Fuel tank distribution

guglielmo6160 · Post by **guglielmo6160** » Wed May 13, 2009 8:03 pm

I have some issues with fuel distribution on my F31
the problem is it seems that if I allow the engines to use fuel equally from both fuel tanks, it seems that one of the tanks depletes faster than the other. My tanks are situated one in front of the other, at the stern.

I guess my question is do you guys know if it is common to have dedicated fuel tanks per engine or do you share tanks. I can manually select either way, but it seems when I do that one engine starves and the other runs constantly, after a half a tank or so. If I leave it so they share both tanks, all is good, and every thing works fine. I know every engine is different and each does its own things, but it seems extreme to have that much fuel left in one tank , and the other to be empty. All filters fittings, and tubing/hoses are ok, with no issues. Im overlooking something. Today I noticed that my starboard gas fill tank was empty, Im not sure which tank it is, I didnt look, and I dont know which engine it supplies, but I do know that my port gas gauge showed 1/4 and my starboard gas gauge showed empty and, the problem was my port engine ran out of gas. Seems odd, that opposites happened. so basically I just keep all valves open and let the engines draw from which ever tank they want and all is fine, but one tank always seems to run out faster, to the tune of 1/4 tank or so. now that I totally confused you , what do you guys think?

prowlersfish · Post by **prowlersfish** » Wed May 13, 2009 8:17 pm

with all the valves open It not suprizing you would draw more out of one tank . the engines would draw more fuel out what tank has less restriction in the lines/pick up . they will never be exactly the same . also both engines will never burn the exactcly the same amount of fuel . As I am sure you know no 2 engines are exacty the same . I would never trust the gages on how much fuel you used I would go by how much you put in .

ready123 · Post by **ready123** » Wed May 13, 2009 9:48 pm

Have you checked the anti siphon valve at the top of the tank that does not run dry? They can quite easily get partially plugged with stuff. If that was happening I think you would see the symptoms you are... i.e. when the engine is pulling from that one tank it starves and so when joint it takes most from the other.
Looks like you need to find out which tank is at fault and disconnect the anti siphon from the top of it to check if it is plugged.

prowlersfish · Post by **prowlersfish** » Wed May 13, 2009 10:54 pm

How many gal. diffrince between the 2 tanks at fill up ? it there a hight diffrence between the tanks ? if its only a few gal. it very well maybe normal

Ift its a large difrence the anti siphon sure would be high on the list things to check. Good point Michael . I know I have seen these valves cause all types of proplems and is easly over looked .

kevin babineau · Post by **kevin babineau** » Wed May 13, 2009 11:08 pm

sometimes the tank vents get clogged too..what that has to do with what ur talking i dont know but i had two cents in my pocket

captainmaniac · Post by **captainmaniac** » Wed May 13, 2009 11:47 pm

One other consideration for running off separate tanks vs sharing them, is fuel contamination... If you run off separate tanks and one of them has water in the gas (maybe it's on the side of the boat that gets direct sun during the day so you get more condensation, or the weiner at the gas dock doesn't put the gas cap back on right and rain water or spray gets in) or rust issues from the tank corroding, then you only risk losing one engine. If you share the fuel, you risk knocking both engines down. It's easier to limp home on one engine than to paddle your way back!

captainmaniac · Post by **captainmaniac** » Wed May 13, 2009 11:48 pm

By the way... my port engine burns about 10% more fuel than the starboard.

mtwolf · Post by **mtwolf** » Thu May 14, 2009 8:22 am

Bill,

Your rear tank, the one at the very rear of the boat, feeds fuel to your port engine. The front one feeds your starbord engine. I had a problem, before I replaced my fuel tanks, with the port engine running out of fuel. Turns out the rear pick up tube was totally scaled shut. Of course, you can't pull the pick up tube with out pulling or moving the tanks. You could disconnect your rear pick up, at the top of the tube, and try a can of carburator cleaner sprayed down the pick up tube. May of may not clear the tube if that is the problem. Also change your fuel filters if you haven't already done so. My filter for the starbord tank was three quarters clogged after about ten hours running on the old tanks. I had to take two screw drivers and pry the filter out of the canister. Wish now I had taken a picture. Hope this helps?

John
F-31
Galloway Creek Marina
Middle River, MD

guglielmo6160 · Post by **guglielmo6160** » Thu May 14, 2009 9:00 am

thanks to all as usual very good advice,
to be honest I didnt think those valves,
Im gonna check asap
I mean its not a big deal, just trying to understand this system better, I kinda like the idea of running each engine for its dedicated tank, for the reasons mentioned here. I did that on my long trip, (about a hundred miles) and after about a few hours my starboard engine just shut down from fuel starvation , then I opened the valves to all, and it was fine, so obviously something is wacky. but as I said with both, it runs fund and never starves, so Im assuming either my gauges are not correct or I have some type of clog, but Im leaning towards some type of venting action.

gjrylands · Post by **gjrylands** » Thu May 14, 2009 9:31 am

My 69 31'er had the same set up. I normally ran with the crossover valve closed so each engine ran off its own tank. Fuel consumyion was similar. When I opened the crossover valve the syphon action would equilize the fuel level in the two tanks when the engines were off. If your valve is open and your tanks don't level out there is an opstruction in the line or you have an air leak in the lines that is breaking the syphon.

Dan Faith · Post by **Dan Faith** » Thu May 14, 2009 9:37 am

Why not just run out of all tanks? I have a friend that found out on a Skiff Craft by paying a tow. He filled both tanks but had all valves open and it used the fuel from the tank with the least restriction. When that tank got lower it just sucked air not fuel from the tank that had gas. The pump just supplied air to both engines. thought he was out of fuel and was not. As a pilot I never believe the tanks and in twin engines I never run both fuel tanks to both engines or in my single I never run drawing from both tanks, If I starve one engine I should alwas have a reserve in the other tank run on both tanks and when your out of fuel your out of fuel. Leading cause of aircraft accidents is excessive air in the fuel tanks.

guglielmo6160 · Post by **guglielmo6160** » Thu May 14, 2009 9:52 am

good info Gerry
and yes Dan I agree,
Im sure I just have one tank system with some issues and as I mentioned I have not looking into it yet, my engines run so well and all is good , with that exception so I have been overlooking it , until yesterday when I went to get some fuel, before the price soars again, and here its still under 3 bucks. I couldnt fire up my port engine clearly from lack of fuel and the port fuel gauge still showed at least a quarter tank, the starboard tank was empty but the starboard engine fired right up and ran fine,,
I was a little confused, until I filled both tanks and everything was peachy
so , clearly something is funky
I just dont like filling both tanks all the time, I usually put in what I need, my type of boating is fairly local and there are stations everywhere , so I can always get fuel
I like the cash in my wallet, not in my tank

kevin babineau · Post by **kevin babineau** » Thu May 14, 2009 10:09 am

i have an unrelated question....i have electric holley fuel pump and fuel mechanical fuel pumps...r the electric ones just to prime the system or run continuosly? with the mechanical pump i can only think the electric ones are for priming

guglielmo6160 · Post by **guglielmo6160** » Thu May 14, 2009 11:06 am

If your using the mechanical pumps, I would assume the electric ones are to prime the mechanical pump? assuming the electric pump is being used before the mechanical pump,
if its a diaphram pulse type chances are its always running,
usually its one or the other, I was going to convert the mechanical ones to the electric myself, there very easy to install and swap out in the event of a mechanical pump failure which I believe to be common,
you just have to make sure the electric pump is designed for float type carburetors and not fuel injection, otherwise your going to flood the carbs bad

kevin babineau · Post by **kevin babineau** » Thu May 14, 2009 11:25 am

ill take a pic of the set up and everyone can tell what they think thanks