CB125S Engine HELP

natez

New Member
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 :/
 
natez said:
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.
 
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)
 
natez said:
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.
 
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.
 
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?
 
trek97 said:
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.
 
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.
 
trek97 said:
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.
 
trek97 said:
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.
 
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.
 
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.
 
natez said:
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.
 
natez said:
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?
 
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.
 
fMaybe the headlight is the only one you need to unplug if you just cant switch it off.
 
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.
 
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