Carbuator question

[Billman]

A friend of mine has a Honda CB-1 400ccm with keihin carburetors. Lately the fuel consumption went sky high and while the engine is shut off the cylinders fill with gasoline and it then flows back in the sump.

Q: the float chambers seem ok, what could be faulty? Can it be that the needle wears out thus not stopping the fuel from running down? The carburator position is so that they seat in a steep angle.

Any opinions?

[AndyP]

Check the little valve that the float pushes on, they can get worn and not seal properly. If this happens the fuel will continue to flow into the float chamber even when the float is at the top of it's travel. I've had this before on a set of Mikuni carbs.

[AndyP]

Also check that the floats actally float. I had one that had a split in it which let fuel soak into the honeycomb centre and stopped it floating.

[Billman]

i moved the floats up and down and i dont hear anything. they seem empty!

my thought was at the needles (and their guides) as it seems that they are the only moving part.

Lets leave the fuel pouring in away. The jets are the correct original size but the consumption is approx double the stock. What could be the cause?

[newsh]

Almost certainly the one of the float needles is not seating when the float goes up or you have a hole in the float so it will not turn off the gas when the bowl is full. Can you connect a piece of clear tube to each carb drain in turn and check the fuel level in the bowls? If the fuel level is high then the consumption will go up, this is one of the ways of adjusting fueling as well as the jet sizes. It is also possible, if the consumption has gone up gradually over a period of time, that the needle jets are worn. But check the float valves are working first, if fuel continues to flow when the engine is switched off it has to be float related.

[newsh]

The floats should be empty otherwise they won't float! There is a little tab near where the float pivots which lifts a valve to turn off the fuel.

[newsh]

I think the float valves on these have a hard rubber tip which may have been damaged, you need to take the carbs off and check them.


http://www.gadgetjq.com/keihin_carb.htm

[Billman]

the floats seem fine. He installed an aftermarket fuel pump lately thats why i think the gas leaking when the engine is off is due to residual pressure from the pump. (the original had a pressure switch that turned it off when the bowls where fuel and the pressure rise.

the fuel consumption was high even with the older original pump.

[newsh]

Fuel return pipe blocked or kinked?

[byd1380]

Are there overflows on the carbs? I had this issue on a delorto on a wee Montesa trials bike, tortured me for ages, fuel used to run eveywhere, the overflows were connected wrong, the needle valve/jet floats all seemed good.

Ivan

[Billman]

IIRC there is no fuel return. As i said the original fuel pump had some kind of a switch and when the bowls where full (hense the fuel pressure was rising) the pump stopped working. The pump was there only because the carbs are higher than the lowest part of the tank.

Lets say this motorcycle has no fuel pump what can wear out and rise consumption?

[newsh]

Hmm, I think this bike needs the correct pump or it may overfuel anyway. The float valves will not be able to hold back the pressure from a pump running continuously.

Needles willl wear the needle jets (sometimes called emulsion tubes) this happens gradually over a period of time, you can compensate a little by dropping the needles till it starts to surge on cruise then raise one notch but this is not a fix. Only real fix is to replace the needle jets. In this pic its parts called the collar and the holder, I'm guessing the collar is the part the needle slides up and down in which wears, most carbs just have a one piece item..

[Billman]

is there a way or a valve that can stop when the pressure rises above a certain level? he installed a cars fuel pump '(low pressure car pump for cars wit hcarbs) because this one costed 40 euros and the original one costed 180 euros. The original pump had some sort of spring device on it to stop the current when it started to see pressure.

The fuel consumption was high even with the old original pump that why i am asking about the needles.

[Pirvu Eugen]

Not ok! The original pump has that system wich cuts the pump when the certain pressure is reached! The bike carbs dont have the return flow. The car pump will pump too much and will overflow the carbs.
I will try to make a photo from that system.

[simon20vq]

Does it flood on all 4 cyclinders? Must be a float issue. The only way to stop it flooding if its the pump is to turn the pump off a minute or two before turning the bike off. It will then use the extra fuel rather than flooding it self.