Recommended Float Height + Additional Advice

[Devin Vreeland]

Hello, I recently bought a 1981 xs400 as my first bike ever! However, when I bought the bike I knew It didn't run perfectly and have been trying to fix it. Here's the lowdown:
- I believe this bike has the Mikuni bs34's (whatever's stock)
- When the bike is cold it runs as expected.
- When it gets to operating temperature, it starts to struggle to run, eventually dying completely.
- When at operating temperature, I can hold throttle to make the bike stay alive.
- When it's at operating temperature, it will be able to run on choke fairly smoothly.
- Plugs are covered in soot.
- Played with air/fuel, all it did was get the bike to not start at all when set to factory recommended specs.
- Petcock works well.
All these signs, to my knowledge point to a carbonator (air/fuel) issue. But, according to the previous owner, the carbs had just been ultra-sonic cleaned by someone (I'm still doing my own due diligence). I pulled the carbs tonight and ended up cleaning the main jets (most likely not related) and the pilot jets (what I think could be the culprit). I didn't see any real debris in either as I recall but know even a small blockage can make a large impact. I also want to check the float heights but have been getting some mixed answers on the stock float height.

What is the recommended distance between the carb basins mounting surface and the top of the floats (Including or discluding the lip?).

Anything else specifically that I should clean while in the carbs? it all looks pretty clean to me.

Also, if I'm off on my diagnosis, any other ideas as to what the problem could be?

I don't know a great deal as this is my first motorcycle any help is much appreciated!

 

[Durney]

Hello, I recently bought a 1981 xs400 as my first bike ever! However, when I bought the bike I knew It didn't run perfectly and have been trying to fix it. Here's the lowdown:
- I believe this bike has the Mikuni bs34's (whatever's stock)
- When the bike is cold it runs as expected.
- When it gets to operating temperature, it starts to struggle to run, eventually dying completely.
- When at operating temperature, I can hold throttle to make the bike stay alive.
- When it's at operating temperature, it will be able to run on choke fairly smoothly.
- Plugs are covered in soot.
- Played with air/fuel, all it did was get the bike to not start at all when set to factory recommended specs.
- Petcock works well.
All these signs, to my knowledge point to a carbonator (air/fuel) issue. But, according to the previous owner, the carbs had just been ultra-sonic cleaned by someone (I'm still doing my own due diligence). I pulled the carbs tonight and ended up cleaning the main jets (most likely not related) and the pilot jets (what I think could be the culprit). I didn't see any real debris in either as I recall but know even a small blockage can make a large impact. I also want to check the float heights but have been getting some mixed answers on the stock float height.

What is the recommended distance between the carb basins mounting surface and the top of the floats (Including or discluding the lip?).

Anything else specifically that I should clean while in the carbs? it all looks pretty clean to me.

Also, if I'm off on my diagnosis, any other ideas as to what the problem could be?

I don't know a great deal as this is my first motorcycle any help is much appreciated!

Inside the carbs, the needle and seat screw into the carb body. There's a tiny "O" ring around the seat that screws into carb shrinks and even with floats set up perfect it runs rich. I wrapped plumbers tape (teflon) around the threads and and it solved the problem.

 

[LeBouc]

On my 1981, the float is of the plastic type and the recommended height for that type is 22mm from the base of the gasket (removed) to the top of the float when the carb is put upside down. There is another type, in brass and it's 28 mm if i remember well.

 

[tstidham]

When I was doing the float heights on my 78, the manual listed a few values based on the carbs and years. The one value was 32mm, but that was clearly too much bending of the float tab to be correct for my carbs. I settled on 26mm for mine (brass floats) after reading many posts stating it was the optimal height and haven't had any issues with that.

 

[Revvy Kevvy]

I have mine set to 27 and its running terribly, gunna try 23 next and try the clear tube dummy check with it.

 

[LeBouc]

I have mine set to 27 and its running terribly, gunna try 23 next and try the clear tube dummy check with it.

You have to verify which type of float you have; if it's the platic one, you have to put it at 22 mm but if it's the metal one, it's 28 mm.

 

[Revvy Kevvy]

I have the plastic one, shaped funny not round like the metal one shown in the picture. So, 22 it is going to be.