'98 YZ400F - Fuel leak from carb overflow hose - help!

OK, so I broke down and reassembled this old YZ400F to some extent (not completely, but much of it). Among the things I tweaked were the carb, of course. I replaced worn out accelerator pump pieces, gaskets, fixed choke issue, etc. and cleaned everything fully, although it was already fairly clean through the jets and passages.

I adjusted float height to spec (9 mm), but I was very confused by the matter because the float height could change dramatically (~3 mm) if I just applied a tiny light touch to the floats. The first mistake was probably that I measured the 9 mm from after I had "touched" the floats down (down in terms of having carb upside down).

In any case, the problem now is that fuel is leaking from the overflow after the float bowl fills. This confuses me more because to me it seems that having pushed the floats down slightly during measurement, if anything I would have a LOW fuel level -- not overflow. Could the floats themselves be no good and simply need replacement?

Please help! Any suggestions are greatly appreciated.


PS: What the heck is the name of the big knurled screw that bends up and adjusts idle? I thought that was called the "idle mixture screw," but other posts are mixing me up. Also, I would really appreciate it if someone could explain the "default" setting for that screw because the PO had it turned all the way in...

Quick update. I felt inspired tonight (and after reading a lot I realized a definite mistake I made with the pilot screw), so I went down and pulled off the carb again. I set the pilot screw correctly, adjusted the float height exactly to 9 mm, this time at the point where the tang just barely touched the float needle without compressing the needle spring. Fuel is still leaking out through the overflow after the bowl fills. :smirk:

The needle valve appears to be in good condition and I can't really tell the condition of the needle seat (almost looks like it's not there?). Should I just put in a new needle valve and seat or is this not likely the problem?

I have just experienced the same problem with my '98 YZ400F.

I had a spare float valve kit so changed this and problem solved.

What I did notice however, was the fit of the old valve body into the carburettor was not as tight as the new body i.e. the o-ring was worn. I suspect part of my problem was fuel bypassing the float valve because of the o-ring leaking.

My suggestion is check this first before parting out for a new valve assembly.

I was starting to think that was it. Even though only one of the pieces might be bad, I just went ahead and ordered all of them to be sure (needle valve, o-ring, valve seat). Cost from Sudco was $21 total, probably going to be closer to $30 with shipping. Worthy investment if it fixes my problem... Will let you know.



Well, I'm positive the leaking was the needle valve now. I replaced seat, valve, and o-ring and that's OK now. :smirk: Had to adjust the float height back to where I had it because then that was producing an overflow. So at this point, I have all that dialed in. Carb is no longer overflowing, floats are good, carb is getting proper fuel flow (tested that independently just to make sure after all was said and done).

Only now the bike practically exploded when I kicked it over once, choke on. With choke off, it wouldn't start. I tried it again choke on and bam, huge loud explosion kicking it over. Turned idle screw way down (it was all the way in), replaced spark plug, and it sounds like it really badly wants to start... But it doesn't.

I think now I'm going to go back in and check the valve clearances. Please tell me if I'm stupid.



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