put them in, should've given it rings then. Lessson is learned this time. Found a standard size set of rings for about 100 bucks a pair. Thanks guys.
 

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It's possible (I'd imagine) that the other brands of Japanese motorcycles(Honda,Yamaha,Suzuki) would have rings that could fit. Don't forget the Triumphs and other British bikes. If it were me, I'd take your rings along to the bike shops and see what might fit(bring along a micrometer or caliper for measurements).

Another thought is to visit the independent service centers for parts.
 

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Discussion Starter #10
Hey, thanks for the suggestion MFolks. That's a great idea, hadn't thought of it. If the dimensions are the same they'll probably work!
 

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The only major difference I can think of might be in ring thickness (width of the machined groove in the piston).
 

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Discussion Starter #12
If I went that route, with rings from a different bike, I will certainly painstakingly triple-measure every dimension before I commit. On another note, I got the head gasket in the mail today, and when I said before that reusing my beautiful one was certianly an ignorant statement as it isn't beautiful at all. That white gasket material that goes around the cam chain area isn't even on the gasket that I'm using, it's pretty much just steel. Interesting, and certainly a possible cause of my problems, but I'm still going to be patient and measure these up. 10 hr days and a nickel-and-dime you to death Jeep is hurting progress.
 

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Discussion Starter #13
Well an interesting find today. Dismantled bike to measure piston to wall clearance and what not to see if I need to go bored-over and oversized rings or if I could get away with stock replacement and a hone-job. First thing I noticed were two parallel lines worn into my cylinder wall that run the length of piston travel, and about a centimeter apart. These are new, so naturally I cursed and shook my head. Then an examination of my piston showed me the culprit, a little chip in my right cylinders top lip of the oil ring..and another small crack a centimeter around the circumference from it. Wonder where that oils coming from? hahaha. Well the cylinders I wore those grooves into were from the replacement bike so I've still got my original cylinders that have been honed and taken care of, that I was going to save for replacement later down the road. Ring probably got that crack from amateurishly compressing them with a slot screwdriver to get them in the cylinder. Plan as normal, new standard rings and hone cylinders, install new head gasket and valve guides.
 

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Discussion Starter #15
No sir I have not yet but thank you for reminding me! Once our shiny new guides arrive I'm taking it to a shop to have them remove and lap valves, install guides, and reinstall valves. The last time I tried to remove valves, one got bent so I'd rather pay a shop to do it right, and I know the guys pretty well there, they do good work.
 

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Discussion Starter #16
The beast lives! And without plumes of white smoke!!!! woohoo!!! haha. There was some white smoke out of the left pipe, but it didn't smell like the oily smoke, and my plug was black as night and dry as..something realy dry. Add that to the fact that just to get it to run I had to pull the float bowl off and clear the float needle, and I'm fairly certain that smoke is a fuel related problem, far too much fuel to air. What do you guys think? I'm going to look for a carb rebuild kit in that list of parts resources after I post this. I've got a long way to go yet, but man am I excited to have that smoke gone!!!
 

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Discussion Starter #17
Inside the carbeurator, offset of the main jet there is a little rubber stopper with two jets inside of it. Is that rubber stopper supposed to completely seal off what's inside that tube from the rest of inside the float?
 

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Discussion Starter #18
Well I bought some rubber stoppers from a shop near me, from a Honda...called them carbeurator passage plugs. They were too big so I turned them down on a lathe a bit and fit nice and snug. then was still chasing my tail when I found my dad didn't put the jet keeper back on the left side and the jet and needle jet fel into the bowl. Still couldnt figure my high idle problem after that until I looked at one of the most obvious answers, THE BUTERFLIES! they were misaligned somehow and stuck open too far,she is running like a champ now, no white black or blue smoke and lots of power for my 140lb self. Thanks to all who helped. Thanks to the forum.
 
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