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Thread: Oil In Exhaust.

  1. #1
    First time CB owner Best_Coast's Avatar
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    Oil In Exhaust.

    Hey all,
    Just bought a 1980 CB750, since I have some spare time on my hands (Thanks Covid...).
    It came in a variety of parts, and its hard to tell how long it hasnt been running.

    I got it fully built yesterday and ...it runs! Which is pretty good, not knowing anything about the history on it.

    Anyway,
    I've been looking at posts here about finding oil in the exhaust. Its a decent amount that sprays out of the left hand side (the left hand exhaust is also louder, has some grey smoke coming from it, and there is a ticking noise, see video). I noticed the oil on my shoes and pants after the ride, and saw where it was leaking from the muffler connection. Looking at other posts on here, it seems like the most likely culprit would be valve guide seals?

    So, having never torn into an engine before, and not really knowing what I'm getting into - should I order a top end rebuild kit, remove the heads, and go from there? Or is there a better place to start?

    https://youtu.be/9dvh2TG6Bvk

    Thanks!

  2. #2
    CB750 Guru amc49's Avatar
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    Usually oil seals unless not taken care of then rings. These are somewhat complicated engines and not good for teaching newbies on. At the least EVERY part must be marked as to where it goes or it will not go back together correctly and lots of work for nothing. The valve oil seals that come in gasket sets do not last. The valves cannot be ground and go for $20 apiece at least.

    Engine comes out of bike and then tears down. The head will not remove with engine still in frame.

    Mileage? If high the cam chain tensioners will be cooked and $$$$$$. The engine is one of the most expensive to rebuild.

  3. #3
    First time CB owner Best_Coast's Avatar
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    It has around 32,000 miles on it.
    I rented a compression tester today but, of course, its the wrong size for the plugs.

    Reading your responses to valves on other posts - it sounds like my best bet is to do a compression test, then remove the head and inspect the valves?
    And if the valves need work, to take it to a machine shop? (there are tons of vintage shops here in Portland, thankfully)

  4. #4
    CB750 Guru amc49's Avatar
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    Flex the tensioners and look for small cracks in the rubber doing it. They need to be replaced if cracking. Here in Texas with the heat they are often dead at 30K miles.

    Yes on the bike shop but not all do as good work as they claim. BTDT and ruined vintage parts to show for it.

  5. #5
    First time CB owner Best_Coast's Avatar
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    I ran a compression test and the cylinders were within 140 - 153 lbs each, which seems alright, considering the milage.
    The valve clearances are all between .002" and .006". Reading your comments on valve clearances, amc49, I know they need to be adjusted, but they're closer than I expected.

    I'm guessing that the rattling is probably the cam chain - I didnt get a chance to tension it today, unfortunately. But, am I okay to drive it until I get the valve oil seals replaced?
    Or is oil in the exhaust still a sign the I should hold off on riding it?

  6. #6
    CB750 Guru amc49's Avatar
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    The rattling could easily be spun bearings when it ran out of oil due to eating it too. The valves are almost always closer than expected and how they burn.

  7. #7
    CB750 New Member Kevhed's Avatar
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    Amc49 do you offer carb rebuild service? Sorry to be off topic.

  8. #8
    First time CB owner Best_Coast's Avatar
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    I'll post a quick update for anyone else that has this problem.

    After checking the valves and compression, I decided it was safe to ride (at least for a little while).
    The knocking went away, and the oil in the exhaust seems to have diminished.
    Maybe oil had leaked from the heads when it was sitting for extended periods, and was just being expelled upon first starting it?

    Either way, I'll keep this updated.

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