When I was cleaning the carbs of my bike I noticed the float valves left over from the previous owner were a lot shorter than the ones in my random ebay rebuild kit, and some of the springy seats had frozen, so I replaced them. I checked the float height (carbs sideways so that the valve just barely seated and the float is in contact with the pin) and I remember it was correct (13.7mm iirc).

Before I put the bike fully back together I ran it with an external gravity fed fuel tank, runs fine. I tried it into the fuel pump, ran fine. I even tried the actual fuel tank connected to the fuel pump but off the bike and it ran fine. However, when I put the fuel tank on the bike and connected to the fuel pump, the idle was much lower even though my idle screw had not changed. When I skipped the pump and ran it off gravity, it ran fine for a while.

However, now, when I turn the petcock to the on position, the idle will drop, just like when I had the fuel pump on. I'm also getting about 25mpg's cruising 45mph without much stopping so I think I'm definitely running very rich. I also notice that when I brake, or brake hard, the rpms will drop. It feels like something with the floats is wrong but I've cleaned them many times already, the slide real smooth, the seat is clean, and the rubber tips are fresh.

the jets are stock as far as I can tell. I have a brand new air filter on, so that shouldn't be clogged up. Exhaust is stock.

Anyone have any ideas what could be going on? I found an old thread where IDoDirt took a picture of his float valves to be 16mm, so I'm thinking of pulling my carbs off again to measure the float valves to see if they are the same length, and cleaning it again. Somehow I am getting a rich condition and I have no idea why.

 

I would say yes check the float valves again.

 

I swapped my float needles to another set of aftermarket needles, no dice. Swapped it to a set of used OEM needles in good condition, no dice.

Somehow now the 4th cylinder doesn't fire or is firing very poorly. When the other cylinders are up to 200C it's barely breaking 100C.

I've tested the floats they seal against my external fuel tank all night the level didn't change, I just dk what could be going wrong. 😭

 

Seems to be maybe a fuel line is being pinched once the tank is placed back on. How about the main jet and idle jets? Take those off and verify they are the correct jet sizing (#138 and #135) for 49 State.
Have you synchronized the carburetors and balanced the idle circuit? Those are two separate adjustments with a synchronizer gauge and a digital rpm gauge.
also, check the air injection hoses. Those may be pinched off or clogged. Those are my suggestions. 🙄

 

I figured out that every one of my cylinders is firing relatively normally except for #4. I checked compression before it's fine, the coils are good and the spark is strong. If I pull the vacuum port on cylinder 4 it runs, so I know it's way too rich.

Could this be the result of the air cutoff valve stuck closed? How can I test this?

 

What is the air cut off valve?

 

If you look on carburetor #1 and #4 it's the little plastic thing on the right side.

It's purpose is to richen the mixture when you chop the throttle to reduce afterburn. From what I read it's explain like this. When you chop the throttle at high rpm you can create a lean condition as the high rpm sucks a lot of air with only the pilot circuit amount of fuel. The air cutoff valve basically works to close an air hole in the idle circuit when there is sufficient vacuum pressure (such as from a throttle chop).

The valve itself works kind of like the cv slide, it's actuated by vacuum pressure and moves a valve to close an air jet. The usual case of failure is the rubber diaphragm hardens and doesn't close (causing afterburn on deceleration) but I'm thinking maybe mine got stuck in the closed position, enrichening the mixture.

I'm thinking this is the case because when I was testing with the airbox off, I can get the engine rpms to drop by covering the slow air jet on each carb except for on cylinder 4. Cylinder 4 seems to be be not firing, but if I open the vacuum port it fires just fine so I'm thinking it's way too rich. I don't think this is the pilot screw out of adjustment since they're all at about 2 turns out across the board, and with something this drastic I don't think it's mixture screw issue. The compression is only lower on #2, so it doesn't make sense for #4 to be misfiring due to lack of compression. I think the only way to be running this rich is due to some air port being closed when it should be open, and when cleaning the carb I was able to stick wire thru the entire slow air jet into the pilot fuel jet, so that orifice must be clear.

 

I did not know that. All I can say I had issues after cleaning the 49 States/Canada carburetors until I ultrasonic cleaned them. Had gasket sealer in the chambers, stuck in a screen a mess. Synchronized them and pilot screws started at 2 turns out and went to 3/4 to get to 1,200 for Canada version. No problems. I’m not sure how to test that air valve.

 

Hey there,
Reading your last post, can I offer something? When dialing in your pilot screws, the proper procedure is:
Adjust the #1 pilot screw until the RPM no longer increases. It doesn't matter what that value is, just adjust until it no longer climbs. Once that has been obtained, back it off 50 RPM.
Then, adjust your idle adjustment **** to 1,200 RPM.
Repeat this for the remaining carbs.

The only trick in this is that it is hard to see that small of an increment with the bike's tach. It is easier with a bench tach.

Once that is complete, assuming the sync was correct and the carbs are spotless, your setup is just the way the factory recommends.

 

After taking the carbs back off and cleaning them about a million times I finally found the issue. The petcock and float thing was a red herring.

I inspected all the pilot jets and found that some looked to be slightly larger than others, even though all marked factory Keihin 40S jets. Maybe they got worn larger or rusted out or whatever, or somebody went at them with a drill or steel wire. Either way, that's what was causing the rich condition, and I'm guessing the petcock thing is basically forcing a leaner condition with the 'lower' effective float level. Once I replaced all the pilot jets, it runs so much better. I re synced the carbs, followed the mixture screw adjustment prescribed in the manual, and now it runs like a dream.

hope this helps somebody in the future and thanks for the help everyone