Carb #2 overflows-Tried EVERYTHING

1983 xj900rk
Just when I thought I figured out my carb overflow problem it came back to haunt me...and it's ONLY with carb #2.

I bought new float seat assemblies for all 4 carbs and installed them. At first I had no issues and then within a month carb #2 started to leak AGAIN out of the overflow tube. So I exchanged the entire float valve assembly with carb #1. The problem continued with Carb #2 even after exchanging the float seat, needle, and float with #1. Carb #1 still worked fine. I spent the day fiddling with it and I came up with a makeshift clear plastic carb bowl for #2 so I can see what is going on. When I initially poured gas into the carbs the float assembly did it's job and there were no leaks, however, as an experiment I drained carb #1 and then put more gas through the main gas line and sure enough Carb #2 started to overflow even after it initially stopped the flow of gas.

I've been dealing with this issue for a while now. There are no visible cracks in the housing. I tried swapping the float bowls. It started after I completely took apart the carbs and broke the rack. I ultrasonic cleaned each carb, got new throttle shaft seals, the whole thing! And now I'm left with this leaking carb #2 that will not go away. The leakage seems to happen mostly after riding at higher speeds on the freeway. If I'm just riding around town it's less likely to happen although it HAS happened. Any suggestions would be appreciated. Thanks.

 

Could there be lower pressure in the overflow pipe of carburettor two? I doubt it's possible. Are there any splits in the rubber where the pipe joins the brass stub? Are the air jets the right way round in carburettor two?

Could it be leaking between the needle valve seat and the carburettor body?

 

If you take the carburettors off. Remove bowl two, press down on the needle and force air through the fuel inlet does it leak? Lift the needle out and do the same thing mabe a hairline crack in the carb body.

 

Hi Franz. Since I don't understand high and low pressures when it comes to well anything, I don't quite understand your pressure question. I did try the air test by turning the carbs upside down so that everything just sits in place and I couldn't blow any air through the main gas line nor could I hear any air leaks whatsoever.

I'm curious of your air jet question. What do you mean by "right way round"?

 

The pressure was about low pressure drawing fuel out of the float bowl via the overflow tube at high speed. I read about it somewhere but then if that's possible why are the other three bowls not affected?

I am trying to find a picture on the jets at the carb throat where it joins the air filter. The setup in the Haynes manual is wrong. They are in the wrong places. I will keep trying to find an answer.

 

While not likely, it is possible that there is a small crack in the float seat housing.

 

That might have been me you read about. I once had a situation where gas was pouring out of the overflow tube on the 4th carb. I didn't have a tube attached to the overflow port and it was just exposed to high wind while riding causing a venturi effect and drawing gas out. It was immediately solved by attaching a tube. That was roughly 2 years ago when I first acquired the 900 and I wasn't having any difficulty with the 2nd carb at that time. I thought this might have been the issue with #2 carb but I don't see how because #2 is not exposed.

 

That's what it was the tube was off the overflow stub.

 

Indeed - or a damaged/old/flattened/wrong-sized valve seat o-ring.

 

It"s amazing how often it's somthing so simple. Been there.

 

I wouldn't overthink that and just go ahead and install an overflow tube (on all four) if you haven't already. They need to be there anyway to keep fuel off the hot engine if and when it does overflow

 

Why don't you try a slightly thinner pin than the float pin you have on carburettor two? You changed the needle valves between one and two and the floats. Did you also change the float pins? Needle rising slightly off square not seating properly sometimes? Is the needle clip moving freely on the float tang? Is there any burrs on the float tang edges?

 

Yes we all have lol.

 

One carburettor flooding solved. But you changed the parts between two and one so this may not help.

https://xjbikes.com/forums/threads/flooding-carb-problem-solved.18604/

 

https://xjbikes.com/forums/attachments/2021-11-12-17_33_17-clipboard-jpg.44121/

 

Does it ever overflow at idle? Are all your slides shutting at the same time?

 

I think I narrowed it down to the float. I think. lol. It's the only thing I haven't replaced. I switched the float from Carb 2 to Carb 3 and sure enough #3 started leaking. There is absolutely no tangible reason for this except that maybe these old floats can be warped just enough to cause issues. So I replaced the float with this cheap one I bought off ebay and it worked. The only thing is that the new float is a couple of millimeters smaller.

Can the old floats go bad? One forum mentioned that an old float can get heavier because it can become ever so slightly porous and gas can get into those crevices. I'm also wondering if the size difference is caused by the float just sittig in gas for over 40 years causing the float to bloat a little and not functioning properly.

 

Roger that, over time floats can absorb fuel and sink.

 

So looks like a simple solution as @tabaka45 noted, happens again.

 

Open the old float up to see if a tiny amount of fuel is inside.

 

For anyone who is interested:

I ended up buying a new float from Yamaha and the differences couldn't be more obvious. The old float is bloated and a couple millimeters bigger than the new float. I guess that was just enough to offshoot things causing overflow issues. Anyway, see for yourself.