Planet Minis

aaron6920

Today I got around to trying to start my GPX 125 engine in my ct70. I am having some problems. I can only get the bike to run for about 5-7 seconds at a time. It doesn't seem to matter choke on/off etc.....

I have tried the only screw I saw (mixture screw?) from all the way in to out backing out a 1/4 turn at a time. The exhaust I run is basic for crf 50.

Any Suggestions, am Missing an adjustment here?

Carb is 22mm mikuni I believe..

briansX19haulsass

TBolt USA, LLC » Pit Bike Engine Parts » CARB KITS » Mikuni 22mm Carb

briansX19haulsass

this will solve everthing

schofell84

or you should check for kinks or a clogged filter / petcock ...

aaron6920

The carb in the link is the same carb I have. It is brand new....

mexicanyella

What was the carb jetted for before the GPX125? If it's only running for a few seconds, and you're sure the fuel's getting into the float bowl fast enough, maybe the former motor needed a smaller pilot jet than what you need now. Any slide carb should have two screws on the side; the sort of flush-mounted one is generally a pilot air screw, which you adjust the mixture for the idle to 1/8 throttle range. But if the pilot jet is wrong you may not have enough adjustment range with that screw.

There should be a more sticky-out screw on the side of the carb, too, which is the idle speed, which works by limiting how far down the throttle slide can drop when you release the throttle.

So, yeah, what were you running before the GPX? If you somehow had a 22mm jetted for a 70 motor, you probably are too lean now.

A lot of people here have talked about CT fuel line routing and problems with fuel flow; make sure your float bowl is getting filled before entering the dark and mysterious world of carb jetting.

aaron6920

Thanks for the help, the carb has one obvious screw on the side and I am unsure where the other may be. Previously I was running the stock engine and carb on my ct70. I bought the gpx 125 and it came with this carb. I would assume the jets would be set for the gpx 125.....

What should I check further?

mexicanyella

Try pulling off the float bowl, hooking the fuel line up, and turning the fuel on. Make sure fuel is coming out the float's needle valve at a decent trickle when the carb is installed in its running position. Gently lift the float until the fuel flow stops, then release it. Make sure the needle isn't hanging up in its seat. If it's not, you can probably be sure the float bowl's getting full. This is after you're sure the tank's full, the petcock flows, and the hose isn't kinked or looped with an air bubble trapped and whatnot. Yes, I have disassembled a carb and made up some new cuss words and then realized I was out of gas. Just shut up.

I can't see any screws on the photo of the carb in that Tbolt link. Wrong angle. Could you by chance take a photo of the carb, from each side?

dirtbkr188

That 22mm Mikuni clone carb seems to run the best with a baseline setting of the jet needle clip in the 3rd or 4th position from the top, a #17.5 pilot jet, and a #100 or #105 main jet. The PITA part of that carb is the fuel screw, located in the front of the carb bowl, underneath the intake manifold going into the head. Try setting that at 1-1/2 to 1-3/4 turns out from seated, and make your adjustments to it in 1/4-turn increments. Normally, you have to literally undo the carb from the manifold to tilt it and adjust the fuel screw. Turning it OUT richens the mixture, and turning it IN leans the mixture.

aaron6920

Holy Cow I see the other adjustment screw you are talking about. Wow, it is really hard to get at. I think I will have to take the carb. off just to get at the screw. So, given my running situation do I start leaning or richening the mixture?

Also, to try and keep the bike going when I had it running for the short while. I did try more throttle and it killed. Would this be the fuel mixture screw or could be jet related?

mexicanyella

From your description I'd say try opening the fuel screw (I have yet to come across a bike carb with a pilot fuel screw, this is exciting) a quarter turn or so and see if that at least gets it to run longer than a few seconds. If you just now found that then what you were turning before must have just been the idle speed adjustment (throttle stop screw).

If my general carb info is muddying the water here say the word; Dirtbkr188 knows the carb and I don't.

aaron6920

Thanks much mexicanyella and dirtbkr you both have been very helpful. I will try later today and see if I can get more adjustments made.