Keihin PWK 28: Setting the float height

Remove the Carburettor from the bike and clean the outer surfaces off, we use a tooth brush with a bit of the petrol from out of the carb and then blow it dry with compressed air.

Remove the float bowl, turn the carburettor upside down, remove the 2 securing screws and then gently pick the float bowl up from the back right hand side first (as you are looking down at the carb in the upside down position). You need to lift this corner up by about 5mm with the carb upside down, then the float bowl should gently come right off, be warned it isn’t easy till you’ve got the hang of it and don’t force it as you will bend the floats or the overflow pipe.

The carb needs to be tilted so the float just touches the the float valve, the float valve has a sprung loaded pin on it so if the carb is held horizontally upside down the weight of the floats will press the spring down and give you a false height. Keihin recommend that you set the floats so they are parallel with the gasket surface of the carb as shown by the 2 red lines in the picture.

The float height is critical to the way your bike runs, setting the floats further away from the Gasket surface shuts the fuel of earlier leaving the level of fuel in the float bowl lower which in turn weakens the mixture of the carb. Setting the floats closer to the gasket surface allows more petrol in to the bowl and therefore richens the mixture of the carb slightly but you risk losing more fuel out of the overflow pipe at the base of the carb. Because of this you can use the float height as a extra way of tuning your carb but to help keep things simple we recommend keeping the floats set at the same height while you get a good base setting.

We find on most trials bike the Carb is held at a downhill angle towards the engine and as such they tend to overflow eaisly so we prefer to set the floats so the top of the float lines up with the casting mark on the carburettor as shown in the picture by the 2 red arrows. Again this needs to be done with the tab on the floats just touching the float valve.
If the float height isn’t right or you wish to change it, bend the tab show in the picture with a small flat bladed screw driver till the desired float height is achieved.
If you are having problems with fuel constantly poring out of the carb overflow it is likely that you have another problem, we recommend checking the float valve is clean by taking it out and giving it a good blast with compressed air or carb cleaner.
While you have the float valve out check there is no marks on the rubber tip and that the sprung loaded plunger presses in and returns easily to the same position.
On reassembling your floats make sure you have a fuel filter and it is in good condition, we recommend using these excellent Mann filters as they filter out much smaller particles than most other filters we’ve tried.
The overflow pipe is another part that can cause the floats to stick, when removing the float bowl it is very easy to bend the overflow pipe if the correct removal procedure is not used. When overflow pipe does get bent it can rub or catch the floats stopping them from closing the valve properly.
Some of the latest Sherco, GasGas and Beta trials bikes using the PWK have a much smaller hole in the end of the overflow pipe pictured, this helps to reduce the amount of fuel that splash’s out of the overflow on these carbs. Beta did a modification piece to fit in the end of the pipe but unfortunately it isn’t available at the moment but if you have access to a lathe it is easy to make one up.

Many people complain about the amount of fuel these carbs leak and they nearly all mention that the Dellorto carburettor doesn’t ever leak, this is because the Dellorto PHBL, as fitted to many trials bikes, does not have a overflow pipe so if the floats stop working for some reason it will flood your engine instead. If you really want to totally stop your carburettor leaking you can block the overflow pipe off, you run a slight risk of flooding your engine when dropping your bike or parking it up at a silly angle while walking a section but we have run a bike with the overflow blocked for 5 or 6 years without problems, another bonus of blocking the overflow is the brass overflow tube that stick out of the floats can be removed making it much easier to remove the float bowls.



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