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Checking Cylinder Compression

Discussion in 'XJ Technical Chat' started by gremlin484, Mar 28, 2007.

  1. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    Hindsight is always 20-20, and he's learning (quickly, I might add.)

    Take a look at his PhotoBucket links, you'll see what I'm talking about, and thanks.
     
  2. ski84

    ski84 Member

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    Let me try again... The compression was low after adjustment, some carbon or other debris was holding the valve open. He then adds oil and the debris is dislodged now causing the valve to properly seat. The valve is still too tight so his compression numbers rise to an abnormally high number. Moellear states heavy oil at exhaust header and carbon build up. Could be a rich condition from a possible exhaust valve not seating properly causing incomplete combustion (ie carbon build-up). Seems feasible to me what say you?

    RickCoMatic had a good suggestion on page two of this thread: "You can take a Head off the Bike and "Level-it" up-side-down, fill the Combustion Chambers with Rubbing Alcohol and look for Bubbles or a Leak to detect a Burned Valve". Have you tried that?
     
  3. moellear

    moellear Member

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    i have not. just pour the alcohol on top the valves in this shaped bowl area? i would think that it would drain down the plug hole. but sitting in the house when the head is in the detached garage makes me cringe thinking about walking in the freezing cold is just not right so I could be wrong...also, the alcohol won't harm anything i hope, correct?

    [​IMG]

    P.S. I got this cosmetically a little cleaner, ready for a new gasket, and the tachometer housing on the head is refurbished with correct rubber. That Kroil stuff REALLY works. I was amazed
     
  4. ski84

    ski84 Member

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    One other point... There is an excellent adventure story on ADVrider about a bunch of young east coast guys that took some 30 to 35 year old bikes on a cross country trip to California and back. On the way to Kansas, the TX 500 started running very badly with low compression. The TX is notorious for developing a crack in the head so they assumed that's what it was so they were going to scrap the bike. The mechanic of the bunch developed a hunch just before they gave up on it. He surmised correctly that one of the valves wasn't seating properly. He scraped off the debris and they continued on for another 6500 miles in 30 days. An inspirational read!

    http://www.advrider.com/forums/showthread.php?t=366117
     
  5. ski84

    ski84 Member

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    Put the plugs back in and tighten them before testing.
     
  6. moellear

    moellear Member

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    duh!

    ski84.. i've read the ENTIRE story a year or so ago when I became a member on ADV!!! it was truly quite an experience, just from the literature the same aged guys as myself did that. upon graduation from college in May I was thinking about doing something like this too.. but now we're getting way off topic and I'm gonna re-read their section about fixing the valve in a strangers shed.

    on topic: with the compression #'s across the other three cylinders, would anyone suggest replacing all rings? it seems borderline whether or not the pistons would need new rings across the board. suppose i don't have to think too hard about it now Fitz, just wondering and will make purchases after determining a solution. one step at a time
     
  7. snowwy66

    snowwy66 Member

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    improperly seated valves won't cause his compression to go up when he does a wet test. by putting oil into the cylinder. that's a sign of rings.

    doesn't alcohol vaporize?

    you do have to fill up the whole chamber so you can test the entire valve seating.

    if you only do one cylinder. then you've gone from one dead cylinder being worse then 3 to haveing one cylinder run better then the 3.

    the smart thing would be to do all 4. if things don't look good then you'll be kicking yourself in the arse for not doing a thorough job. youi'll have to take it apart again. and buy all new gaskets AGAIN.

    i know fitz is concentrating on one cylinder. to find out why. but only doing 25% of the motor isn't worth it.
     
  8. TIMEtoRIDE

    TIMEtoRIDE Active Member

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    You're supposed to get a metric spark plug for the spark plug hole. :p
     
  9. ski84

    ski84 Member

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    I suggest you do and my point is, don't assume the worst!
     
  10. moellear

    moellear Member

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    [​IMG]

    TOOTHPASTE! hilarious. for more details see link
     
  11. Polock

    Polock Well-Known Member

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    forget the Rubbing Alcohol just lap the valves and be done, in the process your going to check the valve springs and the valve stems
    and put on new stem seals and clean that carbon from the exhaust ports then clean the head and cylinder mating surfaces like a operating room floor, the block and bottom of the jugs too
    your not halfway done don't think about putting it back together yet
     
  12. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    Exactly.

    But that's the course he's on, and although we've been trying to figure out #1 (which sounds so far like it may need to get a new piston) replacing all 4 cylinders' rings is indeed the only way to do it 100% right.

    Plus everything you just listed for the head.
     
  13. Polock

    Polock Well-Known Member

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  14. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    No way. That stuff is for cars; there aren't that many 10K rpm automotive motors out there, except maybe in racing.
     
  15. Polock

    Polock Well-Known Member

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    That's why you need two cans ... :)
     
  16. moellear

    moellear Member

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    so by lapping the valves, I'd probably need to replace all shims then too? this is getting mighty expensive..
     
  17. iwingameover

    iwingameover Active Member

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    You shouldn't remove enough material to need new shims if they were in spec correctly to start with. Too tight to begin with then you might.
     
  18. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    Relax. You just shimmed the valves, and the lapping process won't remove enough material to make a difference.

    Further, your bike has such low mileage, that the valves shouldn't require much lapping at all.

    Have you had a chance to pull the suspect piston, scrub it clean, and inspect it with a magnifying glass yet?
     
  19. moellear

    moellear Member

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    yes I pulled it off this afternoon and will scrub it clean after finishing this post. pics on photobucket link again :). the theory that maybe PO placed a different piston in cannot be correct. pictures show that it is in fact a Yamaha piston for my bike 4H700. on the same picture you can see "094" is this the production # of that particular piston? now whether or not the other three match this piston, idk. everything seemed to be in good shape with nothing out of the normal, from my point of view. i will admit: i didn't get a magnifying glass out but will take another glance at it soon.

    snapped a view pictures of the connecting rod just to show nothing seems to be out of place there as well. on the flip side, concerning the head, I am getting it cleaned up for when a new gasket is ready to be in place. do normal auto stores carry the valve spring compressor and rod/suction cup to spin valves in place for lapping? i'm hesitant to do the valve job primarily because this problem deals with rings/pistons via wet compression results. i wouldn't mind doing it but with my mileage it shouldn't matter either. then again,, i do have it torn apart this far lol.
     
  20. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    Re: Piston replacement--- sure it can. It could have come from another 550, off eBay, and not been properly fitted.

    You need to get it scrubbed clean and examine for cracks developing on the underside, or in the ring lands; they aren't always easy to see/apparent which is why the magnifying glass.

    Re: valve spring compressor: That's generally a "borrow" from someplace like AutoZone; and they MAY have the correct type. You can't use the "paddle" type, it needs to be the barrel type, with a relatively small diameter, long "barrel."

    The suction cup tools are super cheap; I bought a "set" (two tools, each with a different size cup on the ends, giving you 4 different sized cups) for under $10 some years back, they're still that cheap. They probably have those to loan, too.

    Why bother with the valves? They were all at least somewhat tight, so it's worth a look at the seats, and a quick lap to "freshen" them if nothing else. Valve stem seals are cheap, easy to replace and yours are a quarter century old with 18K miles on them. And you have the head off already. NOT doing the valve "service" just wouldn't make any sense; especially if you get it all back together and discover that at least part of the carbon in the one cylinder was from a bad valve stem seal. Just not worth it.

    Plus, lapping valves is fun, it really is (well I think so anyway.)

    NOTE: VALVE SPRINGS HAVE TO GO TOGETHER, AND IN THE HEAD, IN A CERTAIN ORIENTATION, direction of spring-coil wind wise. I didn't see it in the Haynes; so be very careful on disassembly, and I'll get you the page from the factory book.
     

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