Author Topic: CB125s compression issue (Read 9599 times)

y2kc0wb0y · « **on:** November 28, 2011, 10:28:18 AM »

Picked up a survivor level CB125s a few months ago. Bike has 595 miles on it and was basically kept indoors most of its life. The bike has super low compression from sitting. I think 70psi is the best I've managed to get it up too.

She does run if I pull start it and get some heat build up in the combustion chamber but it runs poorly with lots of blow by. Just curious before I tearing into her if there are any advanced tricks I could use to help loosen the rings up? I've tried SeaFoam in the crankcase/spark plug hole as well as Marvel Mystery Oil. I hate to crack her open if I don't have too and any advice would be appreciated.

grcamna2 · « **Reply #1 on:** November 28, 2011, 03:33:21 PM »

Are you sure the valves are adjusted good (.002 ea.) and that they're sealing ?
If so I guess it's easy enough to pull it apart & replace the rings & hone it.

y2kc0wb0y · « **Reply #2 on:** November 28, 2011, 06:46:57 PM »

Double Checked, Triple Checked.

MoMo · « **Reply #3 on:** November 28, 2011, 07:24:49 PM »

When you tried Seafoam did you put an ounce or so in the plug hole and let it sit for a couple days? If so and compression did not come up you are in for a top end rebuild-on that bike it is super easy. And that is a beauty...Larry

y2kc0wb0y · « **Reply #4 on:** November 28, 2011, 07:34:33 PM »

Thanks Larry. I did not try SeaFoam but MMO sat for weeks with the same result. :|

MoMo · « **Reply #5 on:** November 28, 2011, 07:38:04 PM »

Quote from: y2kc0wb0y on November 28, 2011, 07:34:33 PM

Thanks Larry. I did not try SeaFoam but MMO sat for weeks with the same result. :|

I have saved at least 4 bikes from top end tear downs with Seafoam-it seems to have a knack for freeing stuck rings. Can't hurt to give it a shot...Larry

y2kc0wb0y · « **Reply #6 on:** November 28, 2011, 07:50:38 PM »

Larry; that's encouraging. I'll give SeaFoam a shot at freeing this

grcamna2 · « **Reply #7 on:** November 28, 2011, 08:11:22 PM »

What year CB125 is that nice one ? I had an 82,which was the first year they had electronic (CDI) ignition & it had the same style kick starter on it. I bored it out to 135cc w/ a Powroll piston kit & installed a little Supertrapp exhaust on it. It ran well & would top out at 70+mph.

y2kc0wb0y · « **Reply #8 on:** November 28, 2011, 08:16:38 PM »

Hey there. Its a 1978.

grcamna2 · « **Reply #9 on:** December 02, 2011, 04:47:28 PM »

Cowboy,
Did the compression come up at all ?

Flying J · « **Reply #10 on:** December 04, 2011, 10:00:29 AM »

If do end up doing a rebuild they have cylinder with piston and rings for like $70 on ebay.

y2kc0wb0y · « **Reply #11 on:** December 05, 2011, 03:40:47 AM »

Quote from: grcamna2 on December 02, 2011, 04:47:28 PM

Cowboy,
Did the compression come up at all ?

Feels a slight bit better but it's still way down on compression numbers.

Flying J · « **Reply #12 on:** December 05, 2011, 10:39:25 AM »

I have never done it but you could do a leak down to see where the air is escaping from.
Here is that kit i was talking about.
http://www.ebay.com/itm/Honda-CB125-Cylinder-Bore-56-5mm-CB-CL-SL-XL-124CM3-/180659817775?pt=Motorcycles_Parts_Accessories&vxp=mtr&hash=item2a102a0d2f

y2kc0wb0y · « **Reply #13 on:** December 05, 2011, 11:33:20 AM »

Thank you sir. Amazing you can get the parts for the price of dinner and a movie.

MoMo · « **Reply #14 on:** December 05, 2011, 02:06:33 PM »

Quote from: y2kc0wb0y on December 05, 2011, 03:40:47 AM

Quote from: grcamna2 on December 02, 2011, 04:47:28 PM
Cowboy,
Did the compression come up at all ?

Feels a slight bit better but it's still way down on compression numbers.

What is the latest reading? ...Larry

y2kc0wb0y · « **Reply #15 on:** December 05, 2011, 08:47:19 PM »

65 cold. Too lazy to pull it around the block again to warm her up. Lots of smoke kicking it. That about 100 psi off of normal. I don't think she's going to come around.

MoMo · « **Reply #16 on:** December 06, 2011, 05:35:22 AM »

Quote from: y2kc0wb0y on December 05, 2011, 08:47:19 PM

65 cold. Too lazy to pull it around the block again to warm her up. Lots of smoke kicking it. That about 100 psi off of normal. I don't think she's going to come around.

Probably not as it should be at least 120 cold while kicking over with the throttle wide open. It is an easy rebuild and parts should be inexpensive...Larry

Davidov · « **Reply #17 on:** December 12, 2011, 05:58:57 AM »

You should be able to leave the bottom end alone, and avoid opening the case.

Measure the cylinder and piston to make sure it's not out of specs. If not you can install new rings, and hone the cylinder. Lap the valves too.

After that it should fire up on the first kick! Nice bike!

y2kc0wb0y · « **Reply #18 on:** December 12, 2011, 07:02:28 AM »

Quote from: Davidov on December 12, 2011, 05:58:57 AM

You should be able to leave the bottom end alone, and avoid opening the case.

Measure the cylinder and piston to make sure it's not out of specs. If not you can install new rings, and hone the cylinder. Lap the valves too.

After that it should fire up on the first kick! Nice bike!

RE: First Kick - That would make my day. Valve lapping is a good idea too. The last time I did that was on a old VW engine.

Davidov · « **Reply #19 on:** December 13, 2011, 08:19:08 AM »

The 79 CB125S in my avatar was a "first kicker". Hot, cold, choke, or no choke. It had a nice "THUMP, THUMP" with the aftermarket exhaust.

In regards to valve lapping. It is important to go both directions (counter-CW, and CW), and not spin the valve in one direction.

Flying J · « **Reply #20 on:** December 13, 2011, 08:55:26 PM »

I got my 78 125 motor fired up. It has a decent knock to it. When i turn the motor over to adjust the timing i can hear the cam chain click as it tightens and slackens. Not really sure how to adjust the cam chain on this one or if mine is even working.

y2kc0wb0y · « **Reply #21 on:** March 31, 2012, 09:23:59 PM »

So we pulled the top end down today and I'm glad we did. No marvel mystery oil was gonna save the burned/cracked piston we found. appears like something went thru the engine. Piston is toast cylinder wall was lightly scored. Lots of tail tell signs on the head. Gonna pull the valves and have a look. I'll post pictures soon.

Flying J · « **Reply #22 on:** March 31, 2012, 09:27:50 PM »

You can get new pistons and cylinder kits on ebay for like $70

y2kc0wb0y · « **Reply #23 on:** April 02, 2012, 06:26:26 AM »

Roasted; I wonder what the back story is on this damage. I was able to hone the cylinder wall out so just a finger nails size nick was in the wall. Do you guys think I'll generate a large heat spot here if I leave the nick? The rest of the cylinder looks good.

grcamna2 · « **Reply #24 on:** April 02, 2012, 01:45:50 PM »

That piston...I've never seen the like.I wonder if an individual poured a whole can of starting fluid into it..which could have hydro-locked it..and THEN tried to(literally

) "Fire it Up" ??

I want to see a picture of the Cylinder and Head internally.

y2kc0wb0y · « **Reply #25 on:** May 04, 2012, 09:15:23 AM »

Got the bike back together and running. Question: are the CB125 engines on the noisy side normally???

grcamna2 · « **Reply #26 on:** May 04, 2012, 10:04:42 AM »

Can you give us any details about the top end work that you did to it ?

y2kc0wb0y · « **Reply #27 on:** May 04, 2012, 10:51:29 AM »

In addition to replacing the piston and honing the cylinder we did just "basic" valve work seeing the bike only has 600 miles on it. Meaning: We just pulled the two valves, polished, lapped, cleaned the squish/combustion area of carbon and re-assembled per the manual.

grcamna2 · « **Reply #28 on:** May 04, 2012, 12:51:33 PM »

How did your cylinder measure up? did you use an inside micrometer that gave you accurate measurements all the way up & down the bore of your cylinder ?

y2kc0wb0y · « **Reply #29 on:** May 04, 2012, 04:27:30 PM »

Didn't have a meter. Used the compression ring and measured the gap which in ec down the sleeve

Flying J · « **Reply #30 on:** May 04, 2012, 04:58:36 PM »

What noise its it making? I rebuilt my 125 and i would not say that it is a quiet motor.

grcamna2 · « **Reply #31 on:** May 04, 2012, 06:28:40 PM »

Quote from: y2kc0wb0y on April 02, 2012, 06:26:26 AM

Roasted; I wonder what the back story is on this damage. I was able to hone the cylinder wall out so just a finger nails size nick was in the wall. Do you guys think I'll generate a large heat spot here if I leave the nick? The rest of the cylinder looks good.

It's possible the cylinder sleeve could be warped or out-of -round from too much heat in one place and that's causing it to be tight at one point in the stroke of the piston and loose at another point..localized overheating.

y2kc0wb0y · « **Reply #32 on:** May 05, 2012, 07:36:44 PM »

Quote from: ffJMoore on May 04, 2012, 04:58:36 PM

What noise its it making? I rebuilt my 125 and i would not say that it is a quiet motor.

Sounds is hard to describe - rattling and a lot of carb roar.

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Author Topic: CB125s compression issue (Read 9599 times)