What's this tube for? (carb to fuel pump)

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Rhadley

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Have a 1984 120 hp merc, in-line 4. There is a plastic tube hanging loose from the carb, running down the side of the engine. Previous owner says it was always like that, and the boat also hesitatates when accelerating from an idle. I noticed that there is an open port on the fuel pump with a piece of the same tubing material attached! Looks like it goes there.<br /><br />Could the tube cause acceleration problem, or what is that for?
 
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ufm82

Master Chief Petty Officer
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Jul 29, 2003
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Re: What's this tube for? (carb to fuel pump)

The tube is there so that when the fuel pump diaphragm breaks, the leaking fuel goes up into the carb instead of all over the bilge. Bilge area+ fuel vapors+ spark= big insurance claim and your death. That tube needs to be replaced with a new piece immediately, before you go boating again.<br /><br /> As for the hesitation, yes, the carb may need work. That tube has nothing to do with it.<br /><br />UFM82
 
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RomanRegal

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Jun 9, 2003
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Re: What's this tube for? (carb to fuel pump)

What material is the tube made from. I would like to replace mine also.<br />Thanks Roman
 
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Rhadley

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Re: What's this tube for? (carb to fuel pump)

OK-thanks for the info. By the way, how often does a boat explode? I don't recall ever hearing about it happening. To top off this boat's explosion-proness, the exhaust blower doesn't work. (I just bought it)
 
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MFG197

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Re: What's this tube for? (carb to fuel pump)

please don't be a "it happens to the other guy" guy. you're right, boats don't explode every day, i can only recall 1 or 2 in the last 10 years in Cleveland (that made the news). not every fuel vapor problem is going to be a massive fireball, but maybe enough to burn up your boat and maybe put you in the burn center. remember things happen everyday that you never hear about.<br /><br />make it right before you put the boat in the water.
 
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ndemge

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Re: What's this tube for? (carb to fuel pump)

Rhadly... I'd get that blower fixed before starting that boat! Iboats sells the blowers, very cheap.<br /><br />every once in a while I will forget to run mine after sittin in a cove or something, and I get a big lump in my stomach as soon as I realize it.<br /><br />with carburated engines, that lil flash of fire that comes up on the top of the carb if you flood it a lil bit... flame arrestor helps, that's what it's there for, but if there are vapors, it can still ignite from that, starter, alternator, relay, etc...
 
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Ralph 123

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Re: What's this tube for? (carb to fuel pump)

Here is how boats explode (up here in boston we get probably 6-12 a year due to all the boats)<br /><br />Gas vapors are heavier that air. In a boat, the settle in the lowest part of the boat - the bilge.<br /><br />The vapors just continue to build up there (unless you suck them out with a blower or forced ventilation from the boat moving)<br /><br />A stray spark ignites the vapors. The boat explodes. <br /><br />Now here is some good general instructions from boatsafe.com:<br /><br />oats built after July 31, 1980 which contain power exhaust blowers in gasoline engine compartments must have the following warning sticker near the instrument panel:<br /><br />WARNING<br />GASOLINE VAPORS CAN EXPLODE.<br />BEFORE STARTING ENGINE OPERATE BLOWER<br />FOR 4 MINUTES AND CHECK ENGINE<br />COMPARTMENT FOR GASOLINE VAPORS<br /><br />Fueling<br />Proper fueling procedures are very important in preventing onboard fires. Gasoline vapors are heavier than air and can spread rapidly into enclosed spaces. You should check the bilges and all closed compartments for gasoline vapors. The sniff test is the most effective method for detecting fuel leaks.<br /><br />Fueling Precautions<br /><br />1. Secure boat to the dock.<br />2. Switch off engine(s). <br />3. Extinguish all open flames. <br />4. Do not use electrical switches. <br />5. No smoking. <br />6. Ports, hatches, and doors closed. <br />7. Portable tanks should be refueled ashore. <br />8. Make certain all passengers are ashore. <br />9. Determine quantity of fuel required. <br />10. Hold hose nozzle firmly against fill pipe opening. <br />11. Do not overfill. <br />12. Wipe up all spillage. <br />13. Open ports, hatches, and doors to ventilate. <br />14. Turn blower on for four minutes minimum. <br />15. Do the sniff test. <br />16. Start engines(s). <br />17. Re-board Passengers. <br />18. Untie from dock and cast off.<br /><br />And remember, fundamentally, anything electrical, (esp motors), can and do spark. I can walk across a rug on a dry day and build up enough charge to cause a spark....
 
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Rhadley

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Re: What's this tube for? (carb to fuel pump)

Don't worry, I do not intend to become a statistic.<br /><br />But as to the affect of an open tube coming from the carb--wouldn't that decrease the vaccuum pull? I could see that affecting fuel flow.<br /><br />What do you guys think?
 
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pecothern

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Re: What's this tube for? (carb to fuel pump)

I'm thinking that the tube is hooked into the fuel bowl and if the tube was off it would only act as a vent to the fuel bowl and that wouldn't affect engine performance. However you can test that theory by starting it up and putting your thumb over the port and check for vaccum
 
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pecothern

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Re: What's this tube for? (carb to fuel pump)

I would say the most common cause of hesitation from idle would be faulty accelerator pump on your carb look down the throat of the carb when you pump the throttle and you should see a good strong stream of gas spraying hard down the throat into the manifold. If you don't see that there is your problem. Also check your timing if it is too far retarded it will hesitate as well.
 
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Rhadley

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Re: What's this tube for? (carb to fuel pump)

Will do, 1 of 3. Appreciate it.<br /><br />What are the other 2?
 
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akriverrat

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Re: What's this tube for? (carb to fuel pump)

the tube connection at the pump is on the side of the diaphram in a car would see atmospheric pressure so the tube being off effects the pump operation in no way. the other end of the tube is often attached to the spark arrestor, other times to a vacumn port. either place, if the pump diaphram is intact, sees a static load and once again doesn't effect pump or engine operation. if the tube is disconnected from a vac port the engine is drawing in extra air, leaning out the mixture. so it could cause problems with the way the engine runs, particularly at low rpm. the sole purpose of that line is to send any fuel that leaks through the primary diaphram safely into the induction system and not the bilge or crankcase.
 
oldboat1

oldboat1

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Apr 3, 2002
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9,612
Re: What's this tube for? (carb to fuel pump)

think I would replace the plastic line with rubber fuel-line tubing. You might find that there is a little less noise with a line connected. The pulsating air sounds like ticking (sound of a tappet) -- at least mine was that way.<br /><br />I use the blower on my inboard, but I have to admit being impatient (4 minutes is a long time). The old smell test is a good safety precaution, and lifting the engine cover to make the test also airs out the compartment -- not a bad thing, and a good idea to leave the compartment open when starting.
 
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