So my carb was a MESS. Pretty sure I have a Mikuni BST 34. ATV would backfire all last winter when plowing and was running no so great. So i took it out and rebuilt it. the top cover had a hole in it, so i replaced that, the side cover was broken so i replaced that. I put all of the new parts in it, EXCEPT the bottom throttle set screw (i think; its the one you can adjust from the outside)
I took it all apart and soaked it in carb cleaner and then rebuilt with all new parts. Ran like absolute crap. So as we changed things around, it got better and worse depending. In the end, i have all of the old parts back in the carb now, except the fuel float needle as that was broken. The new side and top are fixed, but other than that, its the same it was before. Now it'll start and idle fine, but as soon as you give it throttle, it immediately dies and stalls.
Only way to get it to throttle up is to cover the air inlet from the air box and choke out some air. i would assume that means its running very lean, and i make it run rich by covering the air inlet.

really pissing me off here to be honest. anyone have any pointers? Not sure why now its running bad when all of the same parts are back in it the same way as before. Is there an internal jet that needs to be adjusted that i ran all the way in when i rebuilt it?

 

You need to remove the 2 jets from the carb and clean them out if you haven't done so already, especially the pilot jet.

 

I had new jets in and they made it run worse. This way it at leasts starts and idles.
I'll take it back apart and see what I can do and report back.

Thanks

 

Were the new and old jets the same size? How about the float height? Are you running the airbox lid? If not you need bigger jets. Also if the engine is sucking air from somewhere, that could lean it out, like around the carb intake boots

 

I'm not sure about the jet sizes but I'm pretty sure they are the same. But again, I have everything back to how it was, but no vacuum leak where the top cap and side cover are now. Only think I can think of it that it was tuned to compensate for the leaks and now that there isn't one it's running differently. But the bottom jet (or whatever it is) that you adjust from underneath is stripped and can't be adjusted anymore.
Air box is on and tight. I can't find any leaks anywhere.

 

Yes it's lean if it runs better when covering the air inlet. Too much air or not enough fuel. If the carb had been tuned for the leaking parts then I would have to think it would now be way rich, which does not sound like your case. How do you know the jets that you put in are the correct size? They should have numbers stamped into them. I would verify that it has the correct size jets.