Bike coughing, then dying when tank gets to about a third.

[Suntoucher]

As the title states, really can't get my head around the problem, basically once I get to about a third tank whilst riding 80km+ in strong winds/uphill (or both) the bike will start coughing and slowing til I'm ringing it out, then eventually die.

Pull over to the side of the road, wait a minute or so, occasionally hitting the starter will see it spring back to life as if nothing happened, if the winds are still strong or still going uphill it will wash and repeat til I'm going under 80 in which it's never a problem.

It's fantastic on the motorway going back to Browns Bay from the City with a couple decent hills at Upper Harbour highway etc. Maybe 2-3 times I'll be sitting on the side of the road for a minute.

But if I take East Coast Road she runs like a dream.

My thoughts say an issue in the fuel line since it never has a problem when there's more than a third tank, but it's inexplicable where the problem would be as I would think until it's down to dregs and rolling around the bottom of the tank the fuel pressure would stay pretty constant into the pump.

Vacuum stopping air into the tank? Any advice would be perfect, quick fix would be even better.

Stops to the petrol station confirms it's around the third tank mark as I can never squeeze more than ten litres into it when I finally give in and fill it up and they have a 14L tank(2 reserve but switching it to reserve doesn't change anything).

[Snails pace]

I would suggest muck / water in the fuel system. If there is a fuel filter clean or replace. Drain the tank and refill. If the bike has a carburetor you should drain the bowl. I have read that if you have water in the tank to do the following.
"1/2 cup of methylated spirits added to a fresh tank of fuel will bind the water and fuel to get pumped through together. The bike runs crappy and jerky for a while, and then all should clear with the next load of fresh fuel."
I have not needed to try this myself so do not have any experience if it works or it it is detrimental to the bike. Doubt that it would be. I have been told that if you have a metal fuel tank and do not ride regularly, it is best to keep the tank full when not using, as there is less chance of condensation forming in the tank. More a winter problem than a summer I suspect.

[The Stranger]

Vacuum stopping air into the tank? Any advice would be perfect, quick fix would be even better.

Stops to the petrol station confirms it's around the third tank mark as I can never squeeze more than ten litres into it when I finally give in and fill it up and they have a 14L tank(2 reserve but switching it to reserve doesn't change anything).

Were it the vacuum issue wouldn't it be less evident with more air space in the tank?

Had a bike behave similar and it was a fuel pump issue. A previous owner had removed the fuel pump and simply used gravity feed. It was sweet until about a third of a tank too.

But the other one to consider surely is the petcock.

[Bounce001]

Had a similar problem with my slugmaster. Turned out the breather hose had been crimped under the tank after it was put back on after a service. Was hardcase undoing the petrol cap while riding and tightening it again to keep going!

[MSTRS]

I too predict a problem with the fuel tap on the tank. Do you have a reserve setting? If so, I'd guess that'd be worse still, due to more blockage at the lower level of the intake.

[marty]

Or, the reserve tap is somewhat comprimised, only allowing a limited amount of fuel through when on reserve.

Put it on reserve when you have a full tank, and see what it does.

[Suntoucher]

No issues when it's on reserve on a full tank. Can't replicate it at all on a full tank so will try number one first as it sounds like it could be it. Will also check all the breathing hoses again but have checked them several times over already.

Fuel pump is still there, thinking next step will be to drain the tank and pull the petcock and pump off.

[MSTRS]

Your bike has a fuel pump? Most carbed bikes rely on gravity feed, often with the tap being opened by means of a vacuum tube from the inlet manifold.

[Suntoucher]

Says fuel pump on it but that sounds more likely as it isn't mechanically controlled and has a tube coming from the manifold.

[MSTRS]

hmmm...someone who knows more than me will be along shortly.

[kiwi cowboy]

Your bike has a fuel pump? Most carbed bikes rely on gravity feed, often with the tap being opened by means of a vacuum tube from the inlet manifold.

FZ 750 had carbs and a pump.
Not sure on other bikes.

[MSTRS]

Yep. FZRs had one too. As did some of the Hondas. I did say 'most do not' and Hyosungs would surely be unlikely to, given they are a budget bike.
OP did say there seemed to be a vacuum hose on his, which would tend to imply gravity feed once the tap is open.