Fuel starvation at high speed

[Wilson]

G'day,

I'm having a fuel starvation issue I'm hoping someone might be able to shed some light on.
I've been off the bike for a bit after a bit of an accident. In that time I thought I'd tidy a few things up, so I had the tank off and was tinkering (as much as my efforts can even be called tinkering) with things, but did nothing of note.

When I fired the bike back up, she seemed to run with no dramas, and I rode around for a couple of days. Then I took it to a track day, and on the way there was my first extended freeway run. 100km/h or so, and after a while (say... 10km), the exhaust note changed to a drone (the "I'm out of fuel" noise) and it started losing power. Still ran for a while, but eventually I had to pull over and she conked out. It did the same thing for most of the day. A mate had pinched my fuel filter when I had the tank off. He gave me a replacement, but we had to dig up a new bit of hose, and it turns out the hose was too long and was kinking. Problem solved?

No. It seemed to run fine and I rode around for a few more weeks. But then my next freeway run. Same thing. Exhaust switches to a drone and it starts losing power. After this point, the problem got worse and worse. It'd run for shorter periods of time, and would start losing power even if I hadn't just twisted the throttle hard.

We tried to diagnose
- Fuel hose is short, straight, and has no kinks
- Fuel tap appeared to be working (sucking on vac hose caused fuel to flow)
- Vacuum appeared to be working (disconnected tank and there was a pulsing vac from the cylinders)
- The problem was the breather. The bottom bit of hose was blocked. We cut the hose and found... a bug had crawled up there and nested/coocooned/laid eggs. Must've figured it was a nice, warm dark place, now there wasn't a constant source of fuel vapours. It had totally jammed the hose closed. We replaced the hose and the problem appeared to be sorted.

Except that it wasn't. This morning was the first freeway run since then. Again, same problem. It didn't take long on the freeway, either. Only a couple of km. Drone, loss of power... didn't quite stall this time, but that's probably because I reached some traffic lights before it did. I'm at a loss. I've checked everything I can think of. Any advice on what else I should be looking at?


I did search and came across this:
viewtopic.php?f=24&t=16935&p=118525&hil ... on#p118525

I've had a few troubles over 80/90mph where it pretty much stops accelerating and then dies in power until I have to pull over and rev it a bit. Is this that problem??

Could be the same issue - fuel starvation as the tube probably perished and finally split/snapped.

It sounds similar... but I suppose I didn't have a really good look at the condition of the hose. I just checked for vacuum and was satisfied that it was working. Is it possible that the hose could work fine at city speeds and then leak under heavier load if it's got a small split?

The problem only occurred after the drop (low speed highside that didn't appear to damage much (apart from my wrist)) and the bike had worked without hiccup previously.

[Cammo]

It's obviously related to a fuel delivery issue, there's a few more things you could look at.

From your description that it's running out of fuel at higher revs/loads (highway speed) it could be that the carb float bowls are not supplying enough fuel for some reason.

- Take the carbs off and check the condition of all floats (no holes etc), that the float height is set correctly and that the float spring is functioning properly. The haynes manual covers most of this.

- Check that the filter tube inside the tank isn't blocked (unlikely)

- Check that the vaccum line has absolutely no very small holes in it - if in doubt replace it.

- Check that the fuel filter inside the fuel tap isn't blocked


It might also be an idea to determine whether the problem is related to engine load/speed (fuel delivery) or perhaps engine temp (usually kinked hoses).

Let us know what you find!

[Wilson]

Cheers for the tips. Carbies do not live in my happy place (my CBR is currently in a million pieces because I thought I'd have a look at fixing my carbs myself, pulled it apart, went "OMG WTF", and kinda left it all on the bench, went inside for a beer and never came out again). So that'll be the last resort. At least it's good to know that's one more place to check if I can't get it running. I found your carby guide (nicely written. A big thank you for that), so I'll head there if/when the time comes.

Fuel filter in the tank looked okay.

I think I will replace the vac hose just to be safe (thanks for the tip).

I suspect it's demand/rev/speed related more than temperature, as it's previously played up fairly early in the game, and this morning it didn't have a great deal of running time before I hit the freeway. I'll keep an eye on that to try and eliminate it for sure though.

[superlite]

We cut the hose and found... a bug had crawled up there and nested/coocooned/laid eggs. Must've figured it was a nice, warm dark place

Been there! It must be an Australian thing!

Sounds like your vacuum hose to me - and yes a perished hose will behave badly at higher speeds when there's more of a vacuum. Could be a few other things, but that'd be my first point of call - and it's an easy cheap fix to try first.

I do believe that Cammo called me from the Hume Highway with a similar issue once...........

[CMSMJ1]

The hose also is prone to collapsing on itself even if no splits when it is hot enough and the vacuum is large enough - so, in Oz and high speeds...

We have some sun over here today btw - was frosty overnight but might hit 10 degrees today...check us out

[superlite]

We have some sun over here today btw - was frosty overnight but might hit 10 degrees today...check us out

Nothing to envy here at the moment! We're going for 0 overnight with a top of 14 in my neck of the woods! So, not much difference besides the fact that it's autumn here and spring in the UK Cold weather = tinkering time!

[Old Painless]

As above float height, vacuum line and tank breather seem likely. Try opening the fuel tank lid when it starts and see if it fixes it. You may get a pssshhhhh noise as the pressure in the tank releases when you open it if its blocked.

[skinnydog0_0]

Another possibility is there is some crap in the fuel tap stopping the diaphram opening fully at high speed. Might be worth a look.

[Neosophist]

Vaccumn pipe might be collapsing in on itself with the suck of the high RPM.

Had a bike do that, replaced the vaccumn pipe and it fixed the problem.

[Wilson]

Thanks for all the input. Late nights at work mean it's difficult to get to it till the weekend (though last night was spent replacing a clutch cable by torchlight... it needed it really badly (fraying and stretching... shifting gears was becoming difficult ). Riding with a new clutch cable is joy ). But I'll definitely try all these things.

I did fill up last night and noticed the bike seemed a bit more lively again. My suspicion is that I've pinched the tank breather when I put everything back together, so the next time the problem manifests, I'll pull the breather hose off and see if that fixes it.

The hose also is prone to collapsing on itself even if no splits when it is hot enough and the vacuum is large enough - so, in Oz and high speeds...

We have some sun over here today btw - was frosty overnight but might hit 10 degrees today...check us out

Winter time here atm, so this is less likely than otherwise. I think we're hitting about 10 degrees as well... but in the other direction. Don't be too jealous. My last track day had 41 degrees ambient and probably closer to 50 on the track!