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Old 11-18-2019, 03:52 PM   #1
Lippy
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Default Dirt bike mechanical question

I have a 2006 Yamaha TTR125LE and it runs great but I canít help but notice a load metallic knocking noise when cranking the engine. It is heard when the engine comes to a stop after making a revolution (without starting). Also it is very load and more frequent when the throttle is opened up while cranking the engine. Once the engine is running the noise stops and the engine sounds great. The noise can even be heard when kick starting the bike so I think the starter is less likely to be a suspect. Does anyone have any ideas on what this could be? It sounds like it could be causing some serious damage to the engine but again, it only happens when the engine is not running so I am not sure if it does this by design or if something in the cranking system is going out. Here is a video of what I am talking about. I took the spark plug wire off so you can hear the noise without the engine firing.


https://www.dropbox.com/s/3xy1jua4dl...4%202.mov?dl=0
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Old 11-18-2019, 04:04 PM   #2
Walker
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Resonating back thru the carburator? That's why it changes when y u open the throttle.
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Old 11-18-2019, 04:06 PM   #3
Graysonhogs
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In
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Old 11-18-2019, 04:09 PM   #4
Walker
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2 or 4 stroke?
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Old 11-18-2019, 04:49 PM   #5
JayB
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That's a 4 stroke.

The only clicking sound I am picking up is the sound of spark from the plug cap striking against the head.
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Old 11-18-2019, 05:13 PM   #6
Walker
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Pull the sparkplug out and see if it still makes the sound.
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Old 11-19-2019, 09:01 PM   #7
Lippy
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I took the spark plug out and it does not make the noise, even with the throttle open. So I guess it has something to do with the compression. Could the timing be off but the engine still run as smooth as it does?
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Old 11-19-2019, 10:44 PM   #8
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Could you try a higher octane gas or even alcohol free gas. I think you might be getting pre-ignition from high compression.

Last edited by Walker; 11-19-2019 at 10:58 PM.
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Old 11-19-2019, 10:50 PM   #9
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Could be you need to adjust your valves.
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Old 11-19-2019, 11:55 PM   #10
RifleBowPistol
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It might be a decompression cam lobe, I do not know the correct name for them. I just rebuilt my 4 wheeler. It turns out it has a piece on the cam, that opens the exhaust valve some when cranking to kill compression when cranking to make it easier to crank.
I found that piece works very well, I adjusted the valves the way I have been taught to adjust valves on race cars. Which is to adjust a intake valve as the exhaust valve for that cylinder just starts to open. Then turn the engine over till the intake valve is just about closed and adjust exhaust valve. Well it turns out that does not work with, ATVs and some bikes, such as four stroke dirt bikes. If you adjust the valves the way I was taught for cars, the decompression cam lobe does not work. So the compression is not bled off when cranking, so it is almost impossible to crank the engine over. If you have a high compression four stroke ATV or dirt bike with a decompression cam lobe, you have to adjust with the piston at TDC for the compression stroke. At TDC, the decompression cam lobe does not interfere with adjusting the exhaust valves.
I found I could hear that cam lobe rattling around when turning the engine over. It seems to operate on centrifugal force. It only opens the exhaust valves and bleeds off compression at low engine rpms, basically when cranking. But as soon as the engine starts, the rpm spins the cam lobe fast enough lobe basically retracts, so it quits bleeding off compression. There is a good chance that cam lobe is your noise is what you are hearing and not a problem.

I know my 4 wheeler makes some noise when I crank it over and it does not start. I was thinking it was either the rod or main bearings, which are needle bearings. I am not completely sure the needle bearings are not making noise the noises I hear when cranking the engine. But when it starts, I don't notice any abnormal noises, so I don't worry about it. If you are getting filtered oil to needle bearings, they should be good. Needle bearings don't need much oil, all they need is oil splashed on them, for them to be properly lubricated. So usually roller bearings get oil strayed or splashed on them and not pumped to them to create pressurized oil within the bearings, like a Babbitt bearing, such as most automotive bearings.
You should probably check the adjustments of the valves, it's a good thing to do every year or so, depending on how much it is used. Possible causes could be the decompression cam lobe, valve adjustments, possibly loose bearings, possibly too much piston to wall clearance, basically worn cylinder. If you have a piston or pistons that fit too loosely in the cylinder or cylinders, you will get some piston slap or rattle, till the engine starts to heat up. With most engines, it only takes a few seconds for the engine to heat up enough to stop piston slap or rattle. My wife's car has some piston slap, it only does it dead cold and stops within about five seconds of starting the engine.
Most likely I would not worry about the noise, unless you find metal flake in the oil or find metal in the oil filter.
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Old 11-20-2019, 08:23 AM   #11
crumrw
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Does this bike have a internal belt or chain of any kind? My four wheeler has a belt from the crank to the transmission. When stretched, it does exactly what you're saying because the slack causes the belt to smack the motor case internally.
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Old 11-20-2019, 01:41 PM   #12
Lippy
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Quote:
Originally Posted by RifleBowPistol View Post
It might be a decompression cam lobe, I do not know the correct name for them. I just rebuilt my 4 wheeler. It turns out it has a piece on the cam, that opens the exhaust valve some when cranking to kill compression when cranking to make it easier to crank.
I found that piece works very well, I adjusted the valves the way I have been taught to adjust valves on race cars. Which is to adjust a intake valve as the exhaust valve for that cylinder just starts to open. Then turn the engine over till the intake valve is just about closed and adjust exhaust valve. Well it turns out that does not work with, ATVs and some bikes, such as four stroke dirt bikes. If you adjust the valves the way I was taught for cars, the decompression cam lobe does not work. So the compression is not bled off when cranking, so it is almost impossible to crank the engine over. If you have a high compression four stroke ATV or dirt bike with a decompression cam lobe, you have to adjust with the piston at TDC for the compression stroke. At TDC, the decompression cam lobe does not interfere with adjusting the exhaust valves.
I found I could hear that cam lobe rattling around when turning the engine over. It seems to operate on centrifugal force. It only opens the exhaust valves and bleeds off compression at low engine rpms, basically when cranking. But as soon as the engine starts, the rpm spins the cam lobe fast enough lobe basically retracts, so it quits bleeding off compression. There is a good chance that cam lobe is your noise is what you are hearing and not a problem.

I know my 4 wheeler makes some noise when I crank it over and it does not start. I was thinking it was either the rod or main bearings, which are needle bearings. I am not completely sure the needle bearings are not making noise the noises I hear when cranking the engine. But when it starts, I don't notice any abnormal noises, so I don't worry about it. If you are getting filtered oil to needle bearings, they should be good. Needle bearings don't need much oil, all they need is oil splashed on them, for them to be properly lubricated. So usually roller bearings get oil strayed or splashed on them and not pumped to them to create pressurized oil within the bearings, like a Babbitt bearing, such as most automotive bearings.
You should probably check the adjustments of the valves, it's a good thing to do every year or so, depending on how much it is used. Possible causes could be the decompression cam lobe, valve adjustments, possibly loose bearings, possibly too much piston to wall clearance, basically worn cylinder. If you have a piston or pistons that fit too loosely in the cylinder or cylinders, you will get some piston slap or rattle, till the engine starts to heat up. With most engines, it only takes a few seconds for the engine to heat up enough to stop piston slap or rattle. My wife's car has some piston slap, it only does it dead cold and stops within about five seconds of starting the engine.
Most likely I would not worry about the noise, unless you find metal flake in the oil or find metal in the oil filter.
Thank you thats a lot of good info I will have to look into. It does sound like a part that is working off of centrifugal force since it tends to knock whenever you let off the starter and everything comes to a stop. This particular model doesn't have an oil filter but I did check for metal in the oil on the last change and there was nothing so that made me feel better. The noise is so loud and obnoxious that I couldn't begin to think that it is normal.
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Old 11-20-2019, 01:47 PM   #13
Lippy
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Quote:
Originally Posted by crumrw View Post
Does this bike have a internal belt or chain of any kind? My four wheeler has a belt from the crank to the transmission. When stretched, it does exactly what you're saying because the slack causes the belt to smack the motor case internally.
It has a timing chain but other than that it is gear driven into the transmission.
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