honda cm 125 running intermittently on one cylinder

[northstandwol...]

Hi everyone iam new to the forum because I have a problem that iam hoping one of you fine people can help me with ? also I may be able to help others , I have over 30 years of restoring bikes but this little cm 125 has got me stumped .
it runs fine on the left pot but the right only fires intermittently .
I have so far done the following .. cleaned carb, adjusted tappets new fuel, new plugs, new caps new leads, swopped coils swopped CDI boxes done continuity tests on all the above but still no joy . it starts fine and ticks over but isn't running rite . if I pull the cap off the right pot the engine keeps ticking over if I remove the cap from the left pot it dies ! if I leave it it will tick over but only the left pot gets hot indicating the right isn't working properly
hhheeeellllppp!!!

[Teflon-Mike]

'Swapped Coils'? That's curious; most CM's only have one. Implies you have one of the last ones that have the CB125TD-J derivative engine with 180 crank..... most have the CM engine with 360 crank; but with almost 1/4 century continuous production, with the same basic motor there are an AWFUL lot of revisions and upgrades between fist 6v points ignition, kick-start, and last,12vCDi and e-start examples! So telling us the year could be helpful.

ANYWAY, that aside.... 'Runs' on one or 'Idles' on one?
These little motors are quite wont to start up on just one pot and will idle roughly with one dropping out; it's very common.

There's two factors at play here; and the first is that it's a incredibly short stroke engine; 43x41 dimensions for 62cc a pot, that 41m stroke is actually shorter than a C50 moped's; means very low piston speeds for any given rpm; feature that let the air-cooled, two-valve twin-carb variant in the CB125 rev to an astonishing 14,ooo rpm. BUT means that at low rpm you don't get as much 'suck'.... the vacuum pulling charge to the pot, depends on the speed of the piston falling; so, at low, starting rpm, these engines will tend to be a bit loath to drag a full measure of charge to the cylinder; if the tappets are out of adjustment, the rings or bore a bit worn (and they have very thin rings, and don't need to be very 'worn' just a tad 'blunted' on the corners to loose both compression and 'suck'), and or the valve seats are a bit worn or pitted.... then, starting, idle and low speed running can all be a bit 'fluffy' .

Second factor is the 'Self Exited Ignition'; electric for the sparks is made in the generator, and fed direct and unregulated to the ignition, whether CDi or points; So you only get sparks when the crank is turning, and 'good' sparks when the crank is turning quick enough to 'energise' the CDi. A tired battery or lazy starter, can make a big difference to easy starting on these engines, but once they have caught, if the idle is low, then they are prone to sparks dropping out; may see them in free air, but under pressure in the pot, they just dot have the juice behind them to jump.

BUT... once warmed,and off-choke, rev-em up, they usually 'clear' and with the motor under load, and spinning at drivig rpm's, often ride fine, though maybe a bit fluffy at idle at lights or give ways and such.

Which begs more diagnosis, when you say 'runs' on one, and talk about it idling; have you ridden it? Does it clear under load and revs? And, ridden, what does it ride like; 'cos that will give clue as to whether there is any more worry-some problem to look into, than just a lumpy one-pot idle, that is lkely to go away by doing the tappets and setting the idle above 'drop out' revs.. about 1ooo-1,2oo if you have a tacho.

Otherwise, as said, quite common; and even a good'n, on a cold morning is prone to it, and I tend to wait until I'm getting smoke out of both pipes before I try riding up the road!
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[Ariel Badger]

Have you tested compression on both pots?
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[northstandwol...]

Thanks very much Mike for the advice and possible faults . I forgot to mention that its a 82 model and I think maybe someone in the past has put on twin coils ? I haven't done a compression test but if I put my finger in the exhaust it has good pressure
I have taken it up the road but not very far certainly not far enough for it to get real hot so maybe this needs to be done I have had it ticking over with the occasional rev for 15 mins on one with the other pot cutting in and out but it didn't clear one downpipe very hot the other warm .

[Teflon-Mike]

82 'model' or an 82 'plate'? Could be quite large differences between an 82 manufactured bike, and an 80 manufactured one, that sat in the show rooms for 18 months before being sold on the hump of the the new 125 Learner-Laws.

Twin coils & CDi's though, implies a Super-Dream motor may have been fitted; of the Benly family, that was the only one util they started t phase out production, to have a 180 times,'one-up/one down' crank, rather than 360 timed 'both up' crank. 360 crank motors only needed a single coil, single trigger ignition, firing both pots together on a 'wasted spark'.

Some more investigation may be required; and the discrepancy does beg suggestion, to check that this is't some curious hybrdisation of badly mix & matched Rebel & Super-Dream bits that are seeing a 360 timed CM/CD engine get 180 Timed CB sparks, or a 180 timed CB engine get 360 timed CM/CD sparks!

Why on EARTH would you be sticking your finger up the exhaust Do you mean spark-plug hole? And exhaust.. singular? Has it got a two-into-one on it?

As Ariel-Badge suggests, a proper compression test could be useful; but, if you do, get a screw-in type tester, with correct adaptor, and take care screwing it in. 10:1 on a 62cc pot suggests that the combustion chamber is only 6.2cc or so; but add the volume of the spark-plug threads to that, using a push-fit, car type tester, and the readings can be so inaccurate as to be worthless.

Quick Query; how many carburettors has it got?

The Super-Dream has twin PD24's, the CM/CD a single PD26.

If it has had a Super-Dream lump slotted in; then could have either on it; but if the Super-Dream's twins have been mounted, they wont offer up to the air-box for a single carb, and they are on manifolds that angle them 'in' towards the bike's centre line, where there is a 'hole' between the frame rails on a Super-Dream, for them; where on the CM & CD, there's the main frame spine tube....., possible that the manifolds have been swapped from side to side to make them splay out rather than 'in' which would pull the choke linkage apart, and that's notorious to begin with and critical to get correctly set up on twin-carb engines. The carb inlet rubbers are also notorious for being torn by folk trying to take carbs off without first removing the air-boxes; worse as they get old and hard ad start to perish, giving rise to manifold leaks.

If it has a single carb? IS it the simple slide PD24/PD26? Have a feeling that the '82-84 model year CM had a rather wampy 'CCV' not quite a CV carb...

The Super-Dream motors will run relatively happily on a single carb, on a CM/CD branch manifold (Good way to get round tor rubbers problems), even a single 24. But, those CCV carbs are a bit 'flakey' and don't seem to get better with age.

Would expect a carb issue to effect both pots.. but... I have had similar ruing issues with a rather tired CB engine with the wampy CCV carb on it, and it doesn't make fault finding any the easier!

Another 'check' is your exhaust(s); check the headers for pin-holing; check the gaskets for leaks at the head..
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[northstandwol...]

Mike you are the oracle. man what you don't know about these bikes ain't worth knowing !
seams I will have to do a bit of investigation to find out exactly what bike I have here , I can say that it has a single carb , and yes I did stick my finger in both exhausts and plug holes . maybe your right , and all it needs is a good run ?.......I contacted the seller who confirmed that it had been standing in a house under the stairs for 15 years so maybe it just needs a run ?........also the starter clutch screaches like mad, can these get dry and need a lube ?.
I will check the engine and frame numbers again and the carb number to and the log book of course and post them on here later
thanks for all your help mate much appreciated

[northstandwol...]

Hi Mike
I have had a look at the numbers ....
engine is jco6-5000388 which I think is an engine from a cb125 td-c
frame is jc05-5000736 cm 125
carb is a Keihin pd
so do you think the valve timing has been done wrong ?

[Teflon-Mike]

15 years?
I'd put eve money that the conclusion being it needs a top end rebuild.
But I'll let you work though the Diagnostics.
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'