Stuck valve?
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Thread: Stuck valve?

  1. #1
    Member tmasters's Avatar
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    Stuck valve?

    Need some help with valves. Here is a little backstory:

    Bike was running fine, I went to start it one day and it was only firing on one cylinder. I don't have a compression gauge and it's given me no reason to believe the pistons are bad to this point, so I worked from the other side of the troubleshooting tree: carburetor is definitely clean, fuel is in the bowl and getting to the cylinder (spark plug is wet after running on one cylinder for a few minutes. Spark is powerful and blue if the plug is outside and grounded on the engine; likewise, if I switch the two plugs, the fouled plug runs fine on the other cylinder. Checked timing and it's been as spot on as ever (I have an electronic ignition installed).

    So today I went to check the valve clearances and using my finger over the spark plug hole to feel for the compression stroke, I realized that the right cylinder has no compression whatsoever (the left side, the running side, felt normal). I found what should be the compression stroke for the right side based on the finger test on the left side, and then rotated 180 degrees to the T mark.

    When I opened the valve covers on the right side I was able to jiggle the intake rocker arm with my finger, but the exhaust valve is completely stiff. If I undo the adjuster locknut and wiggle the eccentric adjuster the valve doesn't gain any play at all. My best guess at this point is that the exhaust valve is stuck open, which would explain the complete lack of compression and also why I was able to get the right cylinder to backfire if I revved the engine long enough; gas vapor was still making its way into the cylinder and out through the tailpipe.

    Any thoughts or solutions? Haven't come across this problem in all of my searching yet. Thanks in advance!
    1970 CL350 - Log

  2. #2
    Senior Member 76TWIN's Avatar
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    No short cut really to solve this. You're going to have to open up the top end to see what's going on with that valve.
    I am guessing it may be bent.
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  3. #3
    Member tmasters's Avatar
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    That's what I figure. When I rebuilt this bike I did everything except crack the engine, which seemed to have been well taken care of. Can I get into the top end without taking the engine out of the frame again? If I end up needing a new valve should I replace all four at once?
    1970 CL350 - Log

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  5. #4
    Senior Member 76TWIN's Avatar
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    I haven't worked on the cb350 engine before but I believe you CAN get to the top end without removing the engine from the frame.

    As far as the valves are concerned, there is no reason to have to replace the good ones. I would just do a leak test on all and replace whichever ones failed.
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  6. #5
    Supporting Member Yendor's Avatar
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    NO - the 350 does not have enough frame clearance to access the top end without pulling from the frame.

    Is the OIL full?

    The Valve Stems are SPLASH Lubricated.
    If your oil level went low or became contaminated with fuel you may have lost pressure a starved the top end.
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  7. #6
    Member tmasters's Avatar
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    Oil level is fine, though if you want to weigh in on this I'll take an extra opinion: a mechanic down here in Texas told me that 10w-40 was way too thin for our hot climate and recommended I put 20w-50 in, which I've been running for a little over two months.
    1970 CL350 - Log

  8. #7
    Super Moderator outobie's Avatar
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    20W-50 is on the thick side but that's not causing your issue.

    it does sound like you'll need to pull the motor and do a top end job though.

    before pulling it it would be good to do some more testing to determine cause. you may also have a holed piston.

    are you getting oil to the top end? pull the oil inspection bolt on the left side of of the cam case above and to the rear of the points end cap...(8mm socket) crank the motor to see if you are getting oil there.

    I'd also want to get a compression gauge to check by the numbers.
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  9. #8
    Senior Member mike in idaho's Avatar
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    If the valve stem is bent it would have a LOT of clearance at the rocker, not none. Did you, for sure have the crank turned to the right spot to check it? Sounds like it still needs to come apart in any case.
    ancientdad likes this.
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  10. #9
    Member Konkkaronkka's Avatar
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    Just for sure crank the engine until you see from spark plug hole inlet opens and piston goes down. Continue cranking until inlet closes and piston comes up. Adjust to t-mark and check clearance of exhaust. If zero we have to think more.

  11. #10
    Member tmasters's Avatar
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    I dove in this morning and it took about an hour to get everything clear of the motor and the motor out of the frame. I need to run and pick up a new rubber mallet before I start banging around on this engine and trying to get the top end loose, but here are a couple of questions:

    Short of using a compression gauge (which wouldn't work on the cylinder with the stuck valve anyway), will there be any visual indication on the inside of the top end whether I need to replace piston rings and/or pistons? I'm pretty sure that they are all good based on how the bike was performing up to now, but I would hate to get my valves rebuilt and everything back together before realizing I need to tear it apart again.

    If/when I get new valves, I understand that I have to get them lapped? Where would I go to do that and what do I need to bring with me other than the valve itself?

    My cam chain tensioner is 47 years old and the bike has 18k miles on it... should I replace while I'm in there?

    Basically I'm trying to figure out if this is gonna be a ~$100 project or a ~$500 project, and although there's really no way to know until I open everything up, I'm trying to gird myself, haha.

    I promise I will take some pictures and post in this thread until the resolution; there's nothing more frustrating than someone getting some advice and then never returning with answers. I spent forever searching for answers before posting this thread, and now the bike's been sitting for 2+ weeks and the weather is just getting great around here.
    fillmarr likes this.
    1970 CL350 - Log

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