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

[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.

[OldVet66]

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?

[OldVet66]

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?

[system9]

The carb boots, to my recollection, are about 4 years old? I am almost sure I replaced them when I got the bike. But I have had a few bikes, and I'm getting old, so I'm not exactly sure. I checked them with the bike running by spraying some carb cleaner all around the boots, no change in rpm, I know thats not a definitive test, Today I will take the boots off and inspect them, also swap them and see if the issue moves with the boots.
The valves were done also about 4 or 5 years ago, but that is the only thing I didn't do personally. My son had a class at Washtenaw community college motorcycle program, he took it there and did it. But could be a bad valve. I'm also going to do a compression test today. I'll let you know what I find out. Thanks.

[system9]

SOLVED!
Ok, this is the madness. It goes back to the concave brass floats. SOmething had caused my floats to kind of implode. They aren't broken, but somehow the weather or some weird air pressure issue caused them to compress a bit. This was causing the left carb especially to be rich, which is why it was best with the screw all the way in. I didn't suspect the floats initially, because why would floats suddenly change shape? So after I ruled out the mixture screw itself, I decided to re-set the floats to 22-23 mm based on the advice of some people. I also decided to put in some different pilot jets, to rule that out, so I grabbed some 17.5 pilots and put them in. After that the fuel was spilling out. At this point I decided to use the clear hose method attached to the bowl cap to get an actual float reading with the carbs on. They were waay off. I set them correctly, hoping all would be good. Final measurement was around 26mm. So carbs back on, no fuel overflowing, floats are great, But the bike is still idling best with the damn screw all the way in. I remove the carb boots, clean them check for cracks, they look good. I remove the choke mechanisms from the carbs, check them for leaks, they look good, and then I swap the carb boots and the choke mechanisms to see if the problem moves with them. Carbs back on, still idles best with mixture screw all the way in. I'm just staring at the bike in dismay. I decide to check the plugs again, and I notice they are dark! They have always been tan, never ever this dark. And they are both dark. Now I'm really confused, cause the floats are great, I have put in even smaller stock 17.5 pilot jets, and the screws are all the way in. I think, damn, I'm going to have to put in pilots that are smaller than stock? So i go to my jet drawer ( I have a lot of projects) and while I'm searching I come across a pair of 17.5 pilots. Same as what I already have in the bike now. And then I'm looking at them, and I think to myself "Why do I have 2 pair of 17.5?" cause these are the stock size pilots. I remove the carbs, take out a pilot and look at it. Really squint and look and I realize what happened. I had recently been tuning a Yamaha xs650. It uses the same style pilots. What I thought said 17.5, was actually stamped with a very faint 27.5!
So what I did was solve the original and probably only problem, which was the incorrect float level due to the misshapen floats. But in my attempt to return the carb jets to stock for further testing, I created another problem with similar symptoms at the exact same time I fixed the first problem, that was putting in pilots that were too large! I'm a super dingbat!! On the plus side, when I put the real 17.5 pilots in, the bike ran like a bat out of hell again. So, why would the bike idle best with the mixture screw all the way in? When the floats are sitting too low, and also when your pilot jet is too large.

[Joe H]

Good news and an easy fix…I have found my "ingenuity" has often lead me down strange paths.

[DimitriT]

Thanks for documenting the whole process. Strange that the floats would change shape like that. Maybe something to do with temperature swings when the bike was stored and the age of the floats.

[system9]

I live in Michigan. The temperature can go from 30˚ to 60˚f in 12 hours sometimes. I can only guess air pressure did that to the floats, as they seem very evenly compressed, both sets.