YZ400 float level / fuel leak


9 replies to this topic
  • wobble

Posted December 16, 2007 - 12:55 PM

#1

Hi - I left my 1999 YZ 400 in the garage for about one month (before that it was working perfectly). I have had the bike since new in 99 - used it for many enduro's and MotoX events and amazingly this is the first problem so far. All I have done is standard servicing. When I turned the fuel knob on - it started leaking out of the carb overflow. The bike would not run at all - (obviously far too rich). So - I checked the float / fuel level - far too high. But why would this change all on its own? I replaced the float needle and seat. I checked the float was not binding. I checked the float was not punctured. None of these things made any difference. I then bent the float tab to change the float level - I had to bend it quite a lot. It now has nearly the right float level (I cant bend it anymore) and just about runs - but very rich. Occasionally it still leaks out of the carb. I have a feeling that I have missed some underlying problem - why would this suddenly occur?
Any help or suggestions?

  • grayracer513

Posted December 16, 2007 - 04:22 PM

#2

The Viton rubber tip of the needle valve is dried out and shrunk. It may recover (in which case the float level will be low) but if it's that bad, it more likely won't. It's a result of an older "rubber" carb part being left to sit for a long time.

  • wobble

Posted December 17, 2007 - 01:09 AM

#3

Thanks for your reply - but I replaced the needle valve assembly with a new one and it made no difference. The float level remained high.

  • grayracer513

Posted December 17, 2007 - 06:59 AM

#4

If dirt in the fuel (even extremely fine dirt) is not involved, there are only a couple of things that can raise the float level.

> faulty needle/seat

> binding float pivot

> "heavy" or "sunk" float (saturated with gas)

Take the float out and put it in a can of gas or mineral spirits. 1/3 to 1/2 of the floats should be above the surface, and both floats should be at the same level.

  • wobble

Posted December 17, 2007 - 08:24 AM

#5

Again - thanks for the suggestions but I have tried all these things to no avail. I will just take it apart again and try and look more closely.

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  • earthad58

Posted December 17, 2007 - 06:07 PM

#6

im looking into buying a 99 yz400f from a friend of mine that just bought it suposedly it has a big bore kit on it and it has fmf power bomb header and supertrapp pipe and everything but it starts up and idles with the choke on but pops and snorts then every single time i turn the choke off it immediately dies if i keep it reved up and turn the choke off it will stay running but still snorts and pops and will not idle and i noticed the header gets red hot....i nor my friend know really nothing about this bike except for it has the big bore kit im wondering if this sounds like a jetting problem... as we dont know what its jetted for and also it has regular gas in it im wondering if maybe cuz of the big bore kit it might need to run race gas? id love to buy the bike it has tons of power when i rode it just wont idle and is poppy and i can get it for dirt cheap any help?

  • Caseus

Posted December 18, 2007 - 07:34 AM

#7

Wobble - When you say you replaced the needle valve assy, did you mean the float needle along with a new seat? It sounds as though there is somthing blocking the float needle from completely seating, which would explain the need to "over compensate" the float level. I would look closely at the seat just below the needle float assembly for some crud blocking it. When I encounter a float level problem I will have the carb off the bike. I invert it and secure it creatively in my vice and closely inspect the float. I move it up and down manually with the float bowl off and make sure the whole works is not binding through its movement. I had this problem on a RM125 I bought. After looking very closely i found the float was hitting a liitle piece of the carb casting and hanging up in the carb. It looked as though the float arm had bent slightly when the previous owner was adjuting the float level. It took quite a lot of looking to find the problem. :banghead:


earthad58 - I have learned through personal experience that these bikes have a carb which is notoriously difficult to tune. The first step is to take the carb off and clean it throughly. Now this will be no ordinary carb cleaning, what I am talking about is the most comprehensive and intricate carb cleaning ever known to man. I attempted this after having my yzf sit for a few months and had to go at it three times before I got it right.

Also, no amount of tweaking and tuning will fix a broken (leaking) intake boot. Inspect the rubber boot that holds the carb to the cylinder, it may have cracked. You mentioned you have a big bore kit on it as well, which will of course need the appropriate jetting to go along with the increased performance. For this a more technically savvy person will need to make sure that the jetting is correct for those types of mods.

  • SXP

Posted December 18, 2007 - 02:38 PM

#8

I replaced the float needle and seat.


How about the o-ring that goes in between the seat and the carb body? I can't remember if that comes with the new seat or not. If that has deteriorated then gas is going to leak around it even with the needle valve firmly in place against the seat.

BTW, this first gen FCR carb sucks!

  • wobble

Posted December 18, 2007 - 11:18 PM

#9

Caseus - yes - I replaced the float needle along with the seat. I suspected what had happenned was just as you have described: 'there is somthing blocking the float needle from completely seating, which would explain the need to "over compensate" the float level'. But the new needle and seat made no difference. And when it first went wrong it wasnt as though I had done anything to the carb. I will follow your advice and recheck the float movement. If the needle and seat are ok this is about all it can be... Its interesting that you had this on another bike.

  • wobble

Posted May 20, 2008 - 02:31 AM

#10

Well - I think I have finally fixed the problem. I replaced the float (£20) and everything seems ok. I should have done this at the start! I can see no difference in the old float and the new one. When you 'float' the floats in petrol they both float at the same level. But something is different, as with the new float it runs fine.





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