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84 XJ750RL - Very low compression due to leaking/stuck intake valves

Discussion in 'XJ Technical Chat' started by Penguin, Jul 29, 2019.

  1. Penguin

    Penguin New Member

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    Hi everyone, I've been lurking for a while, but I need a bit of advice on something I wasn't able to find clear direction on in my searches

    The bike: 1984 XJ750RL with 41,000 miles (66,873 km). It's the Canadian model (44X model code). Built by Yamaha with a XJ750 engine with an XJ900 chassis, airbox & electrical. I have the Haynes & Yamaha service manuals for BOTH the XJ750 and XJ900 (so 4 different books for this one bike).

    Background: Was sitting under a tarp in a guy's shed for the last 10-15 years. Has not run in 10-15 years. While it was kept nice & dry, a bunch of mice made it inside and ate the whole air filter, they made it all the way to the carbs and were only stopped by the throttle butterfly valves (as far as I can tell). I've pulled the carbs off, and also removed the rubber intake manifolds from the head (as there was some cracking, had to verify the cracks didn't go all the way through.

    Problem: Very low compression. Here are my compression results:
    Cylinder Test #1 Test #2 (after pouring some oil in the cylinder)
    #1 60 PSI 110 PSI
    #2 30 PSI 40 PSI
    #3 30 PSI 70 PSI
    #4 70 PSI 70 PSI

    As you can see, very low numbers with some clear valve issues going on. Possible ring issues, but I'm confident those rings are stuck at a minimum - so doing the ATF + Acetone thing to unstick them, will otherwise just run it to see if they unstick before I jump into replacing piston rings & honing cylinders.

    Additional Details:
    I followed the exact procedure from the - COMPRESSION TESTING: section of [redacted due to forum spam filter - it's the x j 4 ever site]
    For reference (from that page):
    The only thing I did different from that was
    • The engine wasn't warm (not there yet!)
    • I didn't hold the throttle open (carbs are off)
    I took periodic voltage readings off the battery throughout my test and observed very little variation.

    I checked my valve clearances, here are the results:
    Cylinder Intake (inches/mm) Exhaust (inches/mm)
    1 .006 / .152 .010 / .254
    2 .008 / .203 .009 / .229
    3 .008 / .203 .010 / .254
    4 .008 / .203 .010 / .254
    So my intake clearances are 0-2 thous greater than the upper limit, and the exhaust clearances are 1-2 thous greater than the upper limit.

    While this should be addressed, I don’t believe these clearance numbers are contributing to my poor compression numbers. I’m a bit of a novice, so I could be completely wrong here, but my understanding is that if the clearances are lower than expected, that leads to the valves not seating all the way, which could both cause them to burn up, and lead to compression loss due to an incomplete seal against the valve seat. If the clearances are greater than expected, while that could lead to poor performance at high throttle levels due to the valves not opening as much as expected, and perhaps it taking slightly longer to build up compression, the effect on maximum compression would be minimal – especially numbers this close and compression results this poor.

    One thing I’m suspecting is that the valves or valve seats could be covered in carbon deposits or corrosion and therefore not seating all the way, leading to a poor seal. This would explain the higher than expected clearance numbers.

    Root Cause:
    I next did a leakdown test. On all four cylinders, I’m getting a massive air leak past the intake valves. I used 30 PSI, and at that low pressure, I can feel the air blasting out of the intakes at roughly the same volume & force of someone blowing out a candle. The regulator on my compressor shows it can’t even maintain 10 PSI, and if I leave it connected, the compressor has to cycle several times per minute to maintain tank pressure. I’m amazed my compression numbers were as good as they were. Noticing 0 leakage anywhere else, and I have very good hearing (although it can be hard to hear with that typhoon coming out of the intakes!)

    I stuck a borescope down the intakes, here's what the intak valves look like. A couple notes when looking at these
    • The valves are wet with oil due to the splashing from the wet compression test
    • In some pics it looks like there's a piece missing out of the valve. That's not the case - if you look closely, my borescope just has a low dynamic range and the actual sealing surface of the valve is darker than the rest and so it blends in with the background a little bit.
    • The first cylinder is just at the beginning or end of an intake stroke, you can see the valve slightly open - this is expected. Of note, you can see the portion of the valve stem that was up inside the valve guide is perfectly clean.
    Cylinder 1a.png Cylinder 1b.png Cylinder 2a.png Cylinder 2b.png Cylinder 4.png

    My Question(s):
    Is this one of those things that if I get the engine running (poorly, I know!) that with engine temperature and everything cycling thousands of times per minute, that the valves would re-seat themselves and possibly resolve itself?

    Should I try to spray some throttle body cleaner on the valves (with them in the closed position so that way it can soak)?

    Or is this a non-negotiable pulling of the head and cleaning/lapping the valves?

    On one hand I don't want to go through the work of putting the manifolds/carbs/tank back on only to have to take them out again, but on the other hand I don't want to take apart the head if I don't have to.
     
  2. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    Soak with the ATF/Acetone mix for a week or so.
    Check and adjust valve clearances.
    Start it, run it. See what happens.
    Head work should only be considered if compression and leakdown tests do not show marked improvment.
    If the heads do have to come off, consider splitting the cases to pro-actively replace the alternator chain guide (and rebuild the starter clutch) before it breaks (they age out).
     
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  3. Penguin

    Penguin New Member

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    Excellent, that was what I was hoping to hear. I've had it soaking in ATF/Acetone since Sunday night, I'll periodically turn the engine over by hand every few days.

    While I wait for that, this will give me time to tear apart the carbs.

    Based on my clearance numbers so far, going to leave the valves alone, as I'll have to adjust the clearances anyways when the valves seat properly once I get it running. If it the compression numbers don't come up and I have to pull the head & lap the valves, that will also change the clearances.

    I'll update this thread with how it goes in a week or two. Thanks!
     
  4. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    This is correct, but, it is also very odd to find an engine....especially an older one....to have such large valve clearances. As miles accumulate, valve clearances get SMALLLER, not larger, and your clearances are rather off-the-chart larger. So unless someone (the dreaded PO) really mis-calculated when doing the last valve shim adjustment, there is either a mistake in your measurements, or...………?

    While it sounds like you are on the right path to resolving the issue(s), if you do determine that the head needs to be pulled, make sure to do a full valve job and check the head (and the jugs) for straightness. If the valves aren't the problem, then you have a bad cylinder head gasket, and that is almost always caused by improperly tightened head stud nuts and/or a leaking gasket caused by head warpage.
     
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  5. Toomanybikes

    Toomanybikes Well-Known Member

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    Looking at the pictures, it appears that corrosion on valve and seat has held the valves open enough to give you bad compression numbers and bigger than normal clearance numbers
     
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  6. firebane

    firebane Active Member

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    Yeah I don't know if its the picture quality but it looks like water got into that area and rusted out those valves bad.
     
  7. Ryengoth

    Ryengoth Active Member

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    You should really pull the heads, clean the seats with some 000 steel wool, buff the valves with a wire wheel, put in new stems seals and lap them in. Those valves and stems look horrible in my opinion. I would never try to run those, you may damage the seats and guides if you do.

    EDIT: the fact that one cylinder is showing compression verifies that cylinder was in combustion stroke, the others were mid-cycle and the rest of the valves were open. They've rusted severely being open and are causing the shim clearance issues.
     
  8. Penguin

    Penguin New Member

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    The parts of the stems up inside the guides are clean and corrosion free, as you can see from this cropped photo of a slightly opened valve (at the beginning or end of an intake stroke):
    20190728_2300005.png

    The source of the corrosion is due to a colony of mice living inside the airbox, they made it all the way to the throttle butterfly valves. This is what the "air filter" looked like:
    IMG_20190714_193723.jpg

    I was fortunate that the bike was stored in a position where all the intake valves were closed - so the insides of the cylinders are completely corrosion free, exhaust valves look good, and the actual sealing surfaces of the intake valves aren't as bad as everything else. The oil and the zoom from the camera does make it look worse than it does in my garage.
     
  9. firebane

    firebane Active Member

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    I would be doing anything until those heads were pulled and the valves and valve seats are inspected.

    There is a lot of corrosion on those valves and that could easily cause some damage that will end up in a head being pulled regardless.
     
  10. sybe

    sybe Active Member

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    If you have one of those fancy tube cameras, snake it into the spark plug holes and take a look inside to make sure. The one concern i have is.... and i cant tell if its carbon build up or rust but it looks like you have chunks of metal missing from your valves. that metal is some where and it can destroy your pistons.

    How can you tell the pistons have no corrosion if you didn't pull the heads off?
     
  11. Penguin

    Penguin New Member

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    They're only $2o on Amazon :) Would highly recommend it

    The actual mating surfaces of the valves are not covered in corrosion, plus they are covered in oil, so they are very dark. If you look closely you'll see that where the corrosion ends isn't the end of the valve, the mating surface is just past there on the edge. I'll see if I can get some better lit pictures from the spark plug hole when they're completely extended.

    Before I turned it over, I stuck the borescope down each spark plug hole and inspected the top of the piston, and the cylinder walls. Everything is in absolutely pristine condition under the valve covers and as far as I can see from the oil fill cap - the oil's in decent condition too. Also looked at the tops of the exhaust valves from the spark plug hole while turning the engine over manually, and they look fine.

    While the corrosion of the intake valves is considerable, the corrosion did not make it anywhere else (except where the mice peed on the electrical relays, but that's a different topic!).
     
  12. Penguin

    Penguin New Member

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    I wanted to provide an update & some closure to this thread.

    With a whole bunch of carb & intake work, I got the bike running beautifully on all 4 cylinders, even took it around the block over the weekend (very carefully - the tires are like 15 years old).

    Here are the compression numbers from a couple weeks ago (before I tried driving it).
    #1 155 PSI
    #2 152 PSI
    #3 145 PSI
    #4 142 PSI

    These are huge improvements, and everything is now within spec. Going from 30 PSI to 145 PSI basically without having to do anything is nearly miraculous, consistent with being just a bit gummed up after sitting for so long.

    I checked my valve clearances, here are the results:
    Cylinder Intake (inches/mm) Exhaust (inches/mm)
    1 .005 / .127 .008 / .203
    2 .005 / .127 .006 / .152
    3 .003 / .076 .006 / .152
    4 .0025 / .063 .008 / .203

    As you can see, the valve clearance have tightened up quite a bit, and with the exception of the #3&4 intakes, are now all within spec. Currently waiting for my valve shim tool to arrive, and then I'll get that taken care of.

    Just wanted to thank everyone to contributing their thoughts, this forum has been an absolute goldmine of information as I work through this bike.
     
  13. Franz

    Franz Well-Known Member

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    The valve shim tool can crack cylinder head if the cam is turned the wrong way. Using the zip tie or wire to keep the valve off its seat avoids this risk.
     
  14. Minimutly

    Minimutly Well-Known Member

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    Looks like you had some great advice on this - which you neatly ignored, thankfully. Well done. You did exactly what I would have done. further to this, I would put it through as many heat/cooling cycles as possible before shimming the valves.
    Got any pics of the finished project?
     
  15. Penguin

    Penguin New Member

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    Thanks for the warning, appreciate it. I've watched a bunch of videos and read the instructions here, I'm aware of the risk. Seems straightforward to mitigate, I will be VERY careful. I really don't want to risk disturbing the mating surfaces of the valves, whether that's valid or not.

    Great call. I measured the clearances multiple times as I worked through various running issues and noticed they gradually tightened. Very likely that once I actually get it on the highway and get going faster than bicycle speeds, things will settle further.

    It's not quite ready for pics yet - I spent yesterday fixing loose connections everywhere, I just got the tail lights & turn signals reattached and working last night. I still have some cosmetic & minor electrical work before I can finish putting Humpty Dumpty back together again, but it's starting to look like a real motorbike!
     
  16. Huntchuks

    Huntchuks Well-Known Member

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    I think photos of your progress are desired, not just of your finished product.
     
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  17. tabaka45

    tabaka45 Well-Known Member

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    I am amazed at your compression numbers after taking a look at the pictures of the valves. I would now run some Marvel Mystery Oil in the gas for a while and see if some of the corrosion and or carbon will come off.
     

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