F-12 fuel line vent question

Tom Fleming · Jun 3, 2008

Can anyone tell me what the fuel line vent (part number 3359DX) does? I guess it should be evident from the name, but it just doesn't make sense to me.

I am having fuel starvation problems at times, and it only occurs after the engine has been running for awhile (approx 4-5 mins each time before it dies). I am going to totally flush out the system, and also rebuild the fuel pump and carb. the fuel tank is pretty clean from what I can see, and the sediment bowl does not have anything significant in it.

I guess I was asking about the function of the fuel line vent, and if there was any maint I should do to it.

Inquiring minds want to know, as I have spent WAY too much time at the school of hard knocks!!!!!

James Williams · Jun 3, 2008

Tom,It has someting to do when people started on gas and ran on distilite.It also lets the excess gas return to the tank,becaues the fuel pump,pumps continually and the carb can not handle all of the gas.I found out it gravity feeds to the carb.which helps it start easier.Going up a steep hill I am always afraid the gas will run back to the tank with the tractors that doesnt have them,causing them to run out of gas

Anonymous-0 · Jun 3, 2008

Tom, James is right. The fuel pump puts out more than the carb can handle. The vent chamber has two openings in the bottom. One has a Tee with the bottom line coming from the fuel pump. The other line goes to the inlet of the carb. The second opening is larger and is the return back to the intake line Tee that feeds the fuel pump. A couple of things to check for your problem; make sure the small hole in the side of the plug in the top of the vent is open. This is a vent hole to the atmosphere. As the vent chamber will not fill up completely, you don't have to worry about gas coming out of that little hole. But it should be open. Next check to see if the fuel line from the fuel pump to the vent chamber is not too close to your exhaust manifold. You might be vapor locking due to the heat. If the fuel pump and carb have never been overhauled, now might be a good time to do it. Hope this helps.

Tom Fleming · Jun 4, 2008

Thanks guys. All makes sense. It sure does act like vapor lock. The guy I bought this from also painted it, and there is a good possibility that the vent is plugged with paint.

I will start with cleaning out the vent hole, and also checking how close the fuel line is to the exhaust manifold. both of those things coupled together may be my issue. The carb and fuel pump rebuild won't hurt either.

thanks again.

Bob Kerr · Jun 4, 2008

James, The little vent tank (Cast iron that sits on top of the hood)is for distillate tractors and is not on the gas only models. It is there so if you kill the engine with kero in the lines and it cools off before you can restart it, you drain out the kero on the petcock on the carb and fill the small tank (and the line at the same time) with gas to get it going again. I run my kero model F-12 all the time in some really steep hill country sledding logs and have never had it run out of gas by going up hill. The down slope on the bottom of the tank is enough to keep it from happening. I do have a problem on mine with fuel level due to a leaky needle valve. If I fill the tank all the way it floods easy and runs rich until the fuel level goes down to just about 2/3 full and after that it runs fine. I just don't fill it all the way anymore. Someday I will get the new needle valve and seat. An F-12 will run without a fuel pump if the tank is all the way full since the top of the tank is higher than the carb.

F-12 fuel line vent question

Tom Fleming

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James Williams

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Anonymous-0

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Tom Fleming

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Bob Kerr

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