Mixture screws stopped working? Idles best with screws all the way in?

[system9]

My bike used to run great, actually it still does run pretty good but something weird happened.
For some reason, the bike started idling the best with the mixture screws all the way in.
I removed the carbs, cleaned them, they look great. I replaced the rubber O-rings on the mixture screws just to make sure, and no change.
Today I checked the float height (someone on another site said 22mm, but that's impossible, cause my floats will bottom out at that) but I tried it anyway to see what would happen, and fuel was coming out of the overflow after I started it (like I thought it would) So I reset the floats to about 25,or26mm.
Now, the bike still starts good, and actually runs pretty good. Its definitely not being starved of fuel, as I can race it up to redline.
So, shouldn't the bike actually stop running with the mixture screws all the way in? Thats what it used to do. I thought it must be getting fuel from somewhere else?, but I can find no leaks. For the hell of it, I swapped out the pilot jets with another set I had (slightly smaller) and still, no change at all. Idles best with the mixture screws all the way in.
I'm stumped.

 

[system9]

Just a thought, could fuel be getting in through the choke mechanism somehow?

 

[DimitriT]

Sounds like a float height issue. Measuring the float height by putting a caliper or ruler against the float itself is unreliable. You need to measure it using an external tube like shown here on pg 14-38. To get my fuel level correct I build a stand where I could level the carbs precisely while accessing the bowls from underneath. The stand was made with some plywood and was clamped to my workbench. I then used four tubes which I held in place with wires to get accurate measurements. Small adjustments to the floats allowed me to get all four within 1mm of the right height. Once the float was set correctly the bike ran much smoother on idle as well as all across the power range.

https://onedrive.live.com/view.aspx...B!1243&parId=2370B62DDF9F0C6B!118&app=WordPdf

The mixture screw on the BS32SS carbs is actually an idle circuit fine tuning screw. Most of the idle circuit gas comes in through holes which are not adjustable. But there is one hole which can be adjusted to fine tune the mixture. If you fuel level is too high more than enough mixture will come in via the fixed holes so the bike will run even though the fine tuning is bottomed out. You can see this on pae 14-30 of the manual.

 

[Joe H]

My bike used to run great, actually it still does run pretty good but something weird happened.
For some reason, the bike started idling the best with the mixture screws all the way in.
I removed the carbs, cleaned them, they look great. I replaced the rubber O-rings on the mixture screws just to make sure, and no change.
Today I checked the float height (someone on another site said 22mm, but that's impossible, cause my floats will bottom out at that) but I tried it anyway to see what would happen, and fuel was coming out of the overflow after I started it (like I thought it would) So I reset the floats to about 25,or26mm.
Now, the bike still starts good, and actually runs pretty good. Its definitely not being starved of fuel, as I can race it up to redline.
So, shouldn't the bike actually stop running with the mixture screws all the way in? Thats what it used to do. I thought it must be getting fuel from somewhere else?, but I can find no leaks. For the hell of it, I swapped out the pilot jets with another set I had (slightly smaller) and still, no change at all. Idles best with the mixture screws all the way in.
I'm stumped.

I would think float height is to high or maybe a air leak in the intake system??
Those screws always seem to give me trouble, they are for very fine tuning at very low rpm and throttle position.1mm variance is a lot for float height, it can make a significant difference. Be precise with he measurement .
You could try to spry some carb cleaner/starter fluid around the manifolds and such to see if there is an air leak somewhere.
You could also try to lower the float height by 1mm at a time until it starts to stumble when you try to accelerate from 2000 rpm in high gear, then bump it up by 1/2mm to 1mm so you know the float height is right on.

 

[system9]

I will give it a try. It says to connect the special tool to the float chamber screw. I have a GS450, I don't have any "screw" but rather a large float bowl cap nut. The special tool is just some clear tubing maybe? But my bowl has no drain holes?
Another question. My floats are brass, and they look smushed in on the sides. Like someone with gorilla hands pinched them way too hard. They still float though. Should I try and get a pair of plastic floats?
Again, my bike ran awesome for like 2 years straight, I'm wondering how the floats would suddenly be off, but I want to make sure the float height is perfect.
The .pdf link you linked to (thank you) says to measure to 22.4 mm. Do you leave the gasket on or take it off for the measurement? I've seen people say both, so i'm unsure.
My floats will bottom out at 22-23 mm. When I set them like this initially, fuel then came out of my overflow hose. I believe mine need to be around 26mm to avoid bottoming out.

 

[DimitriT]

If you use the method of measuring the float height then the 22.4mm is with the gasket off but that value could be different for your bike. You need to find a spec for your bike.

I thought the 1982 GS450 has BS32 (or BS34) carbs which are essentially the same carbs as most GSs. These have a screw at the base of the bowl which can be removed. You then need to buy a special fuel level measuring tool to insert in that hole.

http://www.ebay.com/itm/Mikuni-Carb...ash=item3f4ee4f0d7:g:5WsAAOxyf1dTKD80&vxp=mtr

A picture or two of what you have might help.

 

[system9]

My float bowls have no screws, and no drain system, only overflow hoses. Here is a picture of the float bowl with the large cap removed.
I have an extra cap from another set of carbs, I will drill a hole in that, and insert the clear hose. I'll post the results. I'm really starting to wonder why my brass floats were collapsed on the sides, can air pressure do this? They were not full of fuel, so I suspect no leaks. But, they have less surface area than they should it seems, so I may have to raise them well above spec in order to get them to float properly.

 

[system9]

Ok. So something is very wrong. Both my floats are set the exact same, but I have 2 really different wet test results, both bad. Its weird too, because the last time I set the floats to around 25 or 26 mm. I am going to double check my little ruler with my calipers to make sure. But even so, there is a big difference between the 2 floats.

 

[DimitriT]

Yea something looks wrong. Either in how the measure tubes are functioning or how the floats are set. Are there any air vent tubes or holes on the carbs? It's almost like those may be plugged and the bowl is not venting. If it builds positive pressure it would indicate with crazy fuel levels like that. Or maybe if the fuel overflows and closes the vents it would do the same. Are you seeing fuel in the airbox? Sometimes carbs which overflow will leak fuel back into the airbox and it will drip out. Sometimes they leak fuel into the motor which can be really bad as it breaks down the motor oil.

Those carbs you have are not from the BS family of carbs so I can't say for sure how they need be spec'ed out.

 

[system9]

Ok, so I got them level. Had to set the floats at 26-27mm, measured with the gasket off. Gonna eat lunch, hook it all up and see if its good.

 

[system9]

Left carb, bike idles highest with screw all the way in.
Right carb, bike drops out with screw all the way in (like it should)
So, I have it narrowed down to the left side. I can't figure out why it would behave like this. There is no fuel coming in through the vacuum line. I tried a different mixture screw just to see, and it made no difference, so It's not the screw itself.

 

[gs11ezrydr]

It sounds like that or those carbs are getting fuel from another source, Probably around the main jet needle make sure the needles, and needle jets are not worn or out of round letting fuel pass during idle, ,Also sounds like you might be flooding also with the damage to the floats , Get some fuel in a small container and put the floats in it and see it they float evenly if not they will need repl. as mentioned earlier check the choke system also. that may case a rich condition. Also make sure the needle and seats are shutting off completely at the right height.

 

[system9]

Those pics were before I re-set the floats. Float levels are perfectly normal now.
I'm going to take the internals from the left carb and swap them over to the right side, to see if the problem moves to the right side.

 

[Katarat]

Are the holes under the pilot jet plugged ? Have you tired spraying carb cleaner thru them ?

 

[system9]

The carbs have been removed 3 or 4 times recently and cleaned and I also run compressed air through the passages. But I will be sure to pay extra attention to the hole under the Pilot jet this next time. I have pod filters, so taking off the carbs, cleaning them, and putting them back on is 30 minutes at the most.
Lets say the hole under the jet is plugged. Wouldn't that make the left cylinder stall out completely, because at idle the bike is only drawing from the pilots?

 

[RaiderGT]

Yea something looks wrong. Either in how the measure tubes are functioning or how the floats are set. Are there any air vent tubes or holes on the carbs? It's almost like those may be plugged and the bowl is not venting. If it builds positive pressure it would indicate with crazy fuel levels like that. Or maybe if the fuel overflows and closes the vents it would do the same. Are you seeing fuel in the airbox? Sometimes carbs which overflow will leak fuel back into the airbox and it will drip out. Sometimes they leak fuel into the motor which can be really bad as it breaks down the motor oil.



Those carbs you have are not from the BS family of carbs so I can't say for sure how they need be spec'ed out.

They look like the same BS34 carbs I have on my 450

 

[DimitriT]

They look like the same BS34 carbs I have on my 450

Hmm, I guess they come in different style bodies for the twins.

 

[Guest]

I take it you never completely dissembled the carburetors and soaked them in carburetor dip for 24 hrs. before you started the cleaning process, or replaced the O-rings. If you haven't, you have been wasting your time chasing your tail. And then there is the cheap pods and associated jetting problems. Slides and diaphragms do not get dipped, it will destroy the diaphragms. Order another drain nut for the float bowl, drill it and epoxy a nipple tube of sorts to it so you can attach a clear tube to monitor your fuel level in the float bowls. The level should be somewhere in the middle of the lip of the bowl below the gasket if it is measured the same as the other carburetors.

 

[system9]

I have dipped them for at least 24 hours, and replaced all O-rings. I know it says I'm a newbie, I just went 6 months without logging on to the forum, so I had to create a new account. ;-)
I have an extra drilled nut with a hose for the float bowls, that is how I made them level.
My bike has been good, I maintain it on a regular basis. It has a custom exhaust I hand-made. Its been tuned from the start with the cheap pods, I may change them to something like I have on my XS650 I think they are UNI brand? But It likes the cheap ones. Its much faster than stock, not important to some people, but I love it. People who ride it often say its one of the best running bikes they've ever been on. Thats why this weird mixture screw issue is bothering me. I'm someone who takes time to really clean the carbs, tune the carbs, sync the carbs, replace the carb boots, the o-rings, etc.
The bike still starts good, runs and idles good, but only with that screw all the way in, and I know thats not right, so until I find out (and I will If I can get some free time) the question remains, What would cause high idle with the screw in?

 

[Guest]

Have your boots failed? They usually need to be replaced on all the older bikes. I'm not sure how yours are set up, but there is an O-ring in there on my bike. I have the old VM carburetors so there is little in common with the two types. I imagine vacuum leaks would affect both the same way though. When was your last valve adjustment?