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Test post

Discussion in 'Test Zone' started by Penguin, Jul 29, 2019.

  1. Penguin

    Penguin New Member

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    Ontario, Canada
    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.
    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?

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