Topic: CB 125 Stalls

[zglenn]

Hoping someone can help,

I have a 1975 cb125s that I have been fixing up. So far I've replaced the points and condenser, set the timing, new battery, new plug, and cleaned the carb.

The bike starts on a kick or three with no choke (it won't start with the choke on) and idles fine. It run smoothly but stalls at lights.  It seem to get worse the longer I ride. I've adjusted the clutch to make sure it isn't dragging. The coil and spark plug wire are original. I'm thinking I should replace these two but I figured I'd see if you all might have some advice before I randomly replace parts. Oh one more thing, I did try putting on an ebay mikuni knock off carb and it ran well, easier start, but also dies when off the throttle and out of gear e.g. when at a light or even if i pull in the clutch and coast off the gas at speed.

Really appreciate any advice,

Zack

[Stev-o]

Could be a couple things, start here:

is your air filter clean?
is fuel delivery nomal? [pull fuel line to check]

[zglenn]

Thanks for the quick reply.

I put a new K&N filter on it. I actually put the orig. filter back on today, just trying to get back to basics to see if that might be the problem. Fuel seems ok. If I pull the fuel line, fuel pores out?

Zack

[Stev-o]

If I pull the fuel line, fuel pores out?

Zack

Yes, it's gravity fed. Turn petcock off, have a cup handy, pull gas line and put in cup and turn on petcock. If fuel does not flow freely, that may be your issue.

Did you remove the airbox? Old Honda's do not like that!

[zglenn]

The fuel flows freely with the petcock in the on or reserve position. I did remove the airbox the K&N is a cone style filter. I figured I should be able to compensate with the fuel/air screw and different jets if needed. Maybe I should put it back on and ride it for a day or two and see if that fixes the stalling. It just seems that if it idles fine at start up then the problem isn't too much air. But I'm just learning. Really appreciate the help.

Zack

[Stev-o]

Yes, put the airbox back on and see what it does.

Good luck.

[zglenn]

Ok, it's not the airbox. rode it a bit today with the box back on and no fix. The symptoms are very specific, it starts easily and runs well when cold but after 5 to 10 minutes of riding it wants to stall at idle and is hard to restart. I reset the idle @ 1300 and messed with the air/fuel screw, I think its good there. It gets fuel and air and idles smooth when cold. I'm not sure that the valve clearance is set correctly, I re-gapped it this morning but I'm not sure if I'm using the right gauges. One other thing, turning the headlights on does seem to have an effect, it SEEMS stalls easier when they are on but when I turn off the bike and check the battery its reads over 6v. I have heard that the coil can be effected by temp when it begins to fail? Regarding the valve clearance the clymers says .002 mm, my gauges look like .002 over .5 mm not sure what that means?

e.g.
.002
.5
mm

thanks,

Zack

[zglenn]

a few more bits of info in case anyone has a thought or two?

It is definitely more likely to stall with the headlight switch on. I'm not sure what the draw should be but the voltage after the battery goes from above 7 to 6 when the lights on at idle. This actually seems about right to me, but the idle gets rough and often dies at this point. So if its electrical but the alternator and rectifier are working fine then I'm back to the coil and plug wire. Am I just talking myself into this solution? Anyone?

Thanks,

Zack

[Uncle Ernie]

I'm having a vision;  You're getting a new plug, condenser, and manual.

[kirkn]

LOL!  Now that's funny!


But, his first post mentions new condenser and plug.

Certainly sounds electrical - that bit about being more likely to stall with the lights on is telling.  Maybe I'd try hooking up a 6-volt battery charger when the issue is happening just to confirm that it clears up with the charger giving things a good boost.

So, then you gotta figure out if a coil is going bad, leading to bad spark when it heats up, or if the charging circuit is not working.  And by charging circuit, I'm thinking of the battery itself, especially.  

But, given that you mention the battery was new to begin with, I'd be doing my number one charging system troubleshooting trick - clip a cheapie voltmeter leads to the battery, tape the meter to the tank, and go for a ride.  Watch what the battery voltage is doing as you go.  Is it dipping, dipping, dipping, to the point where the motor starts running badly?  

Then, I'd be metering stuff:  stator unconnected AC output when running.  Stator resistance wire-to-wire and wire-to-ground.  Gotta short?  Gotta open?  Then, meter the rectifier diodes.

Between all of that electrical troubleshooting, you'll likely have your answer.

Good luck with it all, and keep us posted.

Kirk

[zglenn]

Ernie: new plugs i have, a new condenser yep and a clymers yep... but thanks!

Kirk, Thanks for the tips I will try your suggestions and let you all know how it went. It could still be the battery it's new but I may have fried it a bit over charging. Not sure if this is useful info. but I messed around a bit tonight and found that I'm getting about 6.5 volts at the battery while running with the light off, if the light is unplugged and I switch the headlight switch on the voltage jumps to about 7+ volts but if the light is plugged in the drain brings it down to 6 or less volts. Also it definitely idles indefinitely with the light off but stalls sometime immediately when switched on.

Thanks again, really appreciate it!

Zack

[kirkn]

Well, those numbers sound mostly exactly right.

The Honda singles (and twins) of the early/mid 70s had an 'auxiliary' charging coil that was switched in when the headlights were switched on.  That would explain the 6.5v when running with lights off and 7+v with lights switched on (but bulb unplugged).

However, I don't think the voltage should dip that low when the lights are plugged in.  I think your problem lies in that clue somewhere.  Maybe a short somewhere in the lighting wires or in the bulb(s)?  Seems like there's just more drop than there should be.  Is it the stock headlight?  Or maybe it's an aftermarket bulb that's demanding too much?  Maybe something in the switch itself?  Although a switch problem would show up with bulb plugged OR unplugged...

And yeah, 6 or less volts at the battery can definitely cause running problems.  The 6-volt bikes especially like to have a very freshened battery, since all the ignition current is supplied by the battery (and replenished by the charging circuit).

My own '72 SL125 behaves exactly the same way if/when the battery voltage drops off (like from sitting for a month or so).

Keep us posted...

Kirk

[zglenn]

Turns out it was the headlight itself, maybe a partial ground/short. Spent a few hours with the multimeter and narrowed the problem down to the light being plugged in and turned on. The voltage would drop to 4.5. Any other conditions e.g. testing the ground and hot leads with the light out but switch on read 6.5+. Finally I tested using a taillight bulb to complete the circuit and there wasn't a voltage drop. Ordered a new headlight bulb.

Thanks for all your help!

Zack

[Stev-o]

Would of never guessed that, glad you found the issue.