3/8 or 5/16 fuel line for 4HP Merc 4 Stroke?

[olympic]

I just picked up a used 4HP Merc 4 Stroke. It has the integrated fuel tank but I'm going to plumb it into my main tank by tapping into an extra port out of my water/fuel separator. With this small motor does it matter whether I run 5/16 or 3/8 line?

Separately, I see on the side a fuel shutoff switch of two positions that appear to be on or off. Does that control fuel flow altogether or is that on/off just controlling the external fuel source? I was wondering if there was a way to switch between integrated and external fuel source.

Thanks in advance for your help.

 

[Texasmark]

I used 5/16 on my 90 hp and ⅜ on my 125 hp if that helps. Doesn't matter if it's larger than needed just things don't work when smaller. The internal fuel pump will pump what the engine needs.

 

[Chinewalker]

Actually, depending on how far the draw is and how strong the fuel pump is, I might use 1/4 inch line.

To do what you want with a valve, you need a three-way valve ? one for integral, one for off, one for external. I had an early 1980s Evinrude 4.5hp with that sort of set-up. Both lines fed into the valve, then went to the fuel pump, so the pump was drawing for either the internal or external.

 

[olympic]

Thanks for the responses. The draw will be about 4-5 feet it looks like from my fuel separator over to this outboard (will be used as a trolling motor). There is already a second outlet port on the water/fuel separator. One of those outlet ports goes to my current 110HP outboard. The other is currently capped off but my plan is to use that for this 4HP outboard.

The on/off switch I was referring to is located on the site of the outboard itself. It didn't come with an owners manual so was just curious if that on/off was for all fuel or just controlling whether the external fuel source was on/off.

A followup question though is whether the primer bulbs will work as shutoffs in the sense that if I'm trolling, do I need to shutoff the fuel line out of the water/fuel separator to my big motor, or will the bulb in that big motor line keep my small motor from trying to pull anything from that line. Basically, are shutoffs needed at the two outlet ports out of my water/fuel separator such that I only open one at a time to prevent any possible pulling of fuel (and possibly air resulting in going lean) from the other fuel line to the motor not in use? I was thinking the primer bulbs would act as that one way shutoff without the need for separate but I could be wrong on that.

 

[Texasmark]

The big motor, if off, will use fuel till the carb bowls fill and the needle valves therein shut off the flow. At that time forget the 110. The bulb could be anywhere and when pumped will feed fuel to the line(s) down stream from it till the bowls on the connected engines fill then no flow. With the 110 off, the 4 can run on the common fuel line and bulb as long as it wants. If you have a large enough fuel line to satisfy both engines, they both can run simultaneously off a common line/bulb if you want. Does that help?

 

[olympic]

The big motor, if off, will use fuel till the carb bowls fill and the needle valves therein shut off the flow. At that time forget the 110. The bulb could be anywhere and when pumped will feed fuel to the line(s) down stream from it till the bowls on the connected engines fill then no flow. With the 110 off, the 4 can run on the common fuel line and bulb as long as it wants. If you have a large enough fuel line to satisfy both engines, they both can run simultaneously off a common line/bulb if you want. Does that help?

My plan was to run a separate bulb to each...so both motors would have a line with primer bulb coming out of the water/fuel separator. If I understand you correctly, I should be okay without having to put in-line shutoffs to each of the two fuel lines individually, correct?

 

[ondarvr]

Seperate bulbs on each is all that's needed, the check valve in the bulbs will keep fuel going to the correct places. Cheap bulbs fail at times, so get good ones.

 

[olympic]

Seperate bulbs on each is all that's needed, the check valve in the bulbs will keep fuel going to the correct places. Cheap bulbs fail at times, so get good ones.

Thanks!

Something else just occurred to me. There will be times when I won't have the gas trolling motor on the boat. In those instances would a shutoff valve be the best way to handle it? I could look at putting an inline shutoff valve right after the outlet port out of the water/fuel separator in the line to the small motor. The other option is to leave the small motor fuel line in place allowing the bulb to be the check valve and just covering the connector end with a plastic bag. Thoughts? Thanks

 

[Texasmark]

Thanks!

Something else just occurred to me. There will be times when I won't have the gas trolling motor on the boat. In those instances would a shutoff valve be the best way to handle it? I could look at putting an inline shutoff valve right after the outlet port out of the water/fuel separator in the line to the small motor. The other option is to leave the small motor fuel line in place allowing the bulb to be the check valve and just covering the connector end with a plastic bag. Thoughts? Thanks

If you have a QD that doesn't leak where you attach the fuel line to the 4 hp then no biggie. Think about it. You are going boating with a small outboard. You load up your "full" gas tank, vent closed to prevent leaks, and your other gear in the back of your truck. Nice hot sunny day, hour or so to get to the fishing spot. What happens to the fuel inside? Have any fuel on the bed of your truck? Question answered.

 

[ondarvr]

Thanks!

Something else just occurred to me. There will be times when I won't have the gas trolling motor on the boat. In those instances would a shutoff valve be the best way to handle it? I could look at putting an inline shutoff valve right after the outlet port out of the water/fuel separator in the line to the small motor. The other option is to leave the small motor fuel line in place allowing the bulb to be the check valve and just covering the connector end with a plastic bag. Thoughts? Thanks

You can put one on, but it's not needed, the check valve in the squeeze bulb and the one in the fitting are all that's needed.

 

[Texasmark]

My plan was to run a separate bulb to each...so both motors would have a line with primer bulb coming out of the water/fuel separator. If I understand you correctly, I should be okay without having to put in-line shutoffs to each of the two fuel lines individually, correct?

I'd do it in a heartbeat!