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Blood Sweat Tears and Grease => Engines => Topic started by: natez on Apr 02, 2017, 17:34:13

Title: CB125S Engine HELP
Post by: natez on Apr 02, 2017, 17:34:13
I had a perfectly good running cb125s which I decided should have a 145cc cylinder and piston on it because at the time I was bored and needed something to do. Got the engine back together and back in the bike and for the life of me I cannot get it to run right. Starts and idles perfectly and has great power until exactly half throttle when it starts to run like it is hitting a rev limiter. Absolutely no power, missing, but no smoke or anything. Does it in neutral and under power. Carb is clean as a whistle, originally had a 95 main jet in there but have tried everything from it to a 125 with no change. Valve clearances are dead on, points are set, I believe the timing is right. No airleaks on the carb or anything. Ignition advance is working. Can anyone help? Its driving me nuts :/
Title: Re: CB125S Engine HELP
Post by: trek97 on Apr 02, 2017, 17:45:01
I had a perfectly good running cb125s which I decided should have a 145cc

originally had a 95 main jet in there but have tried everything from it to a 125 with no change.

I believe the timing is right

She may be running out of air.  As in...the the 125 carb may be too small to feed a 145.

I dont get a warm fuzzy when you write, "I believe the timing is right"

Welcome to DTT.
Title: Re: CB125S Engine HELP
Post by: natez on Apr 02, 2017, 18:12:42
The plug would end up all dark and fouled though wouldn't it? Perhaps smoke out of the exhaust? Others have done the same conversion I did and only needed to go up a few jet sizes.

If the timing was off, would it start and run beautifully until half throttle? The only reason I am not 100% confident in the timing is when I ended up bolting it together, the cam sprocket ended up 180* off, (arrow pointed down at 6 oclock instead of at 12, where it points to a timing mark on the cylinder head cover but I figured that didn't matter since everything else was lined up correctly, like the crank, and taking the time to fix it would not have changed anything once the sprocket is bolted to the cam)

Title: Re: CB125S Engine HELP
Post by: trek97 on Apr 02, 2017, 18:35:15
Starts and idles perfectly and has great power until exactly half throttle when it starts to run like it is hitting a rev limiter. Absolutely no power, missing, but no smoke or anything. Does it in neutral and under power.

Sounds like it is trying to foul.  Have you done a plug chop?

Find a road clear of traffic.

Ride the bike as as far into the problem rpms as you can and still keep it running.  Keep it going for a good 30-45 seconds in this rpm zone.

do NOT roll off throttle,  simultaneously pull the clutch and kill it w the kill sw or key.  Then safely coast it to a stop.  Pull the plug and see what color it is.

Title: Re: CB125S Engine HELP
Post by: trek97 on Apr 02, 2017, 18:40:59
Also, just for fun check to make sure your coil wires didnt get reversed at some point.  +feed to +term and - to - .

Put a timing light on it to double check and also ensure its not over or under advancing.
Title: Re: CB125S Engine HELP
Post by: natez on Apr 02, 2017, 18:43:03
Yes, I have done this and the plug condition looks lean. Light grey color on the plug. Same results with main jets 95-115. 120 and 125 produce a darker color and a bit of a wet plug. Again, if it was missing that bad because it was so rich wouldn't i get some smoke?
Title: Re: CB125S Engine HELP
Post by: trek97 on Apr 02, 2017, 18:45:18
Probably not enough to see behind you in the wind.
Title: Re: CB125S Engine HELP
Post by: natez on Apr 02, 2017, 18:46:11

Put a timing light on it to double check and also ensure its not over or under advancing.


Can you elaborate on the procedure for this? That is a bit beyond my maintenance manual. I used the timing light to see where it was at idle. Its a bit of a pain to try to use the light with that stator cover off and the bike running. I checked the movement of the advancing mechanism and it seemed to be fine.
Title: Re: CB125S Engine HELP
Post by: trek97 on Apr 02, 2017, 18:47:56
Hmm your plug chop seems to be pretty good.  Should be light tan to whitish for ideal mix.  It will be super clean, bright white if its going too lean.
Title: Re: CB125S Engine HELP
Post by: natez on Apr 02, 2017, 18:48:56
Probably not enough to see behind you in the wind.

But if you just rev it in neutral it stumbles exactly the same at a standstill as it does moving, under a load, no smoke. You can advance the throttle slowly and as it revs, it begins to stumble (sounds and feels EXACTLY like a rev limiter in a car) from exactly half throttle to full. You can sit there with it pinned and it just stumbles on and on not increasing in RPM.
Title: Re: CB125S Engine HELP
Post by: natez on Apr 02, 2017, 18:51:08
Hmm your plug chop seems to be pretty good.  Should be light tan to whitish for ideal mix.  It will be super clean, bright white if its going too lean.


LOL yea no kidding, thats what I am saying! It's not something simple here. I am an experienced enough mechanic to follow the maintenance manual and check everything it points to. I have jetted many a carberated bikes, rebuilt engines, on and on and on. So frustrated.   
Title: Re: CB125S Engine HELP
Post by: natez on Apr 02, 2017, 18:54:03
It is impossible to get across what is going on via a forum and not having an experienced person here to help is so incredibly frustrating. I just hope this has happened to someone else at some point and then chime in, otherwise I'm going to have to give up and throw it in the garbage.
Title: Re: CB125S Engine HELP
Post by: trek97 on Apr 02, 2017, 18:58:15
The rotor has a pair of marks just beyond the timing mark.  Whilst watching w timing light increase rpms to over 3k and you can watch the time advance, the timing mark should fall between these two advance marks on the rotor.  But not over or under these marks.
Title: Re: CB125S Engine HELP
Post by: natez on Apr 02, 2017, 19:00:10
Lets say it does fall beyond these marks, then what? There is no adjustment for that correct?
Title: Re: CB125S Engine HELP
Post by: trek97 on Apr 02, 2017, 19:02:15
I'm going to have to give up and throw it in the garbage.

I understand it can be very frustrating.  do not throw it in the garbage.
Title: Re: CB125S Engine HELP
Post by: trek97 on Apr 02, 2017, 19:07:58
Lets say it does fall beyond these marks, then what? There is no adjustment for that correct?

If it does fall between the marks.  Thats great.  We can rule out...sticky lobe, worn springs or weights.

If idle rpm timing is dead nuts and it does not fall between the marks.  Then you need to investigate why. 

Disassemble the advancer, clean and lube, replace springs or weights.

My bet is its carb related.

Have you experimented w carb needle height or different needles?

Title: Re: CB125S Engine HELP
Post by: trek97 on Apr 02, 2017, 19:10:33
did you test compression after the rebuild?

Title: Re: CB125S Engine HELP
Post by: trek97 on Apr 02, 2017, 19:13:23
Ooo,  I just had a thought...and THIS COULD BE IT!

Also, it could be your charging system is shot.  Not supplying enough voltage to properly charge the coil. 

Unplug all your lights including the headlight and go for a ride.
Title: Re: CB125S Engine HELP
Post by: trek97 on Apr 02, 2017, 19:16:24
fMaybe the headlight is the only one you need to unplug if you just cant switch it off.
Title: Re: CB125S Engine HELP
Post by: trek97 on Apr 02, 2017, 19:24:15
double check points gap.  If its too large it wont give the coil time to build a proper charge. 
If its to tight then it wont allow the coil enough time to cool between charging cycles and overheat.
Title: Re: CB125S Engine HELP
Post by: trek97 on Apr 02, 2017, 19:25:53
But definitely try turning off or unplugging headlight first cause thats the easiest.
Title: Re: CB125S Engine HELP
Post by: natez on Apr 02, 2017, 19:25:56
Yea I did a compression test the other day. Rock solid 155psi.

Charging system issue is def a possibility, and thats where things start to get confusing. This bike has a funky 6 volt electrical system. Won't run properly without a battery, burns out highlight bulbs if you do try to run it without a battery. These engines always run alittle weird with a bad battery but mine is brand new, and I have checked it and such. I am about to go try to run it without the head light plugged in but not sure its going to make a difference. It is the only light I have on the bike except on very small LED brake light. If the charging system is shot, do I replace the stator or somthing? When I took the engine apart I tested the stator as per the maint. manual and it checked out fine.
Title: Re: CB125S Engine HELP
Post by: trek97 on Apr 02, 2017, 19:27:59
I gotta go fetch a pizza...try the headlight and report back.
Title: Re: CB125S Engine HELP
Post by: natez on Apr 02, 2017, 19:43:14
Tried it. No change.

We can rule out the timing being off when I set up the cam sprocket right? Would it idle and run from idle to just under half throttle so well if it was off? I have also tried different points plate positions to change the timing either way (now back to stock position) with no change (only thing that would change is it would be very hard to get started in one direction). So it's either got to be the advancing unit not advancing the timing properly (inspected the unit, cleaned, lubed etc) or its got to be some sort of coil/power issue. Wires on the coil can't be reversed, I never disconnected the coil when I did the engine, stayed on the frame, only disconnected the spark plug wire.
Title: Re: CB125S Engine HELP
Post by: trek97 on Apr 02, 2017, 21:19:01
Dang sorry to hear that didnt work.  Let me think on it for a bit.
I am assuming you still got the selenium rectifier?
Title: Re: CB125S Engine HELP
Post by: natez on Apr 03, 2017, 20:50:23
I converted it to the radioshack one a while ago....
Title: Re: CB125S Engine HELP
Post by: natez on Apr 03, 2017, 20:57:32
If the timing (talking about the timing chain/can sprocket, not the points plate) was off one tooth in either direction, then the engine would run like crap at all throttle settigs righr? I was to rule that and/or anything else that I may need to disasemble the engine for.... like i said it starts and has great power up to half throttle position
Title: Re: CB125S Engine HELP
Post by: teazer on Apr 03, 2017, 23:29:19
Could be any number of things but I'd start with taking off the carb and cleaning every orifice again.  Try it without a main jet and see if the stumble is the same or if anything changes.  Have you tried putting on the choke when it's running to see if that made any difference.

I wasn't joking about trying it with no main jet.  At half revs the plug should not be hot enough to burn clean, so the color suggests that it's way lean.  It could be a bent float and partially blocked fuel supply.

I didn't read what you you found with a timing light.  You may need to get a spare alternator cover to drill a hole in to see where the spark is timed without spraying oil everywhere. On Hondas with wet alternator it is a good idea to drill a 1" hole in a spare side cover  to reduce oil spray. And then use an automotive timing light to strobe set the timing.  AT tickover the static pointed should point close to the F mark or T mark and by about 3500 revs it should be between teh two fiull advanced marks.
Title: Re: CB125S Engine HELP
Post by: natez on Apr 06, 2017, 17:26:45
Right but can I rule out incorrect setting of the timing chain/sprocket/cam?

Title: Re: CB125S Engine HELP
Post by: natez on Apr 06, 2017, 18:12:49
Just ran it with the timing light. The static pointer is dead on the T mark at idle. When you rev it up timing advances to the left hand of the two timing marks. It strobe doesn't miss or anything when you start to rev through the bogs so I guess we can rule out an ignition issue.... That damn carb is clean as a whistle, float is set right. I haven't tried it with no main jet yet, but when you move up in jets from 100 (100 is what another person with the same motor and carb went with after he did the same cylinder conversion) to 125, the miss/bog remains the same however it begins to smoke a bit more. Pull the plug and its darker/sootier.
Title: Re: CB125S Engine HELP
Post by: trek97 on Apr 06, 2017, 22:37:54
Im fresh out of ideas.  Seems youve double checked everything.

 >:( frustrating.

Build her back to stock and see if she is still running as good as she originally did.




Title: Re: CB125S Engine HELP
Post by: natez on Apr 06, 2017, 22:43:39
ugh! Kinda over it :( wish I didn't touch it in the first place.
Title: Re: CB125S Engine HELP
Post by: trek97 on Apr 07, 2017, 05:05:07
No, they wont run properly without a battery.   :o  Thats the reason they include a battery from the factory. 

What type of battery do you have?

Im not familiar w the Radio Shack rectifier.  Is is a solid state unit?  Regulator/rectifier?
Title: Re: CB125S Engine HELP
Post by: natez on Apr 07, 2017, 15:06:16
Don't remember the part number off hand, but it's a brand new yausa. Same bat it had in it before I took the motor apart and it ran fine. I even unhooked all the lights and ran it with a 12v battery yesterday (google search cb125 12v conversion, totally kosher to do), starts and idles way better, but unfortunately still loses power past half throttle.

Lots of old threads on the net about the radio shack rectifier. Not sure on its specifics but I did it years ago. I think its only a rectifier. Think I am going to toss the motor and stick a 200CC Lifan in the bike so atleast the whole thing isn't a total loss. Total pain in the ass to rip the motor all apart again just to see if it runs correctly stock, got too much other stuff goin on.
Title: Re: CB125S Engine HELP
Post by: natez on Apr 07, 2017, 15:14:47
Can I rule out incorrect setting of the timing chain/sprocket/cam? If the cam was incorrectly set  by one tooth on the sprocket/chain would it run so well at idle and up to half throttle?
Title: Re: CB125S Engine HELP
Post by: teazer on Apr 07, 2017, 15:16:59
Just ran it with the timing light. The static pointer is dead on the T mark at idle. When you rev it up timing advances to the left hand of the two timing marks. It strobe doesn't miss or anything when you start to rev through the bogs so I guess we can rule out an ignition issue.... That damn carb is clean as a whistle, float is set right. I haven't tried it with no main jet yet, but when you move up in jets from 100 (100 is what another person with the same motor and carb went with after he did the same cylinder conversion) to 125, the miss/bog remains the same however it begins to smoke a bit more. Pull the plug and its darker/sootier.

At idle, the timing should fire at the F mark.  Try to advance the ignition timing until it does.  Then the advanced timing should be closer as well.  That should improve things.
Title: Re: CB125S Engine HELP
Post by: furu14 on Apr 08, 2017, 03:14:09
What about air filter? my XL600 stops dead in its tracks at 5000rpm WOT with those popular steelmesh-podfilters.replaced them w longer runners with foamfilter on end-7500rpm clean..

Try removing the airfilter/snorkel.