1975 CB125 Tracker project

Maritime

Over 10,000 Posts
I fucking give up! I can't get this thing to run right again. What am I missing?

Valves set correct
Cold compression 150 PSI on 3 kicks
Points are new, gapped and timing is spot on with the light per manual
brand new Carb!
Lots of fuel
Good fat spark

Here is the issue:

Set the air screw at 1 1/2 turns out, from cold the bike starts first kick, idles and revs great. As it warms up it goes crazy rich, stops reving clean and starts barfing exhaust you can smell the extra fuel. plug fouls black. It then won't start unless WOT and it won't run except at WOT then it will die and not run at all. Let it cool down and it repeats. I am going to swap back to the oem carb and see if it helps now that we removed the spark plug my assistant put in the intake. What else could it be? I was wondering if a bad condenser could be the issue? Coil is new, condenser isn't but since it starts and runs great cold that doesn't make sense. I have dropped the needle all the way to leanest and I have even turned the pilot out 2+ more turns. nothing seems to work.

EDIT**** I didn't check float height of the new carb. Will do that tonight as even a MM off messes with these bikes.
 
Last edited:

pidjones

Over 1,000 Posts
Only thing I can think of is the new carb (Chinabay?) and the condenser, but usually failing condenser (full open) identifies with burnt points. If the points look Ok, maybe run it until it jacks around and then disconnect the condenser. If it runs good then, the condenser is shorting when hot.

Maybe something closing off the air?
 

Maritime

Over 10,000 Posts
Only thing I can think of is the new carb (Chinabay?) and the condenser, but usually failing condenser (full open) identifies with burnt points. If the points look Ok, maybe run it until it jacks around and then disconnect the condenser. If it runs good then, the condenser is shorting when hot.

Maybe something closing off the air?
OK I'll try that, have to run with the tank off but that will allow a test of fuel flow too. I'm tempted to throw a pod filter on it and delete the air box and see if that helps. It's so frustrating as It ran beautifully for 2 days and then the petcock failed and all went to shit.
 

iatethepeach

Coast to Coast
If the new carb is one of those cheap Chinese carbs, try completely disassembling, then reassembling it. I've bought two from Wincycles and both had minor assembly/quality control issues on arrival. Once they're together right, they seem to be fine.
 

Maritime

Over 10,000 Posts
If the new carb is one of those cheap Chinese carbs, try completely disassembling, then reassembling it. I've bought two from Wincycles and both had minor assembly/quality control issues on arrival. Once they're together right, they seem to be fine.
Yep, Pretty sure it's a wincycle so I'm gonna pull it, inspect it, check float etc. I may just go back to oem as the Sparkplug in the intake from my toddler was what was really causing the issues, I didn't need the new carb to begin with.
 

Maritime

Over 10,000 Posts
OK, even thought the new carb looks nice and almost exactly like the oem, the float height is not stock, I put it to stock and it started pissing out the overflow so it was @30mm 24mm way to high so not messing with it anymore. put the OEM back in, fired it on choke, took choke off and now it's revving better. I let it run for about 3 min and it stayed running, idling and revving. some puffs of white smoke on hard rev so still a little rich but after work I will be able to test ride and see how it goes. Promising so far.
 

iatethepeach

Coast to Coast
There's no time like the present, especially when it comes to test rides.

If you don't mind, please post a photo of the aftermarket carb. I'm curious.
 

Maritime

Over 10,000 Posts
Rode it, it's almost ridable but needs better tune. It didn't stall out but it bogs and I got back fires this time. Can't do WOT and it was only happy at 1/8-1/4 so not sure what's next to do.
 

Maritime

Over 10,000 Posts
Warm engine 1-1/2 turns on air screw. Needle middle clip. OEM carb with new needle seat. Is this rich like I think? or lean. Can't see it being lean but I can and have been wrong in the past.

https://photos.app.goo.gl/dgbmCGD7LyVzcGtJ6

also plug is black and smells like gas.
Aftermarket Carb:
20210923_145342.jpg

20210923_145348.jpg
 

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iatethepeach

Coast to Coast
Yes, that looks just like one of my Winstar carbs. They came in black and yellow boxes labeled "OEMSTD."

I wish I could help with the tuning. Hopefully it will be obvious to someone after watching the video.
 

Maritime

Over 10,000 Posts
Yes, that looks just like one of my Winstar carbs. They came in black and yellow boxes labeled "OEMSTD."

I wish I could help with the tuning. Hopefully it will be obvious to someone after watching the video.
thanks. I got it spot on once. I just need the time to do it. It seems to be responding to inputs with the old carb.
 

Maritime

Over 10,000 Posts
Tried to get it tuned last night, I got it close then changed it a bit and made it terrible. Had to stop, will try again tonight from the baseline. I think I'm turning the air screw too much, need to make very slight adjustments.
 

iatethepeach

Coast to Coast
Yeah, I find it takes a while for the engine to respond and settle after making a mixture adjustment. I usually make it worse before getting it right, too.

I don't know how much it matters, but what's the condition of the boot that covers the throttle cable where it enters the carb? Are the plastic insulator and gaskets between the manifold and head in good shape?
 

Maritime

Over 10,000 Posts
All that stuff is good, No issues with air leaks or anything. It's 100% Carb related. I just need to get it right. I got it really close then made a change the wrong way and messed it all up LOL

Yeah, I find it takes a while for the engine to respond and settle after making a mixture adjustment. I usually make it worse before getting it right, too.

I don't know how much it matters, but what's the condition of the boot that covers the throttle cable where it enters the carb? Are the plastic insulator and gaskets between the manifold and head in good shape?
 

cbrianroll

Coast to Coast
I am the worst at carbs...bravo for your patience And persistence lol. I wish I could take a class so I don't waste so much time on them.
 

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