New here , just snagged a 1980 xs1100

[TDodge7]

every removable jet is 100% clean too .

[randy]

Well, then, there's nothing wrong with the carbs, right?

[TDodge7]

i know . it makes no sense . perhaps they're haunted . i've been buying and selling bikes for close to 15 years now , i always seem to end up with these kinds of unsolvable problems , problems that technically can't exist but do . not one single removable part of carb #3 and #4 is even still inside carb #3 and #4 . i switched everything around with the good carbs . the good carbs still idle , #3 and #4 still don't (3 does alot more than 4) . i've tested every vent and passage with compressed air and carb cleaner , they're all open . they're bench synched , they all have gas , all floats are set to the correct height . i don't think there's anything else it could be . but once i get another set of carbs i'm totally going to band saw old carb #4 in half and see what the problem really was .

[randy]

OK, last comment then I'm out of ideas. My brain is starting to hurt...

Have you tried switching fuel lines from left bank to right bank and vice-versa?

[TDodge7]

i've tried the fuel lines both ways , the carbs don't have a problem getting fuel , it just has a problem sending it thru the completely clear pilot circuit . give it any throttle and it runs , it just refuses to idle .

[Ken Talbot]

all floats are set to the correct height

Like Randy, my head is starting to hurt too.

Even so let's think for a minute about float height. I have never followed the thinking that a number printed in a shop manual 30 years ago would necessarily, or even possibly, be correct for a carb in real life 30 years later considering wear and tear, jet changes, exhaust system changes, etc, etc. Float height controls the depth of fuel in the float bowls, which has a profound effect on how much fuel gets sucked up through the various passages by the vacuum created by airflow through the venturies. My direct personal experience with this was an increase of 10mpg by setting my floats 2mm higher than what they had been at, which lowered the depth in the bowls, which leaned out the overall mixture.

What if the depth in the bowls is too low? As long as the float valve is opening when the floats drop, fuel gets back into the bowls, and at higher engine speed, the stronger venturi would be able to suck up enough fuel to run, even if it might be a bit on the lean side. Maybe, at lower engine speed, where there is less venturi effect, there is not enough pull to run the primaries properly. The problem with this line of thinking is 1&2 seem to be working okay the way they are, but 3&4 are starving the primaries.

Assuming the floats all measure out the same with the calipers, the only thing that comes to mind that would cause two floats to float at a different height would be weight, such as might be caused by cracked/flooded lobes. But, two the same, that's a long shot too.

Something else is coming to mind, but I need to ponder on it for a bit more. Need to let the hurting go away for a while...

[TDodge7]

but when i stick carb 1&2 into boot 3&4 then cylinder 3&4 run just like 1&2 , carb 1&2 have the same float height as 3&4 . so since all carbs have the same float height and 1&2 run great then it can't be float height . imagine how frustrating this is in person lol .

[DGXSER]

Hmm....Yes...have you switched the floats between the carbs? If the ones in 3 and 4 were somehow over heated at some point and expanded just a touch, they may have more air in them and float just a touch higher leaving the fuel level in the bowls just a bit lower. Yes, this is reaching, but per the cat in the hat "the way to find that missing something is to find out where it is not".

[TDodge7]

yes , every single removable piece that was in carb 3 and 4 is now inside of carb 1 and 2 . 1 and 2 still run fine , 3 and 4 still don't (and they're now full of carb 1 and 2 parts which have always worked fine ). thus every single removable part in carb 3 and 4 was and still is good .

[Varnae]

oops, I guess it would help if I read the thread fully. Forget I even posted anything.

[DGXSER]

So it is defintiely in the body of the carbs. I wonder if someone drilled out a pilot jet and opened that passage up on those two on you at some time, or similar event on the mixture screw tip. Only thing that comes to mind at this point. Going bald from scratching my head!

[TDodge7]

all of the mixture screw holes are open and the mixture screws themselfs look great and have their tips . the 3 holes above the butterfly are clear (i tested by blocking them off individually and then using compressed air to unblock them ) so i'm sure they're clear . i know the pilot jet is clean (even the tiny hole on the tip) . that should be the entire pilot circuit . i even switched the mixture screws and o rings just to make sure they wern't some how causing this .

[DGXSER]

I have no doubt you have THOROUGHLY gone through these things. What I was thinking is that someone at some time had a stuck pilot jet in those two carbs and drilled them out but ran the bit past the jet and dorked up the passage messing up the velocity and draw through the jet? Same thing on the mixture screw side, if someone at some point broke a tip off and drilled it out making the hole bigger perhaps messing up the air flow and therefore the fuel being delivered.

Again, reaching deep and not sure how you would check this.

[scalded dog]

i've tried the fuel lines both ways , the carbs don't have a problem getting fuel , it just has a problem sending it thru the completely clear pilot circuit . give it any throttle and it runs , it just refuses to idle .

Not trying to -iss you off, but surely, you have tried the idle screw. I know, dumb statement, but sometimes we tend to overlook the obvious. Later 'Dog

[TDodge7]

it idles on 2 sometimes 3 carbs , i do have the idle turned up a little . to what i can't tell you because i have no tach .