Carbs are flooding like a faucet

[Port]

1981 xs650-sh

My carbs are flooding like hell. The floats had been replaced with plastic ones when i bought the bike. The float needle looks new but the float seat looks like the original. The needle seat o-ring was shrunk up and brittle so badly the seats just fell right out. I replaced both of them but, that didn't fix the flooding. I figure the next step is to check the float height but I wanted to ask some questions before I continue.

- what is the best way to check the float height?
with a caliper or with a clear piece of tubing attached to the bowl drain hole

- can i do this accurately on my workbench?
if so, how what precautions should I take to make sure it's right on the bench and when I put them back in the bike?

- What might be the problem besides the float height?
what next if this doesn't fix it?

 

[jd750ace]

Position it level on the bench and use the tube. You can test it on the bench too.

 

[wherearewe]

Personally i like both. First using digital callipers, or I like to cut a piece of plastic or credit card in an L shape to the desired height and use that. Then using the tube method is like a 2nd check for me. I use a bench vice and some soft cloth to prevent scratches to sit them upright and secure.

 

[5twins]

I've always had success just measuring with a 6" steel rule (graduated in MM on one side of course) but maybe I'm just more familiar with the process than most. There are some "gotchas" when dealing with the BS34 floats. The first year of these carbs, 1980, had normal brass floats just like the earlier BS38s. However, due to the different shape and size of the float bowl, the setting spec was very different from the earlier BS38 specs (by like 2 to 3mm). And to confuse matters even more, when the plastic floats came along in 1981, their setting spec changed again and doesn't match any of the brass float ones. To make matters worse, Yamaha didn't change the spec in the manuals, still listing the '80 brass float spec for later plastic float equipped models.

So, here's the info you need to sort this mess. The 1980 brass float spec was 27.3mm ±.5mm. All the earlier BS38 carbs were in the 24 to 25mm range. The plastic float spec is 22 ± 1mm. It's pretty common knowledge to measure the brass floats to the top edge of each round float "bulb". The plastic floats have an odder shape with a flat area up near the hinge so some confusion often arises as to where they get measured to. They are the same as the brass ones, measured to the top of the float "bulbs", like so .....

The above floats are set slightly rich @ 21mm but there was no overflowing and this carb set was for a straight piped, UNI podded bobber, so I left them. I figured they could use the added richness.

 

[Valkyrie]

Thanks for the info.

 

[Dusty]

Maybe I'm the lucky one but I have messed with these bikes for yrs,and never had a float be out of spec!! I would first suspect the seat or needle or the rubber on the needle before messing with float height. how does that change unless a po hit it with a hammer or played miss match with a set of carbs?? I'm not saying don't do it,I'm just saying check the other stuff as well.

 

[Port]

idk what the PO did but the floats were about 4mm out of speck. I'm not positive i've got it fixed yet, i'm waiting on new pilot jet plugs before i test it. Should be here tomorrow.

 

[Dusty]

I would still pay close attention to the needle,and seat area. Good luck.

 

[webbrs]

I may be late on this thread but I would like to get a clarification of I can. I read that the floats need to be checked hanging at an angle but the pic shows them on the bench upside down. I don't think mine have been apart in the thirty years but extended (manually raised) the floats ride at around 28mm and seated (at rest, with carbs upside down on the bench) at 19mm with the gaskets in so minus one is 18mm. I cant get a good reading hanging them up rite, just too many things going on at once.

 

[5twins]

Some carbs are checked with them at an angle and the floats hanging down just touching the float needle pin. These, from all and any manuals I've seen, are not. They're set upside down with the floats gently resting on the float needle pin. I think the specs given are figured for this and the slight compression of the pin that occurs. That's how I've always set them and they worked fine.

The measurement is taken from the gasket surface so off the gasket and reading 19 would actually have them set at 20 (not 18). In either case, that's wrong. What's right will depend on what the year of the carb set is. The spec is different for different year carb sets.

 

[webbrs]

Yeah I thought about that later and it was 20. Never good at math. So at rest on the bench they should read 22mm with the plastic floats? 23mm with the gasket, correct? Or do I need to lightly raise the floats till just the pin engages? Plus what is the safest way to remove the float pin? I need to check the screen and seal.