Por Apache 125 carb tuning, please help!

[cactusmelba]

Hi all, it's been a while as stupid house projects got in the way of my bike project.



The story so far: I've rescued a neglected 125 (based on the gy series I believe),



and started to try to build a street tracker out of it.



Having ripped the airbox out and installed a pod filter, it wouldn't run well at all, or even start well.. I ordered some jets for the carb, and started working incrementally up in size until a mate tells me that it'll definitely need the biggest one if I've installed a pod filter. So I installed the biggest jet, (I think it's a 115, up from 95), and sure enough it starts first kick every single time.
On the road however, it's utterly powerless. It doesn't seem to want to rev any higher than about 2k revs, UNLESS I reach down and tickle the choke a little, in which case it runs faster and revs freer. So that implies to me that it still needs more fuel, right?
The tickover screw is on the right side of the carb, (ignore the moth, it's been a while since I tinkered..)



but bang in the middle, underneath the carb, next to the rubber inlet manifold, is a small adjustable screw that feels like it's on a spring or something, almost ratcheted. Is this likely to be some kind of mixture adjustment screw? Here's a picture of it, it's the little brass coloured one, opposite end of the pod filter.



Is this the screw I need to be messing with to make the mixture more rich or more lean?

Any help would be greatly appreciated, as the manual seems to only tell you how to take the carb off, clean it, and put it back on again!

Thanks.

[Weldangrind]

If you twist the throttle slowly, does it behave to a point, or does that make no difference?

[rich_e]

I have a 150 with the same carburetor. I have a foam air filter. Running really well with a 105 main jet and a 42 pilot jet. I think you may have a vacuum leak. Your intake rubber seems to have tape on it. Try sealing it up with some silicone.

The street tracker looks great by the way. I love it.

[cactusmelba]

Thanks guys.

Weldandgrind - it does behave if opened up slowly, but only to a certain point, definitely not revving as high as it should. The reason I think it's a mixture problem is because if I tickle the choke while riding it performs miles better immediately. (I've not got a choke cable fitted though so I can't really test this extensively)

The jets I ordered were just main jets, what do the pilot jets do and are you supposed to change them at the same time?

I'll try and seal up the manifold with silicone, I put tape on as it did look a bit cracked..

[Weldangrind]

The pilot (or slow) jet is responsible for idle. As you open the throttle, the carb transitions from the slow jet to the main jet, via the pilot mixture screw. Adjust the pilot mixture screw to eliminate a stumble as you crack the throttle. If you need to open the screw more than two turns, the slow jet is too small. If you have little or no stumble and the bike starts and idles well, the slow jet is a good size.

It does sound like it needs more fuel, but that's based on a tight intake tube. If you have a vacuum leak as Rich suggests, all bets are off. I'd replace that intake tube first.

If you're at sea level, the bike will need a fatter main jet than one at higher elevations.

[cactusmelba]

Mister weldandgrind, that is precisely the kind of information I've been looking for, thank you so much. I'll experiment and report back shortly...