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Does my antique HONDA CB 125 have a serious engine problem, OR is this noise NORMAL to this particular eng. |
Only 1000 miles, young engine, bought used, (possibly abused) by original owner. Engine seems to operate normally, but there is a strange "rattling - like" sound most noticeable at idle and lower rpms, which I think may be related to the timing chain or associated parts. There is a tensioner adjustment screw which was (still is) set such that there is about 1/2 " threads showing above the lock nut. Turning it clockwise, does not change the rattling much but it also makes an additional sound (more like a knocking sound) Turning it ccw, seems to have little effect, perhaps a slight increase of the rattle. I carefully returned the screw to it's ORIGINAL position. I do not think anyone has ever made any adjustments to it. I also have another similar vintage CB-100 which does not make this sound. Is this sound NORMAL to the 125, or predicting DISASTER? Does yours also rattle ? Please give advice on what to do if anything? A nice old bike, and I want to preserve. Yes Saturn,,that's "Normal". Unfortunately. It's Broken. You have an "early style" 125 with a manual tensioner. Mid 70's Honda changed to a "semi-auto" spring loaded tensioner The cam chain tensioner comes apart on the early engines. (There's only One tensioner,by the way) Leaving the mechanism flop around freely,,as well as fail to apply tension to the chain. Which is why amidst the Rattling of the slack chain,, You can also produce a Knocking Sound with the Tension Adjuster. It's an Odd setup. When it BREAKS,,,it automatically applies Some Degree of Tension. It's not even actually "Real Tension",,,,more of a removal of SOME slack. "Tightening" the tensioner does NOTHING. Loosening it actually DOES loosen the chain----but at some point it FREES the Chain to flop around. And at that point the Broken & Un-anchored Tensioner is then free to flail around. Chain slaps at it,,,and bangs it into the engine's innards,,,hence to KNOCK which appears as you Loosen the tensioner. I realize this is too much to read,,and you're probably not interested. Sorry 'bout that. But,,thats the How/Why the Noises change. One thing you DO need to be aware of is that a simple R&R(Remove & Replace) Repair is NOT any actual Reliable Repair. It can Recur at any time. A Solid Fix demands a "Home Remedy" Modification. Honda Never Did update to any actual true solution. They just changed the whole entire Type & Design in order to prevent Self-Destruction AND Manually Initiated failure. The Bikes break themselves on a whim,, and People break them as well----Probably More Often People are the cause. Any average Nutsy~Boltsy type person willing to tackle the job can repair it theirself. The Job DOES require some gaskets,, and a SPECIAL TOOL to remove the alternator rotor. It's possible to remove it without the tool,,, but it's NOT Recommended to do so. Everything at risk under THAT shadetree costs LOTS more than the tool does. The Details of How-To perform the general procedure are covered in any service manual. The Mod which is necessary to stabilize the fault and prevent recurrence is quite simple,,actually rather obvious. There's a few options which allow "anybody" to do it with whatever resources they happen to have on hand. But it gets far more wordy than this to describe in detail. So I'll leave it at that. DO NOT run the engine in current condition. I've seen Many 100's of 100's & 125's that have run as much as a couple 1000 Miles "Broken". Which is why I stress DO NOT RUN as-is. Normally Not a Major Expense when repaired early,,,and a fairly easy job to do. Or,,,take it to a shop. Anyone familiar with Hondas should be able to do a fine & quick job,,at a reasonable price. And end that problem forever. Again,,sorry this is so long-winded. Hope it's of any use to you. .............. *** Valve Adjustment at "Only 1000 mi"?? Absolutely! The Highest Rate of Dimensional Change in all of the details which directly affect valve clearance occurs in NEW engines. Your "lil' single cylinder engine" has about 2 Dozen individual points trying to stabilize their working clearances. A .002" Valve Lash just dont stand a chance against that. >>>>>>>>>>>>>>>> Here's some pictures if you care to look NEWER Style>Spring Loaded http://images.powersportsnetwork.com/fic... ................... Original Style>> YOURS (manual) http://images.powersportsnetwork.com/fic... Item # 16 is no longer clipped to the end of adjusting screw. Chain Slipper/Tension Bar is UNATTACHED at Lower end. And #16 is off wandering around somewhere in the engine. ................... Here is Newer Style,,,just to get a rough idea. *1st Frame shows the Upper Anchor Bolt for the Tensioner at approx "4 O'Clock" to the Camgear,,,on the outer edge of the Access Hole *2nd Frame shows how the Tensioner is anchored at Bottom End,,,and manipulated by the Tensioner Screw. ----In THIS engine the Spring Loaded Lever applies to pressure. ----In YOUR VERSION,,,,The SCREW actually Pulls UP to cause a "Bow" in the Chain Slipper,,removing any slack. You can also see how it's all situated Behind the Rotor,,requiring it's removal http://www.oldrice.com/thunder125.html... ~~~~~~~~~~~~~~~~~~ All those Pictures SHOULD Zoom if you click and/ or hover mouse a moment Usually on that type of Honda engine there is also another cam chain adjuster on the engine case just under the carb close to the cylinder. Check with a manual that you are doing the adjustment completely and correctly. It could also be that the engine is in need of a valve adjustment. it sounds like valve adjustment to me, you access them by the screw caps you were refering to, go to bike bandit.com or oldbikebarn.com they should have schematics and tolerances on one or both of them cam chain adjuster is sticking. it is a spring loaded mechanism, you loosen the locknut, loosen the main nut, and it should "spring" into place against the cam chain, then you retighten However, you have an old bike. Miles are not as important as how long it sat. the rubber parts corrode some, then metal rusts a little, etc.... I would buy a manual for it, and figure out how to remove the cam chain tensioner, and get a new one. Also, check the valves, always a good idea. do not assume, cause it has low miles, that it was maintained, and everything is good. Remember, it sat for a long time. |
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