XS400 carbs frustrating as hell

[randy]

Help!

OK, so my sis has a 81 XS400 special. Ran great last fall. Sat over the winter so I cleaned the carbs and installed new float valves and needles. Also replaced spark plug wires and caps.


Floats are set at 32mm, (stock) according to Haynes. When I opened the carbs, the floats had been set at about 25mm, way too rich.

Pilots are 1-1/4 turns out.

Air filters are new.

Have nice hot sparks on both cylinders.

Here's my problem. It will start and run for a few minutes, then floods and fouls the plugs. Clean the plugs and repeat. It was running fine, then I leaned it out and it still too rich? Doesn't make sense.

[TopCatGr58]

Hey Randy,

Did you make some assumptions like the floats are actually floating?? Maybe they are sinking, and that would flood it?!

Also, don't know where the float bowl vent is, but could it also be clogging, similar to the air vent on the 78 XS11's?? JAT?
T.C.

[randy]

Hey, T.C.

Yeah, the floats are floating. No holes and no gas in them. They are brass, though. I found that strange for such a late model. Even my '79 has plastic floats. The vents on these are routed through the carb body to the air horn and they're clear. These really have me stumped.

[TopCatGr58]

Hey Randy,

Haynes may have printed a mistake, cause according to the SERVICE DATA on the Yamaha Parts site for the 81SH the float height is 27.3 + 1.0mm..not 32mm!

The mains listed as 135 and pilot as 42.5. Did you pull the pilot jets out to verify their style?

They list the carbs as BS34III model.

Also, check for a shared tunnel between the pilot jet tower and the main jet tower. That main size is rather large, and the parts fiche shows a screw/cap over the pilot jets, so if yours doesn't have the pilot tower capped, may be why it's too rich, IF it's supposed to share with the MAINs like the early XS11's!
T.C.

[JWSanders]

I had a problem with mine until I replaced the diaphragms. Luckily, the spare ones from a set of 11 carbs I have fit perfectly. The plugs would be fouled after every ride. One seemed in good shape but the other had a hole in it.

I still dont have mine 100% perfect though. One side runs a tad too rich, the other (which I think has a small exhaust leak) is running a little on the too white side.

I don't think my pipes are mounted as well as they could be, and the copper crush gaskets I bought suck. I need to find the ceramic/metal type like I have on the 11. Wouldn't hurt me to get a new set of pipes altogether, really. Mine were welded to replacement mufflers without the benefit of being on the bike, and are a little crooked.

[randy]

T.C. you son of a gun! right you are, 27.3 mm.

Damn typos! But that still doesn't explain why lowering the floats didn't solve the rich problem

[TopCatGr58]

Randy,

Did you see my last paragraph? Just like the XS11's carbs, IF the pilot jet tower plug isn't there, it will suck in way too much fuel directly from the bowl instead of thru the main jet tower tunnel, and would provide too much fuel, even with lower fuel level!

A closer look at the fiche also showed the Main Nozzle/emulsifier with that little hole in the side of the end that allows fuel to be shared between the main jet tower and the pilot jet tower! I'd check that out, and put the caps on the pilot jet towers IF they are not capped!
T.C.

[randy]

Well, it's like I said, they ran fine last fall, everything is there as before.

And yes, the pilot towers are plugged and the wee hole is clear.

[LoserShoes]

Check the ends for the enriching plungers, and see if the rubber seal is intact. One of the problems I found on the DT3 was the seal was pitted and torn, so the enricher was never really shutting off. Not likely, hey worth a look.

Steve

[randy]

Steve,

Actually,more likely than anything else. That's the only thing I haven't checked!

Thanks, guys.

[mro]

But that still doesn't explain why lowering the floats didn't solve the rich problem

Could have been the float hanging open at that level after engine started.


mro