New carb but bike still running rich

[Cumbria John]

CG125 K1 - I bought a new carb for the bike because I could not weaken the mixture and thought that the old one was worn out, but on fitting it I find that the mixture is still way too rich ( plug is very sooty after warm up. I have set the pilot to the recommended 1 3/4 turns, but can screw it in and out with no effect ( on colour tune } The float height is spot on at .73 and the needle is in the 3rd groove. The air filter is absolutely clean. What else is there to look at ??? I was considering lowering the needle to the second groove but I will wait until I see if anyone has a suggestion.

[Mississippi Bullfrog]

First question - learnt the hard way - are you sure it's the right carb?

I spent ages fiddling with one that the box said was right only to find out it was the wrong carb and there was no way I was ever going to get it set right. That was always running rich as well. The fact that adjusting the idle mix isn't having any effect was the clue.

[fastbob]

In my experience , running rich and a poor spark can often produce the same symptoms . If you have the old Haynes manual it shows you how to make a spark tester from a bit of wood and two nails . What airbox set up are you using ? And have you done a compression test ?

[WD-40]

Are you sure the choke is off? How's it riding? Did you checked the plug after a proper ride or after running it on the stand? It's 6V magneto with points so it's not going to have a very strong spark at idle. The plug might look better after a good run.

[Cumbria John]

Thanks for all the suggestions. Its deffo the correct PD carb. The spark is a good one for a 6v. BUT I did a compression test as suggested and only got 120psi - put a drop of oil down the plug hole and got 160psi !!! The problem is that the bike was a non runner when I got it and have only just got it running ( see my previous problems ) I must admit that the bores looked OK, but obviously they are not. Luckily its a doddle to strip down and do what is required. Thanks again Chaps.

[Cumbria John]

Hi. Here I am again - Been laid up for a while - Still trying to sort out the rich running. Here is the list of all I have checked/replaced -- The bike starts on first kick with choke out, after a few seconds I can shut the choke off - ride a hundred yards and it cuts out with a sooted up plug. Replaced the original carb after doing all the fiddling with float, needle height etc. Yes it is the correct PB replacement. Got exactly the same symptoms.( running with no sponge on the air cleaner so no restrictions there ) - Had the generator rewound and now get a excellent spark (new coil, points and condenser) Checked ignition timing (several times ) and its spot on. New cylinder head, barrel and piston. Checked valve clearances several times.  So basically it is a new top end completely. Any ideas ???

 

[Mississippi Bullfrog]

Did you resolve the compression issue? A sooted up plug could also be caused by burning oil. 

[Cumbria John]

Hi. Thanks for the reply. Yes I bought new barrel and piston, and then rechecked the compression. I got around 160psi. The sooted plug has a dry "fluffy" appearance so I believe it points to fuel. I am going to look into rejetting if I can find some. Trouble is these bikes are so cheap that its difficult to get small parts. If that doesnt work I may try kicking it over ( a lot ) with no plug lead or fuel and then taking the head off to see if any oil has pumped up, but I think that given the symptoms I believe that would be unlikely.

I got this bike as a project because of its simplicity and its turned out to be 100 times more hassle than my last project which was a cb550 k1 which I picked out of a hedge in Anglesea  

[WD40]

You could test how strong your spark with one of these cheap testers

 

[iangaryprice]

Have you checked the ignition timing? Points go outof setting quickly.

[Cumbria John]

Thanks both -will try to get a spark tester, and will re- re -check the timing.