CB125j

[reinax]

Hello,

A couple of months ago i bought an old honda CB125j (late 70's). After opening everything up, cleaning all of it and putting the engine back together, the bike rode for about an hour or two. Then suddenly it died and i haven't been able to get it going again. The new spark plug came out completely blackened so something must be going wrong in the combustion.

My primary concern regarded the ignition timing. I did the whole thing with wiring the light over the breaker points and turning the engine trying to get it to light up on the "f" mark. It seems however impossible to get the timing wright. something that seems completely off: The light sometimes lights up twice on the same rotation. There must definitely be something wrong with the cam shaft controlling the opening of the points. My first thought was that the shape of the spark advancer had worn down, but having a closer look that maybe seems unlikely. more likely there is some play on the cam shaft movement creating an irregular timing. Is there anyone with some experience on an irregular moving camshaft in these old bikes ?

Thanks

[unitynotsocri...]

It does sound like the points , although cleaning the carb bowl and checking the choke slide is'nt sticking can help .
Cos they are old you should check the needle jet is on the right central notch .

Cb100N engines are a direct swap out , having the two peice cyclinder head . Choose your spares carefully as the 125 j engine is a reworked 125s engine and has different components / specs .

What can help is a bible , when carried around all the time it will improve that cable disc 100% , or at least make you think about the afterlife to stop you getting too close to traffic . Good tyres are a must .The origional nylon ones will kill you in the wet . If you have little money then just have the best tyre on the front for improved braking .

the 125 j will take a lot of later honda cb/cg spares and chinese stuff , especially cg clones . The main problem is the points .
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nearly a normal tax paying tosser.......with ferrileness suzi100,cg125,cb125scb100n,cb175,cd100,cj250t,kh250,c15,125 bantam,super 6,rickman gs750,xt500,250rs,dt175,lifan125,dolomite1850,metro,Morris220ld,morrisfg,leyland princess,range rover,corsa,vw camper .now struggling with legs. MORE ORDER = MORE CHAOS

[unitynotsocri...]

one tip that might help is NEVER turn the engine backwards when timing points , as most point models use a mechanical advance/retard mechanisim to ensure clean ignition timing .

Turning the engine backwards can upset the stationary springs that hold the timing cam .
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nearly a normal tax paying tosser.......with ferrileness suzi100,cg125,cb125scb100n,cb175,cd100,cj250t,kh250,c15,125 bantam,super 6,rickman gs750,xt500,250rs,dt175,lifan125,dolomite1850,metro,Morris220ld,morrisfg,leyland princess,range rover,corsa,vw camper .now struggling with legs. MORE ORDER = MORE CHAOS

[Fizzer Thou]

Has the engine run while in your ownership? The reason I ask is because the automatic advance/retard mechanism can be placed 180 degrees out on the backplate.I found this out back when I owned a few early Hondas.

Start off with the points back plate lined up statically.Align the 'T' mark and adjust the points to the correct feeler gauge size (15' if I remember rightly).Is the spark plug a D8ESL?Is the plug cap one of the old metal ones?If so,ditch it and fit a new waterproof/plastic NGK item.Also,check that the air mixture screw is the correct number of turns out (might be 1.5 out from fully in).Is the air filter clean?From what I recollect they were washable foam type that should be washed in a solvent (not petrol) then soaked in a light engine oil and the excess squeezed out).

The Haynes manual is quite handy.Back in 1975 my copy got quite grubby from the number of times it was referred to.
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Just talk bikes.What else is there?

Always have a 'Plan B'