gas from bowl drain tube

[drtaco]

well I have a problem with gas coming out of this carburetor bowl drain tube it started with no input, I swear, out of the blue, I didn't touch nuthin. gas starts spurting out at start up stops when engine is turned off. reading the Haynes manual, it seems the tube is for draining the bowl and also verifying float level by using a clear accessory tube. on page 101,fig.4.1 next to the main jet is a picture of the DRAIN SCREW. I take it that this is a needle valve screw with an o'ring at the tip? and maybe one of the screws o'ring has given up? I tried to tighten screws and they were tight.  but if I am correct (or close) help would be appreciated on sourcing o'rings or whatever else could be the problem maybe screw and o'ring?
thanks

[cvincer]

Fuel coming out of the overflow pipe = carb bowl overfilling = needle valve which is closed by the float is not sealing properly.

The bike has a vacum operated petcock, so with the engine running, fuel is being pumped into the carb bowl until the needle valve is closed by the float pressing up on it.  Engine not running = no pumping of fuel.

Check for grit preventing the needle valve from sealing.    When you are up & running again (whatever the problem was) put an inline filter in the fuel line

[drtaco]

they refer to this tube and screws as "DRAIN". i'am not positive but I thought there was another overflow system and this is a drain. still I can understand having a defective o'ring or seat and bowl being pressurised when running is enough to push past defect and into tube. maybe I should source parts book and find numbers. thanks

[Rikugun]

The drain does double duty. When the screw is loosened, the bowl drains through the tube. However, there is another way fuel can exit the drain even when the screw is tight.

If the carb's inlet needle and seat are stuck or dirty, the bowl will over fill. This would happen when the engine is running and vacuum is supplied to turn the petcock "on". Also, if the petcock is turned to "PRI" (prime), gas will flow even if the engine is not running.

Anyway, there is a standpipe in the carb whose height is slightly taller than the properly set fuel height. If the fuel level exceeds that level, fuel drains through the tube and out the drain hose.

As long as the fuel level is correct, no gas exits the drain unless the screw is loosened.

[QBS]

This might help diagnose your problem:  Engine off.  Petcock on ON.  Open both carb drain screws.  Gently tap each carb float bowl several times.  Turn petcock to PRI.  Let fuel flow to ground for several seconds while gently tapping each float bowl.  Close carb drain screws.  Fuel flow to ground should stop.  If fuel flow stops, problem was trash in the needle valve seat which has now been flushed out.  If fuel flow continues problem is probably a bad needle valve. 

Isolating the problem to a specific carb requires removing the drain tubing from one of the carbs and repeating the test on each carb.  Be very careful to not break off the float bowl drain spigot when doing this.  Plan on replacing the tubing with new product upon testing completion.

Please note that V vacume controlled fuel petcocks are very undependable and prone to failure.  This is manifested by uncontrolled fuel flow when the engine is off and the petcock is turned to "ON".  Trashy and or failed needle valves combined with a failed petcock equal uncontrollable fuel flow to ground.  The ultimate fix is to install an inline fuel filter AND an inline fuel cut off valve.  Both procedures are cheap and easy to do.  The end result will probably be unsightly.  But it's worth it.

[drtaco]

thanks all,l will try separating lines to isolate individual carbs. find out which one or if both are causing the flow and go from there. I did not know of the stand pipe feeding this "drain" I had just thought reading on pg99 section 10 and fig 10.5 referring to an overflow/vent tubing. I have never rebuilt these carbs before, reading about something and working on it can be totally different as i'am proof :)

[motoracer8]

 The needles and seats are probably fine. The little seats have little orings around them and then are pressed into the carb top and held with a clip and screw.  The oring fails from crummy gas and age.

The fuel is flowing around the seats and overflowing the bowls. Yamaha is not the only one. I've seen it happen on several carbs with push in float valve seats.

[Single Carb Vision]

I had the same thing happening on my 83 when it was less than a month old. I had the dealer do a service on it, he had it two days longer than he was supposed to. When I picked it up and was on my way home I smelled gas. I was only a few minutes from home so continued on. When I got home I started trying to find the problem. I ended up taking the fairing, tank, airbox and then the carburetor tops off and then found the problem. Remember now, this was a brand new 83 that was only 1 month old. When I took off the top of the carburetor and started looking at the needle seat I saw something I couldn't believe. The float had been broken in half (no I didn't just do it when I took it apart) and some Einstein put back together with............. a 6-3/4' pan head sheet metal screw! I myself don't ever remember seeing any sheet metal screws that float! When I went to talk to the dealer about it, I told him I think we should talk about something in your office (there were five or six customers in the show room at the time). The dealer/owner was a big boisterous type of person. He said we can talk about what ever it is right here. Well, I made sure everyone there heard me describe what I had found. The owner then quickly decided to finish talking about it in his office. I then had to wait about two months for a new replacement float to come from Japan. I don't know if it came by boat or they just put it in an ocean current that headed to the USA and let it drift here. When it finally arrived, I then I had the fun of trying to remember how the bike went back together after two months of sitting because I hadn't marked or made notes on anything when I took it apart , thinking I would be putting it right back together the same afternoon I took it apart.

Oh, I guess what I meant to say was, you can check your float just in case.

I know someone that has a couple of floats that I gave him after the carburetors were removed from my bike. He sold his bike and doesn't need them. He offerd them back to me but I told him to just hang on to them and maybe someone else might need them, just in case you do.

[QBS]

Sad, but true.  That level of service was not uncommon back in the day.  I'm not sure that it's really any better now.

[Single Carb Vision]

What really got me was the dummy using a sheet metal screw. Did he/they think that if I found it in the future I would think it came from the factory like that???? If he would have used a couple of wooden toothpicks and a little glue it might have worked. Scotch tape would have been a better choice, it might not have held the pieces together but it wouldn't have caused the float to sink!!!!