GS500 Message Forum

Main Area => GS500 FAQS => Topic started by: RedShift on March 22, 2007, 05:03:19 PM

Title: Diagnosing "Both carbs leak Fuel"
Post by: RedShift on March 22, 2007, 05:03:19 PM
Galahs (;u=4123galahs) posted a request for help to troubleshoot his leaking carburetors.  You can read the whole thread by clicking this link (, but he did such a great job of explaining his problem, then solving it himself, I just could not let it disappear into the ether. 

What follows are his pertinent posts so you have a quick study of the problem and the cure.  These are his posts, unedited except tp remove as bit of white-space.

If your carburetors leak fuel, read on...
Title: Both carbs leak Fuel - Floats don't float - Vapour lock!
Post by: RedShift on March 22, 2007, 05:04:59 PM
on: March 17, 2007, 01:35:10 AM Galahs (;u=4123galahs) wrote:

I have had a very similar problem to the member in this post

I was riding my bike for about an hour on a hot day. All was going well until I was stopped at a red light for a very long period.

Whilst waiting there I noticed my idle was slowing to the point where the bike was about to stop. I gave a quick rev of the bike and she sprung back into life but then proceeded to drop revs to the point that she would stall.

The only way I could keep the bike going was to keep its revs at 3000rpm or above.

When I stopped the bike I noticed fuel dripping out of my air filter.

I wont bore you with all the details of my dangerous ride home, but it was scary with the bike well down on power and surging and spluttering quite dangerously. Still if I let the bike idle she wanted to stall. If she stalled the procedure to start her was similar to what you have to do tto start a flooded engine. Wide open throttle when you cranked her.

Now after a quick read on here I decided to check my float heights.

Sheet! They are non existent on both sides!!!

The fuel will keep flowing right on out of the carbs.

So I pulled the float bowls off and checked the float needles. They were both shmick! Not grit and still has a perfect cone point.
I bent the float tab so it would close the valve earlier then tried pouring fuel down the fuel line again. Still it comes out of both sides carbs!!!

I repulled the bowls, and tested the float needle by ever so very very lightly pushing it into its recess and then adding fuel to the fuel lines. It seals perfectly!!!

 :dunno_white: Now I'm really at a loss.  Why would both carbs develop this problem simultaneously and why hasn't adjusting the float tab made any difference what so ever?

Title: Both carbs leaking fuel, continued...
Post by: RedShift on March 22, 2007, 05:06:44 PM
on: March 17, 2007, 03:54:56 AM Galahs (;u=4123galahs) wrote:

Basically this is where I think I have a fuel problem


When I pour fuel into the fuel hose, after filling the float bowls it then runs out of these little holes at the bottom of the carburettor air intake.

This is what I have done so far to try and fix the problem


Bent the float tab up and tightened the screw that holds the float valve seat.
Also cleaned all the jets and the float bowl (though they all looked brand new and no blockages to be seen)

The float needle still looks brand new!


Here's a pic of the carbs with the float removed.


Could the float valve seat be the problem?
Can it be easily removed?
Is there an O-ring there that could be stuffed?

Any suggestions or advice would be hugely appreciated.

The fact both carbs seem to have the same problem is what is really perplexing me. What are the odds they both got the same problem simultaneously???

Is there something else I am over looking?
Title: Galahs figures out "why Both carbs leaking fuel"
Post by: RedShift on March 22, 2007, 05:09:08 PM
on: March 19, 2007, 05:12:23 AM Galahs (;u=4123galahs) wrote:

This has frustrated me for a while so I'll share the pain and tell the full procedure I undertook to make my realisation.

I knew my floats sealed ok when manually lifted so I decided to find out why my floats weren't floating.

They didn't have a leak and their was no fuel in them.

So I removed the float bowls but left the floats attached to the carbs, 
I then filled the float bowls with methylated spirits, and raised them towards the floats whilst peering over the top of the bowl to see f the floats were infact rising. They weren't!!!

Hmmm, so my floats don't seem to float.

So I removed the float and put them in a container of metho. They floated!

Hmmm, I then submerged the float bowl in my ccontainer of metho so it was slightly submerged, then put the float in the float bowl. It floated!!!

So it seems that the float needs a bowl level just a touch higher than the float gasket to get it to float.

When I emptied the float bowl I thought it was strange that the float bowl seemed to hold more fluid than I had noticed when I drained the floats all these times when checking the level.


Something was stopping the floats from filling up enough, preventing the floats from rising. But how could this happen because the fuel is obviously rising up and out of the carb throat.

Wait a second!!!

What if there is an air lock in the float bowl!!!

No way.. it couldn't be!

I then decided to blow down the carb breather pipe.

Ahhhhhhhhhhhhhhhhh! It was blocked!

This would explain it. Both carbs shared the same breather explaining why both sides failed together.

So I removed the breather hose and blow down the breather connection and it flowed.
It ended up the breather pipe had a wasp build a mud nest in it!!!! blocking it.

I cleared the blockage, reinstalled the floats, float bowl for the 1 millionth time then tried pouring fuel down the fuel line.


Ahhhhhhhhhhhhhhhhhh!  :mad: :laugh:

Everything solved!!!

So a simple breather blockage caused all my dramas and was nearly at fault for my death on the road.

I've done a drawing to help people understand what happened for future reference because I don't want anyone going through this heartache. We all take from GStwins and its good if we an all give alittle too!
Title: Graphically - why both carbs leaking fuel
Post by: RedShift on March 22, 2007, 05:12:00 PM
on: March 19, 2007, 05:25:34 AM Galahs (;u=4123galahs) wrote:

Here is a diagram explaining what was happening.


The blockage of the breather hose prevented air from escaping from the float bowl as fuel was poured in.

As fuel entered the bowl, the bowl would fill pushing the displaced air out of the jets.

once the fuel level in the bowl reaches the jet no more air can escape from the float bowl. The pressure of the fuel flowing into the bowl however pushes the fuel up the jet into the carburettor throat, then leaks out the air filter.

The current level of the fuel is not high enough to allow the float to rise and shut the fuel supply off.

So fuel just keeps pouring into the float bowl, out of the jets, into the carb throat and out of the air box never allowing the float bowl to fill sufficiently.

After the vapour lock (air bubble) is cleared by clearing the breather, the fuel is allowed to fill the float bowl to a suitable level where the float will rise and shut off the fuel flow.

Here is a picture of breather hose I had to remove and clear a blockage from.



Really nice explanation, Galahs (;u=4123galahs).  Thanks for all your hard detective work.  Please leave the photos up for all to see.  You never know who may gain from your pain.   :thumb: