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Hi All,

Just trying to get my 1/8th scale hydro going with k&B67.

I am having trouble with small bubbles in the fuel line. I have taken out the fuel filter and replaced the original tank with a new one. I have also replaced all the fuel line with new line.

I have also noted that when the engine stops the fuel seems to track backwards from the carb. You can also see fuel back tracking along the line towards the tune pipe from the fuel tank.

Does anyone have any suggestions as will try anything? 🙁

Hi Matt,
A few things to think about here:
Check that there is not excessive vibration of the tank due to hard motor or pipe mounts.

Check that there are no air leaks / loss of pressure to the fuel system.

Make sure that the pressure feed into the tank bleeds into an air cavity at the highest point of the tank, not blowing into the fuel.

Try to minimise the length of the fuel line from tank to carb.

Check the needle in the carb is shutting off completely, ie screw it all the way in and try to blow thru the fuel feed fitting, nothing should get thru.

make sure the O rings on the needle are in good nick and sealing properly, air can suck past these if they are even slightly damaged ( I run a peice of large silicone fuel line over the outcside of the needle to help seal it off too)

Check there are no pin holes in the fuel line.

Make sure the filter ( 2 piece ? ) is done up tight and not leaking.

Try this stuff for starters !

Cheers, TUI.

Hi Matt

You do not say how far through a run you have got or how much fuel you have used (how much left in tank) or whether the boat stops on the water. It is assumed you are running a plastic tank with a clunk in it.

Reason for asking is I have a smallish problem with my OS81VRM Kiwi 3 hydro. During some racing a few weeks back all would be going well till about the final lap which was probably about lap 7 of running including the mill laps. Symptom was like it was just running out of fuel having just exited the right hand turn onto the front straight and it happened several times, sometimes I would complete the race or stop on the last lap. The tank had about 1/3 of 240 ml remaining so it wasn't emptying it.

So why?

I am running a plastic clunk tank and I am thinking that the centrifugal force on the corner is forcing the fuel to the outside or even forwards (as the boat decelerates in the turn as rudder is applied) for about the three seconds it takes to make the turn and the clunk may not be exactly following the fuel as it should or it may be bouncing around too (especially if the boat bounces when turning) and also it may that the silicone tube to the clunk is too stiff such that it does not sit properly on the bottom. This reminds me of a model aeroplane I fly, at a certain time into a flight doing fairly agressive pylon type turns the motor will momentarily cough 1 to 2 seconds out of the turn as the air bubble passes through the carb, that is my warning that the fuel is getting low (less than 1/3) and it is time to think about landing. Maybe same deal with my hydro except the turn takes longer and the bubble is big enough for it to stop.

My solution as yet untried, upsize the tank to 360 ml so I can finish races, fit a more flexible line to the clunk or maybe even bend the metal tube inside the tank to lightly force the clunk to the bottom of the tank so that it spends more time immersed in fuel.

As Steve says vent pipe should be into air space, this is very important.

Hope this helps and if it does perhaps you could write a wee spiel for Propshaft, hint hint.

Peter 23 😎

PS. Pet hate of mine is people who don't run in line fuel filters, their flawed logic is that they filter the fuel going in so they don't need one. Even if the tank is completely clear to begin with the exhaust pressure line sends its own grap into the tank. Eventually some of it finds its way to the needle valve and motors start stopping and futile conversations about the merits or not of fuel filters begin.

Interesting thought doing away with the clunk, it is not an aircraft so a clunk pickup is not really required and may be a disadvantage if it is bouncing around especially when the fuel gets down. I have run metal tanks for years on all types of boat and the pick up point has been at the rear on the side that fuel gets pushed to in the corners and those tanks pretty much drain to the last few drops. Out comes the clunk.

I use du-bro final filters, they have considerable area of gauze to catch dirt.

Yes I think I have one in my C hydro, was originally called a 'crap trap' no idea who the manufacturer is/was but it is as you describe. I bought it in the 1980's and never used it much for years it just sat in a box till now, seems to work fine and you can see if it has dirt in it. I think I have seen one in Nigel's boat too.

The two cup shaped filters are of different grades, fuel goes through the 'coarse' one then the 'fine' one, the convex surface faces the incoming fuel. Cleaning is by backflushing.

Peter