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Post by chris23413 on Apr 20, 2024 21:13:43 GMT -5
Hello All,
I recently rebuilt the top end of my QMB139 based chinese scooter. All new cylinder jug, piston and head, and an A9 "Performance" camshaft. All tuned, got it to run decently, but I've noticed that there's an excessive amount of what I'm assuming is valvetrain noise. Gets a little bit louder when warmed up, but even when cold it still ticks/ knocks. Went through and adjusted the valves to .002" Intake, .004" Exhaust and triple checked clearances after turning the motor over by hand and they're within spec, Checked and made sure timing is correct and it's dead on also. Just to make sure it wasn't the exhaust rattling, I also went and checked all mounting bolts and they're all cinched down too. took it for a run around today and it runs well, no loss of power, just that persistent loud knock/ ticking. I will add that this is still all within the first 100 miles or "Break-in" period on this combo, so everything is still bedding in. Does anyone have any suggestions? I'm out of ideas and fearing there's something seriously wrong with this motor.
here is what is sounds like when all warmed up youtube.com/shorts/QNcoZmGnw6w?si=rChW1fgR9tpkE3VQ |
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Post by 90GTVert on Apr 21, 2024 6:40:28 GMT -5
You could check that there's no contact anywhere around the stator/flywheel/fan. Those are not uncommon issues that sound bad but are often very simple fixes and it's likely that you'd see some sort of scuffs or scratches.
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Post by chris23413 on Apr 21, 2024 9:52:23 GMT -5
Okay, so I spent the better part of the morning trying to figure out what this noise is. Following 90GTVert's suggestions, checked inside the CVT, doesn't appear anything is hitting or contacting anything, ran it with the CVT cover off, checked if anything was contacting in the fan shroud area, also didn't see anything. Even while running with the fan shroud off (keeping ane eye on CHT's of course) l didn't notice any change in the noise. Drained oil, doesn't look sparkly, magnet looks clean, no excessive metal gunk on it either. I'm stumped, I have no idea why this thing is so noisy when warm. It's consistent though, and tends to start only when the motor is around 130-140 deg F or higher in temperature. When cold, no noise other than a bit of valve tick.
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Post by chris23413 on Apr 23, 2024 6:21:57 GMT -5
Under closer listening with a mechanic's stethoscope, it sounds like the noise is mostly coming from the valve cover area, and sounds much worse than it is. I've tried several different valve lash settings with no luck (.002",.003",.004" for both intake and exhaust) and verified that both the cam and crank are in time with each other. Checked crank bearings as well, they feel good with no excessive play in them. No binding up when warm either. Checked timing chain tensioner last night to make sure it's putting tension on the guides and it is, at least from what I can see/feel. Only other place I can think the noise would be coming from is the rocker assembly. I did reuse the old rocker assembly off of my old head because there was next to no wear on the arms. I'm wondering if that would be my issue here. Beating my head against the wall trying to figure this one out.
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Post by GrumpyUnk on Apr 25, 2024 8:37:13 GMT -5
The valve clearance may be too large. A new cylinder head should have matched the previous with respect to the valve stem length. The rocker arm carrier that goes on top of the head/valves must be one for the valve stem length of the installed valves & springs. I do not know if incorrect rocker arms will work, but if they do, that could be a cause of noise.
A secondary thought is that the valve clearance increases as the cylinder and head, etc, expand. If you measure the clearance on a hot engine, it is very likely the clearance is beyond spec. IOW, close down to a 'close to zero' valve clearance, and likely the noise will be muted to some degree.
I have one that is quiet on startup, but go a few miles and get home and it will be clicking/tapping away. I have not done anything to that engine, but am considering a BBK and a cam... but it runs so well(tappy though) I am loathe to disturb it.
I think these engines operate contrary to what I learned about valve clearance on other engines. These open up the clearance while most others close it.
tom
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