Connecting remote fuel tank to outboard

[Bertolt55]

I'm trying to connect a remote fuel tank to a 4HP 2 stroke Mercury outboard (about 30 years old - the outboard, not me). I get how to plug everything in, but how do you get the outboard to use the remote tank and not its internal one? Is there a switch somewhere, or does just connecting the remote tank prioritise that over the internal one? There is an on/off switch for the internal fuel line. Thanks for any help.

 

[Bilgediver]

I'm trying to connect a remote fuel tank to a 4HP 2 stroke Mercury outboard (about 30 years old - the outboard, not me). I get how to plug everything in, but how do you get the outboard to use the remote tank and not its internal one? Is there a switch somewhere, or does just connecting the remote tank prioritise that over the internal one? There is an on/off switch for the internal fuel line. Thanks for any help.

Your outboard may rely on gravity to get the fuel from the existing tank if fitted in the usual place. Does it have a means of drawing fuel from a tank mounted on the deck? We need more info.

 

[dehlerdave]

My Yamaha has a three position fuel cock, off, internal tank and external tank. There is an internal fuel pump which sucks from whichever tank is selected. Yours may be similar.

 

[crewman]

My outboard is the same, fuel tap has three positions, off to the left, internal tank at centre and remote tank at right position. Not sure if there is a fuel pump, the internal tank is certainly gravity feed, but I think when on a remote tank the fuel is sucked up by the carb suction (Never used it so do not know)

 

[ashtead]

Agreed - I have used a remote tank for first time this year on a 6hp Yam and has a switch as described but you do need to pump the fuel up with the rubber bulb in the fuel line. That said I don’t recall my 3.3 2 stroke merc had a similar switch however I never tried it if it did. Many a google search for owners manual might be place to look to see if it is possible to use a remote tank on a small Merc?

 

[andsarkit]

I don't know which model you have but it could just have the 2 fuel lines connected by a tee.

There are more diagrams on this site. Mercury 4HP

 

[VicS]

I'm trying to connect a remote fuel tank to a 4HP 2 stroke Mercury outboard (about 30 years old - the outboard, not me). I get how to plug everything in, but how do you get the outboard to use the remote tank and not its internal one? Is there a switch somewhere, or does just connecting the remote tank prioritise that over the internal one? There is an on/off switch for the internal fuel line. Thanks for any help.

Just connect the remote tank leaving the fuel tap, which controls the outlet from the internal tank, shut.

As said above you will need to prime the system with the fuel line bulb but once running the fuel pump will continue to pump fuel from the remote tank.

Dont forget to open the vent on the remote tank

You might be able to download an owners manual from the Brunswick Marine in EMEA download center but for a 30 yo motor I'm not confident.

 

[DownWest]

My outboard is the same, fuel tap has three positions, off to the left, internal tank at centre and remote tank at right position. Not sure if there is a fuel pump, the internal tank is certainly gravity feed, but I think when on a remote tank the fuel is sucked up by the carb suction (Never used it so do not know)

Carbs cannot suck fuel. Either gravity or a pump.

 

[Bertolt55]

Thanks folks for all the replies. I will have a look at the engine when next on the boat, but I think VicS is right - I just have to put the fuel switch to off and the engine will be supplied from the remote tank.

 

[jwilson]

Carbs do generate some suction once an engine is running from low pressure caused by the accelerated airflow though the restriction of the carb body. A Swiss gent called Bernouilli discovered this bit of physics about 250+ years ago.

Whether it is enough to suck up fuel from a remote tank I don't know, but quite possibly if the tank is not too much lowwer than the carb.

 

[DownWest]

Carbs do generate some suction once an engine is running from low pressure caused by the accelerated airflow though the restriction of the carb body. A Swiss gent called Bernouilli discovered this bit of physics about 250+ years ago.

Whether it is enough to suck up fuel from a remote tank I don't know, but quite possibly if the tank is not too much lowwer than the carb.

You are talking about the venturi and jet metering . Float chamber is a bit different.

 

[VicS]

Carbs do generate some suction once an engine is running from low pressure caused by the accelerated airflow though the restriction of the carb body. A Swiss gent called Bernouilli discovered this bit of physics about 250+ years ago.

Whether it is enough to suck up fuel from a remote tank I don't know, but quite possibly if the tank is not too much lowwer than the carb.

ITYWF the carb incorporates a fuel pump.

 

[jwilson]

You are talking about the venturi and jet metering . Float chamber is a bit different.

There is a path between venturi and float chamber. It's how the fuel gets up from the float chamber to the jet outlet and into the cylinder. I don't know if the suction is enough to pull up more petrol from a tank, but it could be. I've had lots of things go wrong with small two-strokes in my time but never met a lift pump in anything under 6 hp. Or maybe I just never spotted it....... Now I want to know from someone who has one!

 

[DownWest]

There is a path between venturi and float chamber. It's how the fuel gets up from the float chamber to the jet outlet and into the cylinder. I don't know if the suction is enough to pull up more petrol from a tank, but it could be. I've had lots of things go wrong with small two-strokes in my time but never met a lift pump in anything under 6 hp. Or maybe I just never spotted it....... Now I want to know from someone who has one!

Yep, the fuel has to get from the float chamber to the venturi, but the float chamber itself is usually vented to a pressure neutral area. Use to be straight out, as in Seagulls, where you flooded it until it dripped out of the hole in the side. before starting. Now, with pollution in mind, it is done internally. The float valve controls the level in the chamber. Introducing pressure differences would make things tricky.
BTW, I used a 5hp recently that had a pump for the remote, optional, tank
And, we have an ancient Evenrude 4hp Twin 2T That, apart from no gears, only has an a remote tank. It does have a pump. Dates from the 60s Bought new by my US side.

 

[Mark-1]

Thanks folks for all the replies. I will have a look at the engine when next on the boat, but I think VicS is right - I just have to put the fuel switch to off and the engine will be supplied from the remote tank.

Vic is right and if you take the cover off it's pretty obvious. With tap off it takes fuel from the pipe to the external fuel 'plug' which is sealed by it's own valve when no tank is connected.

 

[scozzy]

for what its worth i also have a mercury 4hp 2 stroke of the same vintage and i removed the external tank fuel plug thingy for this season.For me it was just another place for a leak/s to frustrate me (another story)! I just have the main fuel line and carry a colour coded set of 3 5 lt jerry cans in c'pit locker depending on whether they are in play/mixed or need to be mixed etc.No more leaks and im prepared to top up on the go if im on the motor for a while rather than depending on those crappy little clamps to stop my fuel slowly leaking out onto either sailing club storage bay or the big blue underway...

 

[Caraway]

If you use wormdrive (Jubillee) clips you will probably get leaks.

If your clamps don't look like this, you will probably get leaks.

This sort can work well (they do on cars) but depend on pipe tail size, pipe wall thickness etc.

 

[scozzy]

If you use wormdrive (Jubillee) clips you will probably get leaks.

If your clamps don't look like this, you will probably get leaks.

This sort can work well (they do on cars) but depend on pipe tail size, pipe wall thickness etc.

Cheers for the feedback? but for this season at least I'm working on the least possible points of failure model! I don't go very far in the grand scheme of things so a spare Jerry is usually enough as a back up to internal tank