Carbs still leaking !

[Ultim8pc]

Either way always set your FUEL HEIGHT not your float height as that is used as merely a starting point / guesstimate. C

And +1 for that statement. Actual fuel height can we way different to the float height..

[JimBoUK]

Hi,

After thinking I had fixed my carb issues I still had leaks from the top of the bowls however after more work I finally have the best fuel levels I could get and no leaks.
I did about 100 very enjoyable miles at the weekend in that amazing weather and no problems.

So here is a summary of what I did and my theories. It is all based on the many helpful comments to my posts and my own work getting to where I am .....

The initial problem I had was carbs leaking from the tops of the bowls.
After a full carb strip down, clean and rebuild the first thing I had to get right was fuel levels.

I followed the process of setting fuel heights using the clear tube method.
Whatever I did I struggled to get the fuel levels low enough and consistent. I found I had to bend the tangs up so high it stopped fuel entering the bowl at all.
A few people recommended genuine float valves as I has pattern ones.
So eventually I got some genuine Mikuni float vales. They were not the Genuine Kawasaki ones but Genuine Mikuni ones (effectively the same thing).
The valves came from Allens Performance - "VM26/26-2.0" (much cheaper than The Kawasaki ones)
I noticed obvious differences between the pattern float valves and the genuine ones regarding the sprung loaded pin.
On the pattern ones the sprung loaded pin pushed all the way in where it only went half way on the genuine ones. The movement of the sprung pin also felt smoother on the Genuine ones where I could feel it grating a bit on the pattern ones.
I thought this would solve all the issues but it did not. I still could not get the fuel height as low as I wanted however results were better.

I then had what I thought was a lightbulb moment.
Where I had the carbs on the bench I had a separate fuel feed.
The small tank was hung up high which is much higher from the carbs than the fuel feed on the bike. Surely this would create more fuel pressure going into the float valve.
It also got me thinking about a possible difference between a full and empty petrol tank.
I did some research and found that the fuel pressure is dependent on the liquid density, the force of gravity and the height to the top of the fuel source.
The flow rate is determined by the size of the smallest tube in the journey the fuel takes.
The weight of the fuel in the source tank does not come into play.
So I did some maths and worked out the fuel pressure at the float valve on the bike with a full tank of fuel would be about 0.3 PSI. On my test rig it was 0.9 PSI.
I know it's very low pressures but the float buoyancy was designed I assume with about 0.3psi
Therefore I lowered my fuel source so it was the same as on the bike and tried again.
It wasn't much different
I eventually managed to get the fuel height 3mm below the top of the bowl (not 3mm from the bottom of the carb as the manual picture suggests as the bowl is recessed into the carb body). Results were also consistent.

Even though I had the fuel level about right, I recon that with the forces of gravity and having the bike on the side stand etc, the fuel level will lap above the top of the bowls.
Therefore the bowl gaskets must give a good seal.
Someone mentioned that the paper gaskets can act a bit like a wick and fuel can soak through them.
Someone else said the gasket surfaces on the bowls or carb body could become warped through over tightening.
I checked the surface on the bowls and they were very slightly raised at the corners so I flattened them.
It's more difficult to check the surface on the carb body as it's in a recess but I think they were also very slightly warped. There is not much I could really do about this.
I decided rubber gaskets were the way to go.
The Keyster kits have these but I did not need all the other parts. Someone did sell me some they had that were laser cut but these were not very good and went hard after contact with fuel.
Therefore I bought some fuel resistant 1mm rubber sheet then using hole punches and a scalpel I made my own. I was very please with the end result and soaked one in fuel for a week to make sure it seemed OK.
When I fitted the gaskets I did them up very tight without being ridiculous and since then ..... no leaks

As part of the work I listen to other advice on this forum.
I fitted in line fuel filters between the fuel tap and the carbs. I don't want to take the carbs off again due to some muck getting through.
Someone suggested making sure the fuel tap did not let any fuel through when turned off. Mine did a bit.
If the float valves are doing their job then a weeping tap should not matter but I thought I'd best deal with it.
It had a fuel tap rebuild kit when I did a major rebuild about 8 years ago so I tried a new Pattern fuel tap from M&P for about £20. This was worse than my original !
So I refurbed my genuine fuel tap again taking particular attention to a smooth finish on the tapered plastic bit. It's now sorted and does not let petrol though when turned off.

The carbs are now back on the bike, all balanced and she runs well completely leak free

Here are pieces of advice that might help others :-
Never rely on the dry approach of measuring upside down float heights for the fuel level. It's just a starting point. The only way to get this right is with the clear tube method.
Do the clear tube method with the carbs off the bike on the bench using a home made wooden rig so it's easy to keep removing the bowls.
When setting fuel height on the bench, make the height to the top of the fuel feed the same as it would be to the top of the fuel in the tank when back on the bike.
When setting fuel height, as the bowls need to go on and off multiple times, you only need to secure the bowls with one screw until setup is complete.
Use genuine float valves.
Make sure the surface of the float tangs are dead smooth with no marks to catch on the float valve pins.
Fit in-line fuel filter(s) between the tap and the carb. If you do get some bits come through in the fuel this could be a real pain if it gets stuck in the float valve.
Good luck if you try a pattern fuel tap. Re-furbish an original one if possible.
Use rubber float bowl gaskets - The Keyster carb refurb kits have these or make your own.

Thanks very much for all the advice to get me to this point.
I hope my learning circle helps others.

[Ultim8pc]

Glad you got it sorted and that the advice given on here was helpful.
This is what i love about this club.