Bike cutting out at high speed

[cbfb]

Got a weird problem with my bike (GSF250) that neither myself or the local bike shop can quite put a finger on.

It had been struggling at high revs for a while, most noticeably it was bogging down at around 110 (9000rpm) and wouldn't go any further, maybe up to 120 if you were lucky. Then a couple of times it just cut out completely at around 100ish as if it had run out of fuel - sputtered, slowed down and eventually cut out. Left it sitting for a few minutes then it would start up again (primed it first not sure if that made any difference).

Anyway, I checked the fuel lines, fuel tap, filter etc, the obvious stuff but made no difference. Had no bloody idea by this point so took it to the local shop - they noticed a problem with the choke sticking on, also a suspect fuel line (had gone a bit soft and they reckon it had kinked). Picked it up last night, was running heaps better, straight up to about 130... 140... wouldn't go above that (not sure what these should do, that's as fast as it's gone so far).

Thought the problem was all fixed, feeling pretty happy then it bloody dies again, exactly like it had run out of fuel (yep I did check that!). Came to a stop, left it a few minutes then back to the shop again. I had been sitting on about 130 for a couple of Ks before then... My theory is that it uses up whatever's in the carbs at this speed then runs out of gas because it can't refill the bowls quick enough - some sort of restriction to the fuel flow somewhere (which explains why it's OK at slower speeds when demand is less) - but WHY????

The bike shop are scratching their heads, they've already cleaned the carbs and they changed an o-ring in the fuel tap today but can't make it break down again, meanwhile I'm paying them to ride around on my bike! Nice work for some

Am picking it up again tonight but have a funny feeling it's not gonna be long before it dies again... So does anyone have any ideas what to look at next? I'm stumped and it sounds like the shop is too at the moment

p.s. Yeah I will get a bigger bike but got a few months till my Full licence, on 6R atm.

[Flip]

Check the fuel tank is venting properly and not forming a partial vacuum.

[ducatilover]

Bring it over, I'll thrash the fuck out of it or something.

[nzspokes]

Check the fuel tank is venting properly and not forming a partial vacuum.

I agree. My guess is fuel tank not venting also.

[maggot]

Check the fuel tank is venting properly and not forming a partial vacuum.

That'd be my guess too. Next time it happens, pop the fuel cap and see if it's difficult to open, or makes a sucking sound when you open it.

[DrunkenMistake]

Check the fuel tank is venting properly and not forming a partial vacuum.

+1

Bring it over, I'll thrash the fuck out of it or something.

yah it will be wheelie wheelie fun..

I agree. My guess is fuel tank not venting also.

Do you have a tank bag?
I have herd of them suffocating the fuel cap on some bikes that chock it

[hayd3n]

check you breather hose

[FJRider]

Check the filter ON the fuel tap (inside the tank) is not clogging up with shit in the bottom of the tank. A trail bike I once had did that ... relined the tank ... SORTED ...

[darkwolf]

I'm assuming there is no noticeable smoke when it starts to cut out (suggesting an air shortage).

Otherwise, I'd be inclined to look at an air lock in the fuel tank. Recently heard a story about the breather valve on a new triumph being installed backwards let air out not in... Very similar symptoms. Bike shop was also stumped.

[cbfb]

Well, got it back from the shop last night and done about 50k back home then to work this morning, been really good actually. Overtaking is no longer a buttock-clenching gamble as to whether the bike will cut out as you run out of road approaching a truck, zips past really confidently now.

Bring it over, I'll thrash the fuck out of it or something.

Back in the shed you, I want that hand and bike fixed by 10th Dec so we can do the coast to coast

Check the fuel tank is venting properly and not forming a partial vacuum.

I had a look at the tank cap and couldn't figure out where the vent was

The shop has apparently checked it but wouldn't mind double-checking, any ideas where I'd find the vent?

I agree. My guess is fuel tank not venting also.

As above, don't know how to check the vent is working?

That'd be my guess too. Next time it happens, pop the fuel cap and see if it's difficult to open, or makes a sucking sound when you open it.

Yep tried that, it was fine. Also tried taking the tank off the bike, then seeing if petrol came out slower/faster with the cap open/close - apparently that will tell you if there's a problem with the vent? Anyway, it was fine.

yah it will be wheelie wheelie fun..

Bwahahaha

Do you have a tank bag?
I have herd of them suffocating the fuel cap on some bikes that chock it

Nope, want one though

check you breather hose

Do you mean on the tank? There's an overflow outside the fuel cap but nothing on the inside that could allow air in/out.

Check the filter ON the fuel tap (inside the tank) is not clogging up with shit in the bottom of the tank. A trail bike I once had did that ... relined the tank ... SORTED ...

Yep, checked that... I replaced it 6 months ago when I rebuilt the top end, looks pretty good to me.

I'm assuming there is no noticeable smoke when it starts to cut out (suggesting an air shortage).

Otherwise, I'd be inclined to look at an air lock in the fuel tank. Recently heard a story about the breather valve on a new triumph being installed backwards let air out not in... Very similar symptoms. Bike shop was also stumped.

Nah no smoke, yeah I reckon the air lock sounds the problem, but would that explain why it only happens at high speed? I guess if there was only partial flow then it would cut out when demand got high?

[ducatilover]

I still think I should thrash it for you.

I'll do my best to get my hand and bike fixed, or swap the Corolla for another bike?

If it was a tank vent problem, it'd make perfect sense.
Have you raped the shit out of it in all gears?
If it's working now, just cross your fingers and hand me the keys.

[Scouse]

ride slower then it wont cut out

[Stirts]

Being the awesome knowledgeable mechanically minded person that I am .........

Where do you get your fuel? This one time, when I fuelled up at Gull, my Hornet 250 run like puss-nuts. Coughed and spluttered and cut out.

Stopped using it and bike became normal again.

[cbfb]

I still think I should thrash it for you.

I'll do my best to get my hand and bike fixed, or swap the Corolla for another bike?

If it was a tank vent problem, it'd make perfect sense.
Have you raped the shit out of it in all gears?
If it's working now, just cross your fingers and hand me the keys.

What sort of bike are you gonna get for the Corolla? If you go with that idea I'm riding infront, there will be shit falling off it all over the place

Aye, so where is the bloody tank vent? I should bring it over, unless you're in barmy Palmy today?

Yeah I bring it close to the redline in all gears on the way to work, it's been OK.

Yep will see how it goes... Yeah sure, there will be a 3 grand deposit though

ride slower then it wont cut out

[cbfb]

Being the awesome knowledgeable mechanically minded person that I am .........

Where do you get your fuel? This one time, when I fuelled up at Gull, my Hornet 250 run like puss-nuts. Coughed and spluttered and cut out.

Stopped using it and bike became normal again.

I usually get Caltex gas... Funny you should say that cause the shop filled it with BP and it's gone OK since then... maybe I should try using BP from now on... actually i boycotted BP so it will have to be Shell. Might be the way to go.