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fuel tank conundrum


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i have a tinny, fuel tank permanently fitted @40l, i rarely use more than 20 in a day.

last week i did a couple of engine hours, i put about 5l in earlier in the week, and 10 more last night, it appeared full with fuel coming up the clear filler hose.

today i did a couple of hours running around downrigging idling etc, i would have estimated 15 litres used 20 max. just near the spit on the way in, she starts running poorly the conks out.. fuel primer bulb is soft. So , i unstrapped the tank, jammed a surfer joe under the other end to get all the fuel to the pick up end, and just make it back to to ramp. again the motor is struggling.

So, on the way home into the servo and guess what only 10 litres goes in???

whats gone wrong???

happy to hear ideas theories or anyones experience.

mother of all airlocks?

blockage in a fuel line?

ps, middle harbour is chocolate,the harbour is dirtier than a week ago. squid are fussy, and scored a 70cm king on a dead squid head while the live yakkas went untouched.

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It is normal for the primer bulb to be soft once the motor is running . When the problem occurred , did u squeeze the bulb , did it go hard ??. What was the effect on the engine performance , ran ok for a while or made no difference

Whilst it may be the primer bulb , if the valve fails it does not prime. Very rarely does the valve close especially when the motor is running.

You don't mention if there is a fuel filter fitted. Most motors have two , one in the fuel line between the tank & the motor & another one under the cowling just next to the fuel pump . These should be checked , cleaned , and , or replaced first .

The other item to check is water in tank or the filler bowl next to the fuel petrol pump

Prior to spending yr $$$$ , by pass the primer bulb. If the motor runs ok then fit a new bulb


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It's done 50 hours since the last service,

Don't think the fuel pump has ever been overhauled.

The fuel filter is not clear so I'm unable to tell what's in it.

When I ran it after the breakdown I had the boat planing and everything seemed OK before it went bad again

I've run it at home and seemed OK

Obviously not though.

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I read above that you have a breather hose fitted -- But have you checked the length of the breather - is it kinked or blocked anywhere?? Replace it if you are in doubt.

I had an intermittent problem where my motor would not rev up from troll speed - when I applied throttle it would tend to die down and splutter like running our of fuel - when that happened I had to try and "goose" the throttle controls and eventually she would rev up... Long story short, after several good mechanics had already gone over and given a thumbs up to the motor, pump and filters I was about to accept this as a "feature" of the motor but then by chance while working on another bit I had my neck in close to where the hoses connected with the tank and notices that the breather was kinked. Changed the breather hose and the problem has not reappeared since. So my conclusion was if everything was flat and level - vapour would get thru the kink and the tank would breathe - hence most of the time everything was fine, but if some fuel had pushed up past the kink (like when you troll through swell) this trapped fuel would restrict the tank breathing and hence added restriction on sucking fuel when trying to rev up.

You also mentioned that when you fill up "it appeared full with fuel coming up the clear filler hose" - was there fuel coming out of your breather?? If not, then again this is pointing to a breather problem.

Edited by zmk1962
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