Author Topic: 1980 CB125S Help, please!  (Read 5008 times)

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Offline spraykruger

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1980 CB125S Help, please!
« on: April 24, 2013, 05:59:53 PM »
Hello everyone, I am having a very troubling time with my 1980 CB125S. The bike has 4,000 miles on it. One day I was riding along and my bike died. Turned out that it was a blown piston so I replaced that recently and ever since that, the bike won't start. I checked the important stuff first. I have fuel going through the petcock into the carburetor, I have thoroughly cleaned the carb and everything looked fine, the spark plug looks good, I have compression, I have gone through all the wiring and every connection looks good and clean, I bought a new battery which did nothing to improve the situation, my points look good. The bike won't jump start. The only thing I have to go on is that the spark I have is a little weak. Its definitely there, but its not as good as it should be. Also, my lights are working when I turn the ignition key into the on position. What is happening? Any advice, input, experience would be greatly appreciated!

Thanks, guys!
'80 CB125S

'74 CB550

'03 CR250

Offline grcamna2

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Re: 1980 CB125S Help, please!
« Reply #1 on: April 25, 2013, 01:05:22 PM »
I had a similar experience in the past & remedied it by removing my spark advance mechanism and checking to be sure that the points cam was oriented correctly;it IS possible to install the points cam on the advancer 180 degrees off time so it fires when the piston is headed downward rather than up-ward  :o.
There is a little line inscribed on the points cam which you can use for reference.
I hope that works..,otherwise we'll look elsewhere for you.

                              Bill
75' CB400F/'bunch o' parts' & 81' CB125S modded to a 'CB200S'
  I love the small ones too !
Do your BEST...nobody can take that away from you.

Offline spraykruger

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Re: 1980 CB125S Help, please!
« Reply #2 on: April 27, 2013, 01:16:08 PM »
Thank you, Bill!
I'll likely be digging into next weekend when I have a full day to really focus on the problem and I'll try removing the spark advance mechanism and double checking the points cam (although, I believe I had it timed properly).

Anyone else have/had a problem like Bill and I have had?
'80 CB125S

'74 CB550

'03 CR250

Offline Stev-o

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Re: 1980 CB125S Help, please!
« Reply #3 on: April 27, 2013, 01:59:02 PM »
Charge the battery and put a new spark plug in it.  Just because it "looks good" doesn't mean it is good.
'74 "Big Bang" Honda 750K [836].....'76 Honda 550F.....K3 Park Racer!......and a Bomber!............plus plus plus.........

Offline grcamna2

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Re: 1980 CB125S Help, please!
« Reply #4 on: May 01, 2013, 07:23:09 AM »
What'd you find on the CB125S spraykruger ?
75' CB400F/'bunch o' parts' & 81' CB125S modded to a 'CB200S'
  I love the small ones too !
Do your BEST...nobody can take that away from you.

Offline spraykruger

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Re: 1980 CB125S Help, please!
« Reply #5 on: August 13, 2013, 06:25:21 PM »
What'd you find on the CB125S spraykruger ?

I still can't figure it out! However, after hours figuring out (and buying new) points, I got the bike to backfire while attempting to push start it. This is more life than it has ever shown, is it telling of anything?
I see the line you are talking about on the points cam, but how do I use that to see if I am 180 degrees off?
When I am setting the timing, am I setting it so that the points are closed (touching each other: grounded) or when they are open (not touching each other) at top dead center? Here is what my shop manual says after putting a test light across the line:
  - "Turn the rotor slowly until the test lamp goes out and check the position of "F" mark on the rotor against the index mark on the L. crankcase. If they are in line, the ignition timing is correct."

When I turn the rotor, I pass the "F" mark twice to get back to TDC. When should the test lamp go out? At "F" near TDC or at "F" opposite of TDC? Right now I have the points closed (grounded/no light on my test light) at "F" opposite of TDC. This made sense to me in relation to the four strokes of a four-stroke engine (intake, compression, power, exhaust). Am I correct? Is it backwards?


P.S. Sorry for rambling, this whole timing process is very new to me and I am only now beginning to understand it.
'80 CB125S

'74 CB550

'03 CR250

Offline toytuff

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Re: 1980 CB125S Help, please!
« Reply #6 on: August 13, 2013, 06:50:45 PM »
The light should come on at the F mark right after the T mark. Not before, not after. This is called static timing.

Have you checked the valve clearance? Might be to tight.

tt


Offline Tugboat

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Re: 1980 CB125S Help, please!
« Reply #7 on: August 14, 2013, 03:37:46 PM »
To reverse the direction of the points cam, remove the points plate. Remove the bolt on the end of the shaft. Manually move both advance weights "out" (you'll see what I mean) and pull out the tube in the middle. That is the points cam. Turn it 180 degrees, reinstall everything. Check timing. Fire it up!

edit: Just did this on my CB100 last night to clean the advancer and install a new points plate. It literally IS this easy :)
« Last Edit: August 15, 2013, 08:19:32 AM by Tugboat »
If it's worth doing, it's worth doing twice.

Offline grcamna2

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Re: 1980 CB125S Help, please!
« Reply #8 on: August 15, 2013, 02:11:42 PM »
The points should just barely begin to open & the test light will turn Off when you hit the "F" mark for correct timing.The points will do this to allow the system to fire..,but it fires just when they OPEN & not close.
75' CB400F/'bunch o' parts' & 81' CB125S modded to a 'CB200S'
  I love the small ones too !
Do your BEST...nobody can take that away from you.

Offline Untold

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Re: 1980 CB125S Help, please!
« Reply #9 on: August 15, 2013, 04:39:28 PM »
To reverse the direction of the points cam, remove the points plate. Remove the bolt on the end of the shaft. Manually move both advance weights "out" (you'll see what I mean) and pull out the tube in the middle. That is the points cam. Turn it 180 degrees, reinstall everything. Check timing. Fire it up!

If I understand you correctly, wouldn't the locator pin / woodruff key thing make this impossible?
1976 CB550K

Offline toytuff

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Re: 1980 CB125S Help, please!
« Reply #10 on: August 15, 2013, 06:40:38 PM »
First, the static timing light is simply a 12V light bulb wired across the points terminal (or spring) and ground. With the ignition on, you should get 12V across the points when they are open, zero when closed. It tells you when the points just begin to open; better than you can see by eye.

1) Position the points plate at the middle of it's travel and snug it down.

2) Turn the crankshaft clockwise to put the rotor on the "F" mark.

3) Adjust the points so they are just closed - this is just to get you in the ballpark.

4) Rotate the crankshaft clockwise until the points are at their max opening; they will open quickly and stay at max open for a good bit of crank rotation; you can just eyeball it, there isn't one point that you have to catch exactly.

5) At max open, set the points to about .014" and snug them down. Don't be a hero here, those points screws strip pretty easily.

6) Again, Rotate the crankshaft clockwise to put the rotor on the "F" mark.

7) Ignition on

8) Now loosen the points plate and rotate it back and forth until the static timing light flickers between on and off and snug it down again.

9) Turn the crank clockwise - the light should stay on for a while, then go off as the points close, then on as the rotor "F" mark again passes the pointer.

10) If the light goes on too soon, the timing is advanced; too late it is retarded.

11) To advance the timing, rotate the points plate counter-clockwise or open the points gap slightly; you can loosen both screws and use the eccentric, or just bend the ground-side of the points slightly.

12) To retard the timing, do the opposite, rotate the points plate clockwise, or close the points gap slightly.

13) Rotate the crank around a few times to be sure you've got it. Since the points cam has two lobes, you may never get both sides perfect, but you should be able to get close.

14) IGNITION OFF

tt

Offline spraykruger

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Re: 1980 CB125S Help, please!
« Reply #11 on: February 22, 2014, 03:06:53 PM »
Its running!
Finally after gathering the gumption to tear into again, I took the engine back off and apart. After re-inspection, I noticed the valves and the valve seats were totally shot. Metal from the exploded piston had melded into the valves and peckered the valves seats up really bad. I ended up re-cutting the valve seats, replaced the valves, bored the cylinder to the +.75 spec size, got a new +.75 piston/ring set, put it all together and it ran 3rd kick! Just wanted to thank everyone for their interest in this. I should note that after putting the engine back together, I was only getting approximately 50PSI on my compression test, but after running for a few minutes, its back to over 150PSI. I also got a new carburetor in hopes to avoid the problem again.
'80 CB125S

'74 CB550

'03 CR250

Offline toytuff

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Re: 1980 CB125S Help, please!
« Reply #12 on: February 24, 2014, 04:20:18 AM »
There ya go!

 8)

tt

Offline grcamna2

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Re: 1980 CB125S Help, please!
« Reply #13 on: February 25, 2014, 07:55:39 AM »
That's great spraykruger !
I'm looking for a 1982' model year CB125S in fair condition;I'd like to get a "barn find" that's complete,if you ever here of one for cheap not too far from the Pittsburgh area please let me know.
« Last Edit: February 26, 2014, 10:01:55 PM by grcamna2 »
75' CB400F/'bunch o' parts' & 81' CB125S modded to a 'CB200S'
  I love the small ones too !
Do your BEST...nobody can take that away from you.

Offline Tugboat

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Re: 1980 CB125S Help, please!
« Reply #14 on: February 26, 2014, 10:59:36 AM »
Well done!
If it's worth doing, it's worth doing twice.