Honda CM125 poor running (on 1 cylinder)

[Mosser]

I thought i would take my 1982 Honda CM125 out for a blast today, it started out running nicely, and could reach 65-70mph which is what it has always done, but about 30 miles into my ride, it started to loose power and was backfiring on 1 cylinder only whilst i was doing about 60 and i lost power and it slowed down to about 30mph, when it suddenly picked up power (the other cylinder kicked back in) and it took off again.

It continued to intermittently run rough all though my 80 mile ride i did today, and at 1 point when i stopped at a roundabout, the left cylinder cutout completely and when i gave it even the slightest bit of throttle, it would splutter and backfire and put out blue smoke from the left cylinder only.

The right cylinder seems to be fine all the time, but when the left cylinder cuts out, it coughs and smokes and backfires for a minute or 2, then clears itself up and runs perfectly on both cylinders again.

I have had this problem for a few months now, but never this bad, I recently fitted new spark plugs and a new HT coil and leads, it had all fresh petrol today which it ran fine on for the first 30 miles, so its not the fuel thats the problem, I have recently cleaned the carb out with carb cleaner and its spotless inside and showed no signs of wear on the slide or needles or jets, the float height is spot on and the mixture screw is at the factory recommended 1 turn out, i did try half a turn either way today, but it didnt help, and i dont think its a carb problem anyway as the right cylinder runs perfectly all the time, and i have had a spark checker plugged in when the left cylinder has been cutting out and it was showing a nice fat spark so i am now out of ideas as to what could be wrong ?

Has anyone got any idea about these old bikes and what i might do next to diagnose or solve it ?

Thanks

[6T]

hi mosser if there is blue smoke it sounds like you are burning some oil,i would do a compression test on each cylinder to see if there is a big difference between the two cylinders,check the valve clearance on the inlet and exhaust if you have a valve sticking or not seating properly this will cause the bike to cough and splutter,if you have low compression after you have checked the valves i would take the head off and check the bores, just a note on the blue smoke if you have been running the bike for some distance misfiring or running on one cylinder you are likley to have a lot of petrol contamination thining the oil so an oil change is advisable.hope this helps steve

[Tursdale_Lad]

Hi,

did you get this sorted because I have the same issue, left plug (usually) misfires / doesn't fire but it does seem to run on only one cylinder at a time and swaps back and forth at times. Thinking that this is related to the stator and pickup coils and meter readings don't appear to be even close to the readings shown in the Haynes manual. Having problems finding a new stator for any year of this model (they appear to be the same for all years). Any suggestions / tips?

Paul

[FJ_Biker]

I see David silver has a stator in stock for the cf model cost 59 quid

http://www.davidsilverspares.co.uk/C...85/part_90414/

[470four]

http://www.ebay.co.uk/itm/93-HONDA-C...ht_1949wt_1139

Stealth bike.

Its the way forward.

[Jon_W]

Someone else had similar on a Honda 125 twin, and it turned out to be a fooked exhaust.

As to the ignition, I'm sure you'll find that both cylinders fire of of the same pickup coil (and at the same time). I'd check the carb and the ignition coils first.

[simongpz]

am i right in thinking its a single carb feeding both cylinders like the bigger cm200 if so carb problem is ruled out if its just one cylinder playing up sounds like possible top end problem if its burning oil could be valve guide oil seal gone sukeing oil into cylinder causeing miss fire and smoke
just a sugestion could be wrong

[470four]

I thought i would take my 1982 Honda CM125 out for a blast today, it started out running nicely, and could reach 65-70mph which is what it has always done, but about 30 miles into my ride, it started to loose power and was backfiring on 1 cylinder only whilst i was doing about 60 and i lost power and it slowed down to about 30mph, when it suddenly picked up power (the other cylinder kicked back in) and it took off again.

It continued to intermittently run rough all though my 80 mile ride i did today, and at 1 point when i stopped at a roundabout, the left cylinder cutout completely and when i gave it even the slightest bit of throttle, it would splutter and backfire and put out blue smoke from the left cylinder only.

The right cylinder seems to be fine all the time, but when the left cylinder cuts out, it coughs and smokes and backfires for a minute or 2, then clears itself up and runs perfectly on both cylinders again.

I have had this problem for a few months now, but never this bad, I recently fitted new spark plugs and a new HT coil and leads, it had all fresh petrol today which it ran fine on for the first 30 miles, so its not the fuel thats the problem, I have recently cleaned the carb out with carb cleaner and its spotless inside and showed no signs of wear on the slide or needles or jets, the float height is spot on and the mixture screw is at the factory recommended 1 turn out, i did try half a turn either way today, but it didnt help, and i dont think its a carb problem anyway as the right cylinder runs perfectly all the time, and i have had a spark checker plugged in when the left cylinder has been cutting out and it was showing a nice fat spark so i am now out of ideas as to what could be wrong ?

Has anyone got any idea about these old bikes and what i might do next to diagnose or solve it ?

Thanks

Did you change the plug caps as well?

Plug gaps?

Valve clearances?

Contact breaker points gap?

Condenser/s breaking down under load?

Single coil & carb would pan out, only had twin carbs on the sports models - wouldnt be fuel related as then both cylinders wouldnt run (Single carb) - sounds ignition related but strange as only one coil - this is new and the problem was the same with the old one...

Have you checked all feed wires from the flywheel are secured well(no corrosion) & in good condition? Would appear to not be getting a good enough "spark" signal to light up both pots... usually a blue / yellow pair of wires on this vintage? Try a continuity/resistance multi-meter test end to end?