1978 CG125 K1 Fuelling Issue?

[darthbuttchin]

Hi,

Basically, I'm stuck. Ive recently got the bike ready to go back on the road properly, after finishing at Uni. (Note - TL;DR at end of post, extant issues highlighted in Bold throughout post)

It all seemed good (aside from failing MOT because of a bust fork seal - Not a major fix though), however, I noticed that whilst riding the bike to its MOT it wasnt idling properly. I was basically having to hold the throttle open to keep it idling, which whilst fun (as it was like being on a classic race bike) isnt an ideal situation to be in.

So, whilst waiting for the new fork seals to arrive, I've spent the last week trying to sort the idling issue.

After consulting my trusty Hayne's Manual (in which it states idle speed should be between 1100-1300 rpm - bloody useless figures when this model has no rev counter), I figured it may be an issue within the carb itself. So, I took the carb apart, cleaned it up and put it back together again. No change, still not right. Fiddled some more with the idle speed screw, but it was either too high or cutting out.

So, I went to the next diagnosis, and cleaned up the foam air filter. I re-oiled it this morning (having spent a cracking day autosol-ing the rusty exhaust yesterday) and re-fitted it.

Fiddled with the idle screw and gave it some almighty revs. Once the engine had settled down, it was idling far higher than it should (i.e. sounded like it was gonna take off), so I screwed it back and got it roughly to where it should be, that is, what it sounded like last time I rode it properly.

Rev'd up again and seemed fine.

HOWEVER, I then blipped the throttle a couple of times and noticed a new issue - when I open the throttle it goes up to 1/8th throttle fine, but after that it stutters - like a wobbly cough.

This stutter has happened on multiple occasions in the past -

Initially, it occurred when it got wet - fixed this by covering the spark plug cap in a water repellent spray.

Then, it occurred at fully open throttle, at high speeds (well, high for this little old bike at least). This was sorted by replacing the spark plug.

The problem has re-occurred and each time been sorted by changing the spark plug.

Whilst this seems like the obvious initial fix for the current problem, I am concerned that the bike has gone through 4 new spark plugs in a year - I've also been advised that this is unusual behaviour.

I have also cleaned up the current spark plug - covered in black. This led to me investigating the air/fuel mixture screw, as it appeared to be running rich. I have now set it as it should be (wasnt quite right) but this has not fixed the issue.

I am, to be quite honest, completely flummoxed, so am calling upon the expertise of everyone here, in the hope someone may be able to help me.

Thanks in advance,

db

p.s. video of the issue to come, when I work out how to post it from the iPad.

TL;DR -

Adjusted Idle speed, Cleaned Air Filter, Set Air/Fuel Mix, Cleaned Carb and Spark Plug.

Throttle opens - stuttering cough past 1/8th throttle.

Gone through 4 spark plugs in a year.
____________________
CG125-K1 (1978) - 35 years old and (mostly) going strong! (Restored and rebuilt by my own fair hands).
Call me Tomsk - I am the Flying Womble after all.

[darthbuttchin]

Video -

https://youtu.be/hvgEZHStgWQ

You may need to turn it up a bit. When I get the bike going for the second time you'll hear it - almost like its always on the choke.
____________________
CG125-K1 (1978) - 35 years old and (mostly) going strong! (Restored and rebuilt by my own fair hands).
Call me Tomsk - I am the Flying Womble after all.

[unitynotsocri...]

Make sure you have a type 1 engine and carb . Type 2 carbs have an accelerator pump and a dual cable .
The jet sizes and needle jet position /wear should be checked , a new jet kit may help and some older carbs have silly micro washers that end up missing over the years . Check the accelerator cable for stiffness ,rubbing ,tatty wire snagging inside cable cover or carb slide , those shhty clips under the carb slide spring need to be perfect . Then there's trial and error float adjustment , but leave that for now !

When cleaning the filter , be carefull . Following the haynes usually results in a filter thats got too much oil that plays havoc with your mixtures causing them to run really rich, and tickover. Keep to standard tickover / mixture screw settings . Wash the filter again with a good solvent ,LET IT DRY OFF A BIT and apply a small amount of oil , then keep squashing it in your hands for a few minutes before fitting . OR , just buy a new one . Old foam tens to powder , like old foam speaker cones , loosing its 'foamness ' , as in internal air gaps .

Mechanical causes for your troubles could be ..valves /valve guides , bent pushrods , but usually slightly out ignition timing .POINTS ahhh so quaint , and tunable !
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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

[mudcow007]

+1 air filter

when i cleaned mine, the bike wouldn't run at all, splutter then quickly die

i resolved this by wrapping the filter in kitchen roll an taking most of the excess oil away - bike ran fine after this

[darthbuttchin]

Okay so,

Tore down the carb and cleaned it. Main jet fell out straight away - might be my problem! Haha. Also, the butterfly clip on the throttle needle was set to the recommended middle (out of 5) grooves, which ahead unfortunately worn out, so wet set it to the next one up. Also majorly de-oiled the air filter - bloody haynes telling me to saturate it!

All working fine now, and pleased to say it passed its mot.

Cheers for your help guys!

Db
____________________
CG125-K1 (1978) - 35 years old and (mostly) going strong! (Restored and rebuilt by my own fair hands).
Call me Tomsk - I am the Flying Womble after all.