Hello all,

Riddle me this - my noggin is running out of ideas.

My bike runs perfectly well from a full tank up until about 130miles - it feels like it runs out of fuel. No matter where I put the petcock tap to - normal or reserve I get no fuel. I then walk.

I've checked the vac on number two when I was balancing the carbs - no vac issue. I've replaced the vac pipe to the petcock. I've cleaned and then replaced the petcock. I've replaced the fuel cap. I've replaced the fuel line. I've checked the carbs - no crud.

When I fill her up there is still plenty of petrol left in the tank and I can only get about 11 or 12 litres in to the brim. When I lift the fuel cap there is no hissing of air being sucked into the tank. I've also opened the fuel cap at 110miles (and re-shut it to ride) just in case there was sealed/vac tank. Tank seems pretty clean and rust free inside.

The only thing that I can logically think of left is the breathers/vents for the tank. But my noggin thinks it's unlikely as there appears not to be a vacuum in the tank when I pop the cap.

Any thoughts?

Cheers,
Glen.

PS: GK73a

You might have debris on the pick up tube. If say the lower portion of the outside is restricted then fuel will have a hard time filling the tube once it gets to that level. When its a higher level it sort of pours into it.

Is that part of the petcock? The two yellow-ish tubes sticking in to the tank?

If so, the old one that came out looked mint (as it would, having cleaned it recently) and sucked through ok. The new one equally looks fine - the gauze section being quite see through.

The only other thing that I can add, is that I can ride on reserve or main happily with less than 130miles on the trip counter. It's only once I get to 130miles that neither are able to replenish the float bowls adequately. As though the fuel pressure is unable to overcome the petcock(s) valve or the float valves.
I wondered if I lowered the idle stop whether the petcock will get more vac and that will open the petcock valve better.

Cheers,
Glen.

do you have an in-line filter that could be slowing the flow down without enough fuel to force it down?

No filter. Brand new motorbike spec 8mm id 12.5mm od fuel line. I have routed it under the carbs to get to the petcock.

that might be part of your problem. if the line is lower than the float bowls it will need help pushing back up into them via fuel weight. I always try to use gravity to my advantage when running fuel lines.

kit352 wrote:that might be part of your problem. if the line is lower than the float bowls it will need help pushing back up into them via fuel weight. I always try to use gravity to my advantage when running fuel lines.

Good shout this. I've had a 250 not revving out because of this - ie low fuel level in carb. Make sure you route the hose so it gradually and continuously lowers until the carb inlet. Use wire shielding if it runs near anything hot.

Hmm, after cycling through in my mind whether I thought this was twoddle or genius I've settled with the later.

Maybe, the fuel pipe does get a tad close to the back of the cylinder head. Add a section of pipe lower than the float bowls. I could be getting a vapour lock which does nothing good to my plight!

I'll look into it. How do we route the pipe to the petcock without going under the bowls? If I lay the pipe over the top of the diaphragm cap it gets crushed by the tank. Of all my bikes this one is worst for room and fitting things. I've still yet to work out how to tighten the centre two clips on the airbox!!!!! - I know, fit to carbs with carbs off the bike.

Cheers,
Glen.

Some bikes have a clever clip arrangement where you only have 2 screws for the 4 airbox hose clips. Ie they have a large spacer between each pair. Maybe yours are missing
Would a really long phillips screwdriver get to the middle ones?
Is the fuel hose meant to run through the middle of the airbox?
The carb inlet is also meant to be pointing up to avoid what kit was on about.

just some ideas.

Hmmm. SP airbox, SP carbs. If the line were to go up it would have to go between the two centre rampipes? Then over the top of the carb (which is tight to the airbox), across the top of carb tops 1 and 2 to the petcock?

For the life of me, I can't see the two centre clips once the airbox is on the bike. At the mo I've set them semi loose and use a smidge of silicone grease when fitting the airbox on. Then tighten clips for 1 and 4.

Cheers,
Glen.