This may not be the right fourm, however I have a OS160 twin in my aircraft which has a engine cowl, thereby negating access to the carb fuel line.
I am using a Dubro tank with 3 fuel line connections.
1 line with clunk from tank to carb
2nd line with clunk to fill/empty fuel dot
3rd line is tank vent
When I fill the tank using an electric fuel pump fuel exits the vent line when the tank is full- normal.However fuel also runs down the line to the carb and floods the engine
I assume the electric pump is filling the tank faster than the fuel vent can handle and therefore over pressurising the tank and exiting fuel wherever it can
I was thinking of using a larger vent line
Any suggestion would be appreciated
Thanks

I bet the fuel is hitting the engine AFTER the tank is full. The pump pumps the fuel in, and when the tank is full the fuel needs to push up and oout of the vent, but is also pushing out of the carb line. You don't know the tank is full until the fuel squirts out of the vent line, but by that time the fuel has been presurizing a full tank for maybe a full second. I'm not really familiar with your engine, but if you close the needle valve(s) before fueling, that might stop the flow while filling up. Another thing that may help (enough) is to put the prop in a position where both intake valves are closed. That will at least keep a ton of fuel out of the engine,and the fuel in the intake tube can just be the prime for the restart. Might work, no?

I had the same problem a few years back with a Saito 90 twin in a cowled situation. I put a resistor in the circuit from the battery to the pump that slowed the pump motor to where it was pumping a lot slower than normal. Then added a switch to short out the resistor for high speed pumping. I would then use the high speed to fill the tank but as it approached full I would switch in the resistor and fill slowly till the tank was full. Solved my problem..........RJ

The best solution for me is to simply do not not use a three line system. Cowling or not I always use a two line system just as on any trainer and always fuel toward the tank in the clunk carb line therefore flooding cannot occur.


Its very simple to loop the clunk to carb line out a slot or two holes in the cowl cutting the line there and inserting a 3/8 inch length of brass or alum tube. Very easy and fewer things to go wrong. I also use it for uncowled engine with rear neecle valves to simplyfy replacing that line behind the engine.

In the topside photo of the red/black Cap you can see the fill line that is just pulled up a little and fueled from there.

This method also provide the added benefit of makine pinch tuning possible, A method much preferred by me.


John

Hi!
I have always used a 3 line system you describe but never had problems with the carb flooding!
Are you sure you fill the tank with the throttle at idle!? With the throttle at this position I have never flooded my carb!
Also be sure to use the Uni-flow tank system.

Hi Guys,
Thanks for your input. I think the problem is that the electric pump is filling the tank so fast that by the time the fuel comes out the overflow even though I switch it off quickly it manages to overpressurise the tank. I am wondering if I should just switch to a hand crank pump where I can vary the volume and the speed which the fuel enters the tank.Thanks

I used to use an electric pump, but switched to a hand crank simply because I have much more control over the whole fueling process. I also like to give it a half turn back when it is full to clear the lines and give some 'breathing space' around the vent inside the tank.

It may be not the problem but I've found that new tanks that use the 3 line system with nipples coming directly form the tank (like hayes or great planes) I mean, not the tanks that have aluminium or brass tubes for the lines, comes with the hole for the vent line so restricted that I needed to redrill it. It happened for example that sometimes pressure in the tank was so high that the line from the pump was disconected and I got a fuel bath.

Hope this helps

In addition to always using two line systems I gave up on electrical pumps years ago. The modern geared manual pump (not the six shooter type) allows any fueling rate you desire and of much more importance it allow resonable accurate accounts of your fuel usage and your predicted endurance just by counting cranks going in and out. Unless you are using tanks over say about 16 ounces then the geared manual is my preferrance.