I recently disassembled my carb to give it a good cleaning and replace stuff just for preventative maintenance. It worked real good until this winter when it was idling low and inconsistent. I smelled gas a lot lately too.
When it finally started after assembly, it was real rough. I adjusted the idle higher than normal just to keep it running. Once warm, I went for a short run and it accelerated and seemed to work well. Once home , I noticed some fuel dripping from the drain hose while running.
Today I tried it again for a little longer run and once I drove fast for a bit then slowed down , it just died. I had to pull the choke to start it, then it didn't want to idle and I limped home. Once home, it idled good?
Upon reassembly , I set the fuel mixture screw to 2 turns out because that's what it was set at when I originally checked it. (The manual says 2.5) I replaced the main and pilot jet with the same ones that it had (stock) ,new bowl gasket and fuel needle jet spring. New float needle and seat. I also adjusted the float a little so it would "lower" the fuel level. I installed a new spark plug and set the valves.
What could have happened to give me such poor idle control now and why is fuel leaking out the drain hose when I have a new needle and seat?
Why doesn't my choke work properly? It didn't work before the disassembly either. Before when I tried to start the bike when it was real cold,I used full
choke. Once it started I would have to push the choke almost off within 10 seconds as it would start to run real rough and want to stall?? Once warm , the bike was ok...

 

Hard carb to keep gas from seeping out of the over flow hose because of the float system design. I've even experimented with different o-rings for the needle and seat to see if it helps and also with the float tab as you have,but for the most part if you have this float adjusted as the diagram shows leaking will stop or at least slow down a tad. Plus I'd retrace your steps. Here's a BST40 Mikuni similar to yours on a Ktm.The BST-40 Bible - ADVrider
Plus don't overlook other things such as valve clearance. If any one is tight it'll run crappy. .006 intake and exhaust.

 

Thanks for the info Polaris tech!

Good link for an explanation on these carbs. I'm not too excited about taking the carb off again but I think I will. I need to make sure the float level is correct and all the internals are ok. Maybe my float has started to leak and fill with gas? I'll do the clear tube float level test also before I take the carb off.
One thing I still don't understand when setting the float level. Even in the carb bible link I read, he talks about not tilting the carb all the way upside down ,but yet hold it on an angle? What would be the difference? If the needle is against the seat , it should read the same upside down or on a slight angle shouldn't it?? Also , when I was checking the float level the first time, it looked way off. I saw that if I pressed on the area where the seat is along the edge of the bowl, then the float went close to where its supposed to go. The carb bowl when bolted on must do the same thing ,so it seems I need to hold it down when checking the float height right?
My choke not working right still bothers me. It makes me think the fuel mixture is not correct because the plunger should richen the mixture, but instead it makes the engine want to cough and stumble ,making me think the mixture is already too rich?

thanks

Kevin

 

Holding the carb upside down allows the full weight of the float to press the small needle plunger all the way down along with the needle valve itself. Turn the carb side ways to where the float tab BARELY touches the small needle valve plunger. At this angle the float seam should line up parallel with the carb base.If off,adjust the float tab to where it is.. Here's what it looks like with it close to being upside down.. P.S. you have to hold the float assy bar with your finger like shown in the pictures tight to simulate the bowl pressure when assembled..

 

ok......I'm still a little fuzzy on that adjustment but will attempt it on my next days off which will be next week. What are you simulating when the carb is tilted a little sideways when having to adjust the float tang? Is that where the fuel would stop coming in? What position is the float when in use and engine running? Would it be level with the carb bowl seam if we could see inside ?
Sorry for the newbie questions but I'm really a newbie when it comes to carbs!

 

All you're doing is setting the float height to where it's just beginning to shut the fuel off. When the carb is assembled and the float is upside down,when the bowl fills with fuel, the float rises and shuts the fuel off. The float then should almost be parallel with the bottom of the carb bowl just like it was when you held it sideways to adjust it..

 

Plus if you continue to have problems with the carb over flowing,you can always use this method of checking fuel level in the carb. Old way of doing it but still works. If the fuel level is higher than the bowl,the float tab needs to adjusted down to lower the fuel level.

 

Thanks for your patience with me Mr. Tech !

I'm pretty sure i understand what you're saying now.I'll probably see it better when im looking right at the float again.

I'm still unsure of why the float would fall deeper into the bowl when turned upside down, versus sideways when setting it? Isn't the needle lightly seated when your sideways and it then can fall down a lot farther when upside down? Is the float arm flexing or bending? Can just the weight of the float make it fall down farther?

So the fuel level should never be as high as the carb bowl gasket right?

I do plan on doing the clear tube test before I disasemble it. I actually bent the tang down a little while checking the level the first time with it upside down ,so i should have a lower fuel level than before. But something isn't right so back in I go.
We'll see....

 

When assembled,the float falls all the way down. When fuel enters the bowl the fuel raises the float which gradually pushes the float needle up into the valve until it shuts the fuel off.This level should be a little less than the top of the fuel bowl and the small brass over flow tube that drains at the bottom of the carb if it does.. Go look at your toilet bowl if it has a float arm system. Same principle when the water shuts off..

 

I'll tear into my carb again on mon or tues hopefully. I realise how the float valve is supposed to work ,but I wasn't real sure about how important the fuel level in the bowl was.
I do have a long brass tube inside the bowl that will aparently drain fuel out the overflow/drain tube. I thought maybe this was the culprit for fuel running out the drain tube, but the fuel level must have to get pretty high.
Is there any tricks of the trade to make sure my newly bought and installed float needle and seat (Pure Polaris) work properly? I've heard some say to compare it with my original needle for length and or maybe sand the edges a little, so it does go "all the way" into the seat??