Trouble Keeping CB125s Running

maxDTM

maxDTM

( ͡° ͜ʖ ͡°)
Hey guys,
I picked up this lovely lady last week. The owner installed a new 6v battery that day since the old one was only putting out 4v, and I rode her home with no problems whatsoever. Started and ran fine the next day too, but a few days later is when my problem started, so here's where I am now:
I can get the bike to start with the choke closed, but it's idling at 5k which is pretty high, and giving it any throttle or opening the choke just kills the engine. PO recommended I swap out the points & plug, so I did, but I'm still having the same issue. He said he cleaned the carb the night before selling it to me too, so I have good faith that it's not a carb issue.
I'm still pretty new at this, so any suggestions would be appreciated! SF only has so many more sunny days left this season 8)
 
Yeah, if he was jacking around with the carb the night before I would be 99% sure it IS a carb issue.

Adjust your mixture screw until you get the highest idle possible, back it off just a hair, then adjust your idle screw down to around...1200-1300 RPM I think?

If that doesn't take care of it, I would look into a vacuum leak. With the bike running, spray WD-40 or carb cleaner around the mating surfaces between the carb and motor. If air is getting in, you'll notice a distinct change in running condition. If that's the case, pull the carb and check your gaskets/mating surfaces for imperfections where air could be getting in/out.
 
Hm yeah I'll mess around with idle screws. A leak sounds plausible - usually I'll get the bike to fire up and it'll rev at around 2k or so, but then it shoots up to 5k for a few seconds before cutting off. Tomorrow I'll have some free time finally, so I'll pick up some carb cleaner.
 
surffly said:
What are you running for a n air filter?
Click to expand...

Really glad you asked...
Opened it up and found an accumulation of about 40 years worth of dirty foam:

nHlWAl4.jpg
 
As nasty as that is that would restrict air flow which needs to be fixed but is not the root of your problem. I would guess as your bike heats up you are getting air in through the carb boots. Replace them or seal them. To test try the wd40 or carb cleaner trick as suggested earlier.

-boulevard
 
SSSHHHEEE LLLIIIVVVEESSS 8)
I finally got her running tonight. I picked up some carb cleaner and sprayed it over the mating surfaces as recommended by coyote, but that didn't do anything. I didn't have any more ideas, so I decided to pull the carb and give the jets a look even though PO said they were totally clean. Sure enough, it had a clogged idle jet. Sprayed some of that carb cleaner in, let it do its thing while I headed off to class, and put it back together when I got home. Fired right up!

I'm guessing the gas was just gunky since it hasn't seen a lot of miles lately. Drained it and put a fresh gallon in, so hopefully that will be the end of my woes.

Obviously a new air filter is in order, though. Just out of curiosity, why should I avoid pod filters?
 
Pod filter is a sure fire why to introduce running issues.
These bikes hate them.
Takes a lot of brass, time and skill to make it runn poorly with a pod....
Not worth it.

Learn to keep it running stock.
 
These bikes are pretty finicky when you stray away from stock intake and exhaust, Honda had it pretty good from the start. Unless you're planning on a bore kit or some other power adder, I would stick with the stock setup and ride the shit out of it.

If you appreciate a good challenge, though, get it running as well as you possibly can FIRST, then start changing things one by one from stock. Check your state of tune after each change, adjust, and go from there.

Keep us updated!
 
Oy vey, this bike gave me a pretty good workout last night, and my troubles continue:
I rode down the street to CVS, but when I started it up the problem was back :'( :'( :'( Had to push her up the hill to my house, which luckily wasn't more than a 5 minute walk.

So obviously my idle jet is blocked again which means there's still shit in the fuel... I think a thorough tank-cleaning is in order when I have time tomorrow, but should I be looking out for anything else? Also, since this damn jet is so tiny, can I just get a bigger one?
 
No.

Pull the petcock and clean out the filter. Rinse the tank our until no rust or particles come out. Add an inline fuel filter. Clean out your carb and jets.
 
Chances are, if you were running it with that crappy old foam filter, the foam is in your carburetor. You need to get all the crud out.
 
Yeah, I figured that foam had to be going somewhere. I'll be getting that inline filter, and a new air filter for sure.
 
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