Honda CB125E - The Mysteries of the Cam Chain Tensioner Explained

The Honda CB125E does not hold too many secrets for a competent and/or experienced home mechanic. This video provides a short explanation of the cam chain tensioner and how to reset it if tension on the cam chain has been dropped.

[@tompatrick2744]

Thank you for posting this video. Researching the cam chain tensioner, I found plenty of sites and videos that discuss removal, installation and function, but this is the first video I found that shows how the device actually works.

[@tatendajesa-zv9gy]

It's a nice teaching for us learners

[@crazydoggentleman7930]

Top video mate

[@RawVeganFruitarian]

My tensionor looks the same but once wound in clockwise will hold itself in place without the cover screw, and is then released by a 1/4 anticlockwise turn.

[@chrisd4813]

Would you be able to put up a video on how to check the timing on this bike?

[@highoctanecommuting3862]

Hi Christopher! Do you mean the ignition timing? On the CB125E it cannot be adjusted, at least not easily. It is a CDI unit that works directly off the flywheel and it should be pretty much spot on for the life of the bike.

[@chrisd4813]

​@@highoctanecommuting3862 That's exactly what I meant. Hrm, I got one of these from a friend and it isn't starting - it has spark and with the thumb test, it appears to have compression - if I spray starter fluid right into the carb, I get a fireball going back to the carb which makes me think that the problem is the spark going off too early. Do you have any suggestions on troubleshooting a no start condition? The float bowl is filling with fuel, its been drained and I have cleaned out the jets so it appears to be getting fuel.

[@highoctanecommuting3862]

@@chrisd4813 That sounds like a fun and challenging problem, I'm a little jealous that I can't get hands on with that one. You have definitely started down the right path. You can check the timing with a timing light, there is an F index on the flywheel a few degrees away from the T index visible through the inspection cap as seen in the cam timing videos. I don't believe this is the problem but without the bike in front of me it is hard to say. How many kilometres has the bike travelled? Has it been well maintained? What were the circumstances where the engine failed to start? Did it simply fail to start one day, did it stop while riding normally or was it something else? Is the spark plug a healthy colour or is there signs of an incorrect mixture or oil burning? Is it wet with fuel after repeated attempts at starting it? Google will help you find a spark plug colour chart. If the fuel in the bike is older than six months drain it and replace it. Buying a new spark plug is cheap and knocks a faulty plug right out of the equation so thats a great place to start. Consider changing the oil if the history of the oil in it is unknown, when you drain it look for tiny flakes of metal or discolouration. Are there any unusual noises when the motor is spun over? Remove the spark plug from the engine, attach the spark plug lead and hold the thread of the plug to the engine, spin the motor over with the starter motor and you should get a strong regular spark. If you do next step check the valve clearances and make sure they are correct. Valve recession can cause the clearances to tighten up and hold the valves open just slightly. This is what I suspect it might be, it might pass the thumb test but not fire. Check that the cam chain is tensioned and valve timing is correct Valve timing will only change if there has been an event such as the cam chain skipping a few teeth or misintentional mechanical contact within the engine so it is unlikely to be the problem but worth checking. While you are checking the valve clearances rotate the motor several times with a socket on the flywheel making sure valves are opening and closing correctly, with the spark plug installed you should feel resistance when the engine is on the compression stroke. The CB125E has a robust little engine, if you get to this point without any hint of what is happening then you have an interesting challenge in front of you indeed. So let us know how it went so we can all learn from your experience (or help if you are still stuck) :)

[@chrisd4813]

That is an extremely well written response - I have replaced the spark plug already, checked and confirmed it does have spark. The bike conked out going uphill, which sounded like a fuel starvation issue (It was a friend's bike, that he's given to me for free to tinker with). It's travelled a small 7700km - but I don't think he did any maintenance to it. I'm picking up some feeler gauges to check the valves this week, because it seemed like compression or timing because it barely coughs with starter fluid sprayed into it. The bike itself cranks fine with the starter motor, but it does occasionally screech if I hit it again before it stops spinning so I suspect that is user error. I will adjust the valves as per your previous video at some point next week, and get back to you. Thanks for the tips!!

[@chrisd4813]

I've just adjusted the valves to spec, and it's still not starting. To rule out fuel i sprayed some fuel directly into the carb and cranked it (it made a wicked fireball but no signs of starting). This suggests to me that the spark is either firing too early, or the intake valve is still open when the spark is firing. Do I need to pull the motor to adjust valve timing or can I just "skip" the timing chain back a tooth?

[@SJP999]

Do I have to rotote the engine a couple Of times once I've released the tensioner?

[@highoctanecommuting3862]

Once the cam timing is done, everything is tightened down and the tensioner has been released (and tightened back up again) then yes, its a good idea to rotate the engine. If it wont turn or turns part of the way then stops then something is wrong!!

[@burt2599]

Then, i wound it out as far as i could n the bike would stall at idle clunk stop so i done it again without adding extra tension but then the tapping was back

[@burt2599]

My bike still has same issue