I've searched through this and many other forums but can't seem to find an answer to my unique set of circumstances. I have my dad's old trx125 which had been sitting in a garage for, I'm not kidding, at least 10 years unattended. When I first got it, I cleaned the gas tank and did my best to clean the gummed up carb, but couldn't get it to do anything but fire off briefly with starting fluid (it has new plug). So, I bought a new carb and installed that along with a new air filter. Then it would start as long as I gave it a little throttle but would not idle or stay running without continued throttle. So I read here and other places that sometimes new carbs still need to be cleaned and inspected. I did that - yanked the carb and blew out every hole with compressed air. Still the same - starts without a problem with a little throttle but won't idle or run without constant throttle being applied. I have turned the idle screw all the way in and then tried at half turn increments all the way out. I have messed with the mixture a little but left it generally where the factory had it (and also where the original carb was set) - about 4 1/2 turns out. So - it doesn't seem to be a carb issue. What else could affect idle other than the idle screw and the slow jet? Where should I turn my attention next as it seems the carb isn't the problem.

 

I think you need to start eliminating other possibilities:
1) Is compression good?
2) Are the valves within spec?

 

I should add, when I cleaned the new carb, I saw that the holes in the main jet were MUCH smaller than the holes in the original main jet. Could that be impacting the idle?

 

Compare the pilot and main jets to the ones in the old oem Keihin carb. If the replacement is a Chinese knock off carb the jets may not be Keihin that Honda uses and may be marked differently along with different sized. You can try to clean up the old Keihin jets and see if they fit in the new carb. The pilot jet is the primary jet that has effect on idle.The larger main jet from 1/4 throttle and up.At least worth a shot on trying this and see if it helps.

 

OPT's advice is very sound...I usually don't offer that advice because I feel it should be SOP (standard operating procedure) to validate jets when rebuilding/installing a carb...in summary "new carb" doesn't always = "jetted for my machine"

 

The pilot looks the same and it was very bad in the old carb so I'm reluctant to try it in the new. The main looked in a lot better shape so I'll switch that out and see and report back. Thanks.

 

Well, put in the old main jet and it won't start at all. Guessing too much gas because of the larger holes? Anyway, returning to new main jet.

The old pilot jet is just awful - clogged in the bottom. I'm going to soak it in carb cleaner to see if I can clean it up anymore, but I'm not holding out much hope. I did notice that it has bigger and more holes than the new one, so who knows. I'll switch it if I can get it clear of gunk but if not, maybe I'll order a new one in a bigger size. It really does seem like the engine is not getting any gas from 0-25% throttle as it will start and run when I goose it but not with just a little throttle.

In the mean time, I'll check compression I guess.

 

Well, I returned the new main jet and everything is back to "normal", meaning it starts with throttle and runs as long as I hold the throttle 25% or greater, but nothing between 0-25%. Is it possible the new carb I got is just bad? I blew out everything again and made sure the idle circuit was clear but it sure seems like gas is not getting through? Or is it possible the idle jet is just too small?

The old idle jet is no good, so I could order a size up. I'm at a loss where to go next.

 

You either can try opening the fuel screw a little or even try lowering the needle clip on the slide needle which allows a tad more fuel on the low end and see if that helps.If not then you can try a larger jet as you mentioned.

 

OK - you caught me. Getting a little beyond my novice level of understanding. The slide needle is the one connected to the throttle, yes? How do I go about "lowering the needle clip"?