1974 CB125 engine noise

The "tensioner" on your bike doesnt really apply tension. It basically lightly presses into the chain to extend its running path to take out the slack, more similar to a chain guide than a chain tensioner. The chain has not deformed from sitting for 40 years. Maybe 4000 years and youd start to see some "creep" in the metal simply from the weight of the chain, but 40 years is not enough to add THAT much slack to a cam chain, even if you did have a spring loaded tensioner. 5 minutes of running the engine puts more stress on that chain than sitting for 1000 years would. Never in my life have I seen a camchain stretched to being unusable with only 5k on the engine.

Like I said, its way, way more likely that your tension has failed. The circlip that holds the adjustment arm to the tensioner bar is known to come loose and fall out on your model of bike. You can go ahead and replace the cam chain but its a million times harder than youre imagining without taking the engine apart first. Its not going to nicely slide over the gears if you link it to the old chain and try and feed it through, and its going to be next to impossible to join the 2 ends without having any slack at all available. Its also very unlikely to fix the problem.
 
Take a video of the bike after its been running for atleast 5 minutes. It should sound a lit better once it gets to temp.
After 41 years my tensioner still works fine and like as the above mentioned, the travel distance isn't as great as youd think. Its very minimal. I know because i did a top end rebuild myself.


I have a 74 cb550 and the slapping noise you hear is similar to mine during a colder start. It sounds right after riding it. Ill take a vid when i have a chance.
 
bobjohnson said:
The "tensioner" on your bike doesnt really apply tension. It basically lightly presses into the chain to extend its running path to take out the slack, more similar to a chain guide than a chain tensioner. The chain has not deformed from sitting for 40 years. Maybe 4000 years and youd start to see some "creep" in the metal simply from the weight of the chain, but 40 years is not enough to add THAT much slack to a cam chain, even if you did have a spring loaded tensioner. 5 minutes of running the engine puts more stress on that chain than sitting for 1000 years would. Never in my life have I seen a camchain stretched to being unusable with only 5k on the engine.

Like I said, its way, way more likely that your tension has failed. The circlip that holds the adjustment arm to the tensioner bar is known to come loose and fall out on your model of bike. You can go ahead and replace the cam chain but its a million times harder than youre imagining without taking the engine apart first. Its not going to nicely slide over the gears if you link it to the old chain and try and feed it through, and its going to be next to impossible to join the 2 ends without having any slack at all available. Its also very unlikely to fix the problem.
Click to expand...

I dont know, I thought if the engine was shut off in the middle of a valve opening then the chain wouldve had spring tension on it all this time. Its really a tiny chain.

Regardless, I've inspected the tensioner by removing the magneto and adjusting the tension while I could feel the tensioner moving and the chain getting tighter. If my chain isnt the problem I dont know what is.
 
bobjohnson said:
The "tensioner" on your bike doesnt really apply tension. It basically lightly presses into the chain to extend its running path to take out the slack, more similar to a chain guide than a chain tensioner. The chain has not deformed from sitting for 40 years. Maybe 4000 years and youd start to see some "creep" in the metal simply from the weight of the chain, but 40 years is not enough to add THAT much slack to a cam chain, even if you did have a spring loaded tensioner. 5 minutes of running the engine puts more stress on that chain than sitting for 1000 years would. Never in my life have I seen a camchain stretched to being unusable with only 5k on the engine.

Like I said, its way, way more likely that your tension has failed. The circlip that holds the adjustment arm to the tensioner bar is known to come loose and fall out on your model of bike. You can go ahead and replace the cam chain but its a million times harder than youre imagining without taking the engine apart first. Its not going to nicely slide over the gears if you link it to the old chain and try and feed it through, and its going to be next to impossible to join the 2 ends without having any slack at all available. Its also very unlikely to fix the problem.
Click to expand...

Also, if the slipper and the tensioner have any noticeable 'grooving' in them, the chain tensioner won't be able to get enough throw to take out the slack. Just putting a chain in by itself is (as has been said) a lot of work and likely a total waste of time.
 
Well I finally got a chance to get into the motor. Here's what fell out of the crankcase last night:


2dijdp2.jpg
 
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Chunks off of the transmission mainshaft?
 
Oh. Well, those teeth looked really wide in the first pic.

How's the availability of gear parts for a 42 year old less popular Honda?
 
Pretty good. This particular motor was actually used in a ton of models in the 70's during which time small bikes were popular due to the oil crisis. Im looking at about $150 for the pair to my door.
 
Finally got this thing back together with all new gaskets and fresh rings and it will not start.

I did not mark TDC before tearing down the motor but all my marks line up when I turn it and I've physically checked this by confirming that when I'm at TDC my valve rockers are both wiggle loose and the points have just begun to open. (the advancer and points plate only go on one way, the cam sprocket can be turned 180deg). In addition I have a wet plug when I pull it after a good kickstartign session and I get a bright blue spark when I ground it, despite having a completely dead battery.

I checked the carb and some jets - its still very clean after sitting a year although I did have to adjust my float height as it was bottoming out.

This engine ran ok before - what am I missing?
 
I think what you hear is "old age" . I had a 74 or 73 cb200 , it ran :) but made noises I couldn't figure out what..
 
Fuel fouled plug, by the sound of it. dry out the plug and cylinder and try again. Hopefully the carb float adjustment will prevent that from happening again. Also verify your idle mixture/air adjustment. Too much choke?
 
So basically I live on a fairly major hill and decided to try bump starting it. On the second time down it fired and I could keep it alive with just the right amount of throttle. I rode it enough to warm it up (was riding sans gas tank) but it would not kickstart. I bump started it. Few times down that hill but it wouldn't give me anything from the kickstarter.

before all this when I was still riding it the choke never worked - it had no effect most of the range and the motor would die at max. I stripped, dipped and ultrasonically cleaned the carb but the bike did sit for 40 years prior to keeping so. I don't know if this is related?
 
Definitely sounds like carb related. Did you check for vacuum leaks? If that's not the problem you may have a hole in the carb diaphragm.
 
bobjohnson said:
Do you have the spark advance set correctly?
Click to expand...

Im not sure....is it not just a weighted module that slides over the part of the camshaft that protrudes out of the head? 1. I did not know it was adjustable and 2. how would it help idle? doesnt it just advance spark at higher rpm?



I messed with the carb this weekend. Stripped it again and tried to see if there was anything in the ports. I inserted guitar string where possible and blew everything eese out with compressed air. I removed all jets and checked that every hole and orifice was clear on each.

Would not kick over. Bump started down the hill and ran ok.

Symptoms I've noticed:
a. choke does nothing. If the bike is runnig and I engage the choke all the way the bike dies promptly. If I engage it partially there is no effect.
b. when idling, if i crack open the throttle quickly to WOT the bike stumbles up through 5 or 6k rpm before revving up to the 10k redline just fine with no backfiring, etc.
c. will not kickstart, without any signs of life

here are pics of the carb:

O9kmQTyh.jpg


8iUbcD8h.jpg


2fM9Bdoh.jpg
 
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Your carbs are set up wrong. Sounds like its too rich based on the choke actually choking it out and the low rpm stumbling. What is it idling at right now? Try dialing in the idle with the slide screw, and then fine tuning it with the pilot screw. Check the needle position, it should be set to the middle notch as a base line, try raising the clip (lowering the needle) if the adjustment screws arent giving you what you need. Also Check the main jet, is it oversized? Is the pilot jet oversized? Try running it without the airbox, does it run better? You can also try sticking your finger into the throat of the carb while the bike is idling, if theres and fuel pooled up around the main jet it might be leaking, I had a pair of 450 carbs where the left one was always leaking, after 2 months of trying to tune it turns out it had a microfracture around the mainjet which just let fuel into the engine unregulated.

Also probably want to check your valves to make sure theyre in spec, the 125 motor needs adjustment every 500-1000km or so, and they need to be exactly in spec otherwise it runs like ****. Even 1 thousandth off makes a huge difference.
 
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