Outboard external fuel tank vertical position

[chris-s]

We recently added a new Tohatsu 6hp outboard to our sailboat and positioned the external fuel tank in the sole locker just in front of the engine but I think it might be too low with the base of the tank about 23 inches below the fuel line connection on the engine as we are having an odd issue. All the pipework, fittings and tank are new too, and new fuel.

We hate motoring but sometimes you just have to and of course as engines do, when they decide to stop they pick the most inappropriate time like the middle of a mooring field.

After running for about 20 minutes, the revs begin to drop off and we need to reprime it, after which it picks up and runs fine again but then needs repriming every 5 to 10 minutes. It's clearly a fuel supply issue. If we don't prime it, then it will cut, but restart instantly after priming. It will tickover with no load for ever, but the problem occurs almost regardless of the revs we have been running at.

We've checked for any obvious leaks, the pipe between the filter and carb was a little and looked a bit kinked, so shortened that no difference. The hose between tank and engine was a little long, so we removed nearly 4 feet so it's now about as short as it can be to go pretty direct to the engine, <3 feet, no difference.

It's not the vent, which is remote as I removed the cap in one test. Sea state makes no difference.

So yesterday, we took a second new tank and hose setup with us to try out. Firstly the existing setup behaved exactly as before, 20 minutes in and splutter. We switched tanks with the second one being on deck, and it ran faultlessly for 40 minutes until we went from <3 knots of wind to 25 and could pull out the sails.

Next time out we will swap the tanks around to eliminate anything specific to the tank or it's pipework. But at the moment it's looking like the tank is too low, which will be a real pain to relocate it to one of the cockpit side lockers which aren't exactly spacious to start with.

Do you think it's too low?

Thanks

Chris

 

[Refueler]

If its the one under the boards ... yes its asking a bit much ... you are asking the engine to DRAW up the fuel ...

I think i your situation - I would have done as I did on the boats I had with Outboards ... Alacrity (Seagull 40+ then 5hp Yamaha) ... Snapdragon 23 (5hp Yamaha then 7.8hp Mercury Saildrive) ... I made sure I had integral tank and no need for separate tank. The reserve fuel carried in cans in the locker. The integral tanks always gave me more than enough motor time ..

The Kormoran weekender I have out here has a Jonhson 4.5 Twin with integral tank and I can get easily 1.5hrs + out of a tank.

I know you already have the engine so I think you are going to have to sort a higher fuel tank .. or assist the fuel delivery somehow ?

 

[peteK]

Sounds like a weak fuel pump,many people run it in a tub of fresh water with the tank on the ground,maybe 3ft below engine.

 

[Refueler]

Sounds like a weak fuel pump,many people run it in a tub of fresh water with the tank on the ground,maybe 3ft below engine.

Possible ... but lets be honest - how long do you run one in a tank ? He says he gets 20mins with this setup - that's at working throttle and load ... that's longer than I run an engine in a tank and certainly not at such increased throttle.

 

[neilf39]

If it runs for 20 minutes without problem then it ought to keep running. I assume so.ething in the supply system is marginal and a warm engine tips it over the edge. May be on the limit for the fuel pump. Can you raise it onto a shelf in the locker so it is just under the floor? We kept our fuel tank in the bottom of the locker on or trailer sailor and that was about 2 feet below the engine. Engine was a new 8.9 tohatsu.

 

[Stemar]

Possible ... but lets be honest - how long do you run one in a tank ?

Personally, a float chamberful. Long enough to get the salt out of the system and run the carb dry.

It certainly sounds like a height issue. One way round it that might be a bit less faff than making a new home for the tank would be to go to a car breakers and get a electric fuel pump wired into the engine output. Always assuming you have a 12v output, of course!

 

[Refueler]

If it runs for 20 minutes without problem then it ought to keep running. I assume so.ething in the supply system is marginal and a warm engine tips it over the edge. May be on the limit for the fuel pump. Can you raise it onto a shelf in the locker so it is just under the floor? We kept our fuel tank in the bottom of the locker on or trailer sailor and that was about 2 feet below the engine. Engine was a new 8.9 tohatsu.

The symptons .... at least in my view .... indicate a full prime and then engine running on the prime .. the fuel drawn up the line ... but once throttle is opened the fuel supply cannot keep up and it slowly leans out till it stops. The give away imho is that priming restores engine running and OP does not report leaks anywhere from priming - which might indicate suspect fuel line.
Warm engine or not makes no real odds in this - its the ability of the fuel system to keep the supply going ...\
Its also a 4str if its a new one ... so it will run much longer on a fuel line and prime than a 2str

 

[Graham376]

It's also an indication that the tank may not be getting enough air through the vent. Have you tried it with the filler cap off to see if the problem's still there?

 

[Refueler]

It's also an indication that the tank may not be getting enough air through the vent. Have you tried it with the filler cap off to see if the problem's still there?

He tried that ... halfway down his post ..

 

[prv]

It's also an indication that the tank may not be getting enough air through the vent. Have you tried it with the filler cap off to see if the problem's still there?

He said in the initial post that he had.

An electric boost pump does sound like a reasonable plan, provided the boat has a good way to power it. My concern would be continuing to pump fuel into a stopped engine - presumably the float valve in the carb would stop it, same as it does for the gravity tank on a smaller engine, but I don't know how tight that valve is. Obviously if the engine has a 12v outlet, which will only be live when it's running, that would be ideal. Relying on a simple on/off switch is just waiting for the day you forget to turn it off in a stressful situation.

Pete

 

[Refueler]

Personally, a float chamberful. Long enough to get the salt out of the system and run the carb dry.

It certainly sounds like a height issue. One way round it that might be a bit less faff than making a new home for the tank would be to go to a car breakers and get a electric fuel pump wired into the engine output. Always assuming you have a 12v output, of course!

I considered a lift pump assistance .. but wonder if the carb would stop overfill ..... its why I altered my original post earlier to : "or assist the fuel delivery somehow ? "

 

[prv]

Personally I’d be looking at one of those little “facet” cube pumps off eBay, rather than some bulky piece of scrap off a car. I don’t think they‘re all that powerful, which is what’s needed here - just a little assistance rather than something that’ll blow past the carb valve. I’d give it a try to see what happens when the pump is run with the engine stopped - maybe it’s fine and just needs a manual switch. Otherwise, it’s time to look at automatic detection of the engine running. Maybe something based on the same principle as those aftermarket rev counters that work with a wire wrapped round the HT lead to sense the pulses.

Backing up a bit, step zero is checking that the engine’s own pump and the vacuum tube to it is in good nick...

Pete

 

[James_Calvert]

As you know, you haven't determined for certain whether it's a tank and hose problem or an engine problem.

Next time, when you run with the new tank in the lower position, I expect it will be fine. In which case I'd suspect the old tank's hose connection at the engine end is letting in a tiny bit of air.

Edited to remove a misleading comparison with my 2 stroke engine, as Refueller points out below.

 

[chris-s]

Thanks for all the replies.

I went out to the boat this evening and moved the tank to one of the cockpit lockers and it sits a couple of inches below the deck level. I wouldn’t have been able to raise it any further where it was and I think if I make a shelf for it (currently sat on pike if fenders and mooring lines) I can manage with the loss of a little space.

The wind was too good to waste an evening motoring around so that test will have to wait for another time.

I considered an electric pump but even if wired to the engine 12v I don’t really like that idea enough just yet.

Cheers

Chris

 

[VicS]

Thanks for all the replies.
I went out to the boat this evening and moved the tank to one of the cockpit lockers and it sits a couple of inches below the deck level. I wouldn’t have been able to raise it any further where it was and I think if I make a shelf for it (currently sat on pike if fenders and mooring lines) I can manage with the loss of a little space.
The wind was too good to waste an evening motoring around so that test will have to wait for another time.

I considered an electric pump but even if wired to the engine 12v I don’t really like that idea enough just yet.

Cheers
Chris

If you are using the engine 12 volt output for battery charging it will be "live" all the time the battery isolator is on

 

[Refueler]

As you know, you haven't determined for certain whether it's a tank and hose problem or an engine problem.

Next time, when you run with the new tank in the lower position, I expect it will be fine. In which case I'd suspect the old tank's hose connection at the engine end is letting in a tiny bit of air.

If the fuel pump is OK, I really don't believe it couldn't suck fuel from the low position. Our 6hp 2stroke Mercury does its best to suck the plastic tank flat if I forget to open the tank vent.

Most 2str are using crankcase to suck up the fuel - giving it an edge over the similar sized 4stroker. TBH - you cannot compare the two.

 

[Fantasie 19]

I'm not an expert, far from it, but could it be an exhaust issue??? The engine looks to be in quite a compact/enclosed position - is the exhaust gas not able to vent properly, and after 20 minutes you get enough build up to cause an issue????

 

[James_Calvert]

Most 2str are using crankcase to suck up the fuel - giving it an edge over the similar sized 4stroker. TBH - you cannot compare the two.

Of course you are right and when I looked at my 6hp, it is one using crankcase which explains the effect.

My little 4hp Evinrude does have a pump for its remote supply though... just checked the parts list to make sure!

I'll amend my post if I can.

 

[Refueler]

I'm not an expert, far from it, but could it be an exhaust issue??? The engine looks to be in quite a compact/enclosed position - is the exhaust gas not able to vent properly, and after 20 minutes you get enough build up to cause an issue????

He already said that using a tank in higher position worked fine .........................

 

[chris-s]

I'm not an expert, far from it, but could it be an exhaust issue??? The engine looks to be in quite a compact/enclosed position - is the exhaust gas not able to vent properly, and after 20 minutes you get enough build up to cause an issue????

Not in this case, it looks more enclosed than it really is plus the exhaust exits thru the prop and repriming before it cut out revived the revs. Thanks for the thoughts tho.