Fuel line issue question

[myusernam]

Ive got a 14.5ft foot dory with a plastic tank right up the bow elevated hard up under the foredeck. That tank then goes through a omc style tank connection to 5/16 ortrec down the length of the boat to a full size racor style 10 micron water separator filter then to the primer bulb then the 50hp 2 stroke Tohatsu.
When I am really sucking the juice the motor starves. If I prime the bulb it's ok. If I bypass out the filter it's ok. It's not the motor fuel pump as the older same brand motor I had used to do this also and I ran without the filter for ages.
It's not the filter media. The filter always looks full, never not has the bowl full.
I bought a barb joiner to jump out the tank connection at the bow as I was thinking even though it works when I don't use the filter, it might just be the straw that broke the back and the issue is with the connection? (And it's wobbly and shxt)
Bulb feels normal doesn't collapse on itself. Was thinking I could change the long run to 3/8 line but it shouldn't need it? Run might be 3m long. Figured someone here will know and save me some fiddling

[Dignity]

3/8 would certainly be better, your connections would also have to be upgraded.I have a spare male connector doing nothing at the moment. You could rig up a temporary line above decks to test it out before trying to fit it all under the floor, in the short term you would get away with the much cheaper reinforced plastic hose.

[Noelm]

As the fuel line gets longer, the diameter needs to be increased, you get what is called pressure drop, the motor fuel pump is really quite weak, and a long hose, plus a filter can create problems, that said, a 50HP motor should be able to draw fuel through a filter no problems at all, are you sure it doesn't need some attention?

[myusernam]

Yeah brand new filter and motor but my old toehy did the same thing. I'll try jumpering out the tank fitting. The reason I'm asking is everything is a bit tight to get to or I would have just done it. Cheers

[Dignity]

Yep, 3m is a long way to pull fuel through 5/16 which is why I suggested you run across the top of the deck for a trial run wit 3/8 Whereabouts are you, I'm at Caloundra so water is close and 2e can do lots of testing between fishing, that is if the weather will abate.

[Triple]

What size breather on the tank? Any restrictions on it?

[Noelm]

I find it interesting that the primer doesn't suck flat, it's the last thing in the line in your setup, almost any size motor will flatten the bulb if there is a restriction, I kind of suspect something is going on!

[Fed]

I think I'd fiddle with the tank fitting first, OMC / Yamaha mismatch?

[rickmarlin62]

try the bulb next to the tank and go bigger diameter...rick

[tunaticer]

Depending on the age of the motor your problem could lay with the motor's fuel pump not getting enough negative / positive pressure. You say you pump the bulb and it works again, in doing you you increase the pressure supply to the fuel pump.
3m is a fairly long run for 5/16 hose, but i have seen 6m run to feed a 130hp with no problems before.
Check the compression on the cylinder that powers the fuel pump, if it is down the pulse will be diminished for the pump.

[Noelm]

try the bulb next to the tank and go bigger diameter...rick
Where the bulb is should make little difference, once it's primed the bulb plays no further part in the fuel line system.

[myusernam]

Thanks everyone and thanks dignity for offer but i'm 16hrs away.
Because its so hard to get to everything im going to replace the connectors and run from the tank to the filter in 3/8 in one go. Then the filter to motor in 5/16 shouldnt be issue and if it is i can get to that easier. Have bought all the gear.

[ozscott]

40hp 2 stroke and fuel from 4m away and then through big filter separator then primer bulb to motor. 5/16th but very thick wall...no probs at all at WOT. Cheers

Sent from my SM-G930F using Tapatalk

[Fed]

I remember when forum member 'Splash' bought a new Yamaha motor and plugged it into his old OMC fuel tanks, the fittings sort of fit but they don't work properly.

[Noelm]

There is quite a few fuel fittings that look the same, but, they are not, some fit but leak, some fit but don't allow fuel through, some simply don't fit, but look the same.

[myusernam]

Thanks all. I suspect it was the tank connection but as its been going on for so long i ran 3/8 to the filter and was out today with no issues.

[littlejim]

Dunno if relevant to your situation but a few years ago I had a similar situation with a 70 Evinrude supplied via rubber fuel lines.
Worked fine at medium revs but soon starved at high throttle when the carby emptied.
Eventually tracked it down to a section of the hose where a hook had gone in. Nothing visible, wouldn't leak except under really high high pressure. (That's how I finally spotted it.)
However at high revs air sucked in and caused the problem.
Replaced hose and problem went away.