Odd fuel delivery problem, clues provided...

[Oakie]

My bike has had a variety of fuel problems over the past 32800kms, ... and now it has come up with a new variation to puzzle and confound me. (I'm sure it's a female bike)
Symptom. Starts fine and I go happily toodling down the road until about a kilometre later when it will from time to time slowly die pretty much as if I had not turned the fuel tap on. If I wait a couple of minutes at the side of the road then hit the start button it'll fire up pretty quickly then proceed with no more problems. It is as if that time allows fuel to flow from the tank into the carbies but to the best of my understanding that shouldn't happen as it has a vacuum hose which means fuel shouldn't flow unless the engine is running. Interestingly, the problem seems worse when the bike has been out in the hot sun (think ... petrol expanding in the heat).
Other factors:
>My mechanic noticed that there is a tiny amount of fuel weeping around the fuel tap. He was actually fixing a different problem so didn't have time to look closely at it.
>I've noticed a couple of times that expanded air from the tank is escaping from the key hole of the petrol cap whereas I thought there would have been some overflow tube that vapours would exit from more easily. (Blocked?)
>That vacuum hose is quite worn around where the clip thats secures it to the engine sits. (Leaking?)

If anyone can add all these things together and come up with a reason for my bike misbehaving, I'd really appreciate it as I have 13 days off work to fix it and not much to occupy my time with (eg, I sorted our CD collection into alphabetical order this morning).

[nallac]

i would say its your vent hose/s ,sounds like it could be vapour locking?.

No idea what they are like on your bike on mine its got a vented gas cap.

you could always try riding it with the gas cap loose to see if it helps it
/.

[dino3310]

its a bit different than my bike so it got me buggard but with 13days off
you could strip her down and give it a good going over,good luck with it ,will be interesting to see what the prob is.

[dino3310]

mines got a diaphram in the fuel tap that works of a vacum,if its weeping gas it could be a buggard diaphram

[Ixion]

Well, it's very easy to test. The tap will have a "prime" position. That bypasses the diaphragm . Turn to prime , ride off , see if problem occurs. If yes, it's not the tap. No, it is. Nasty perverse things those damn taps. What was wrong with the good old sort wiv a bit of cork in a hole.

[homer]

I may be guessing but id say the float level is out of sync
I had a bike did this and it would then drip fuel from the carb hose on to the road
while it had some revs on it was fine till left at idle

[Oakie]

Well, it's very easy to test. The tap will have a "prime" position. That bypasses the diaphragm . Turn to prime , ride off , see if problem occurs. If yes, it's not the tap. No, it is. Nasty perverse things those damn taps. What was wrong with the good old sort wiv a bit of cork in a hole.

Nah. Just good old 'On', 'Off' and 'Reserve'
Hmm. Cork. I just opened a bottle of bubbly 40 minutes ago and the cork is out on the street somewhere....

[Oakie]

mines got a diaphram in the fuel tap that works of a vacum,if its weeping gas it could be a buggard diaphram

It seems to weep above the tap so I'm guessing it's where it connects to the tank rather than the tap leaking itself. It doesn't always weep though so I keep coming back to pressure somewhere screwing things up. (ie, pressure somewhere forces it to weep when the pressure is great enough)

[sAsLEX]

The vent in my tank was well clogged up so may pay to check that since yours looks a similar vintage

[Oakie]

Clogged vent? I certainly reckon that could be part of the problem.

We went out for a 30km blast to shake down the Xmas dinner and the bike didn't miss a beat. It had been parked in the garage in the shade. I'll leave it in the sun for a while tomorrow (if the sun is out) and go for a ride and we'll see what happens.

Of course it may have behaved today because it was threatened with replacement last night and I even looked at new bikes on the net. (Brand new Suzuki GS500F K7 for $7700. Not a particularly exciting bike ... but then I'm not a particularly exciting rider!)

[Ixion]

Nah. Just good old 'On', 'Off' and 'Reserve'
Hmm. Cork. I just opened a bottle of bubbly 40 minutes ago and the cork is out on the street somewhere....

If it;s On, Off, reserve, then it's not a diaphragm type. Look elsewhere. Does the tap have a filter bowl? If so have you cleaned it ?

[Oakie]

If it;s On, Off, reserve, then it's not a diaphragm type. Look elsewhere. Does the tap have a filter bowl? If so have you cleaned it ?

No filter in the bowl but there is a filter in the line further down. It was checked a month ago as part of it's regular service. I will have another look though.

[Max Preload]

Interestingly, the problem seems worse when the bike has been out in the hot sun (think ... petrol expanding in the heat).

More the air above the petrol expanding, which should be expelled through the vent. But either way, any increased pressure in the tank won't prevent the flow of fuel - it will assist it.

[awayatc]

More the air above the petrol expanding, which should be expelled through the vent. But either way, any increased pressure in the tank won't prevent the flow of fuel - it will assist it.

Yes but heat will evaporate fuel in carbs.... so it starves if it doesn't fill up again....
It should refill automaticly till float shuts it off again.
Also if it is supposed to work on vacuum, it may just be a sad/ blocked or porous hose..
Otherwise check your floats, they may "stick" ..clean your carbs and replace old hoses. That should take a lot less then 13 days.....
Good luck

[Max Preload]

Yes but heat will evaporate fuel in carbs.... so it starves if it doesn't fill up again....
It should refill automaticly till float shuts it off again.

Not on a bike moving through the air - the airflow alone is plenty enough to stop the petrol boiling or evaporating off faster than it's filling unless the fuel tap strainer is partially blocked, the vacuum operated diaphragm is not being actuated due to lack of vacuum or the fuel hose itself is kinked or clamped due to incorrect routing.

I suspect it's not a vacuum operated fuel petcock anyway and that's really the first thing that needs to be established.