Fuel Starvation - An ongoing issue

[Nipedley]

Hi guys, I have a 1989 GS500E over in the UK. I've had it now for 2 years and done 10,000 miles on it (now up to 50,000) Ever since I've had it, it's had a recurring issue of fuel starvation by which I mean - anything over 60mph, you can feel the bike start to 'back off' on the throttle, until it eventually starts to kangaroo and slow down and slow down until you get down to 10mph where the bike cuts out and comes to a stop, when it comes to start the bike it's clear that there's no fuel in the carbs and you have to crank and pull some through before it will start again. I've had this problem, 5 times in the 2 years that I've had it, each time resolved by doing something different but with the issue later to return.

The first time I had it, I had restriction washers in (to restrict the air/fuel mix for a 33bhp license restriction), once they were removed it was fine
The second time I had it, it was a blocked fuel filter, replacing this with a new one sorted it out
The third time I had it, the fuel line I had replaced during earlier work had disintegrated (terrible stuff) and blocked the carbs. Another set of new fuel line (better stuff) and a clean of the carbs sorted this one out.
The fourth time I had it, the fuel petcock had partially dissolved on the vacuum side (the small black rubber bit), a brand new OEM petcock solved this one in April 2014, so just over 7 months ago. At the same time, I also removed and cleaned the in-tank fuel filter which looked absolutely fine, and fitted a new rubber o-ring to the fitting.

Now we come to the fifth time, this morning it started bogging down at about 60mph until I came to a stop. Riding it home at 30-40mph was just fine, so I could swap to another bike. So we have recently cleaned carbs (spotless I can assure), an almost brand new petcock, good fuel lines and a recent inline fuel filter. The bike runs perfect otherwise.

So I'm thinking my options here are: Remove the inline fuel filter to improve the fuel flow, and if at all possible shorten the lengths on the fuel line (though I was already aware of this last time and they are quite short as is). And if that doesn't help, the only thing I can think of is to perhaps remove the fuel tap altogether and fit a fuel pump in place? Has anyone else done this, if so would they know what kind of fuel pump pressure would be best to avoid any overflow problems?

Thanks for reading and any thoughts on this Also for your information, turning the petcock onto PRIME makes no difference to this whatsoever unfortunately, though the petcock is operating normally.

Kind regards,
Phil

[Suzuki Stevo]

Remove the inline fuel filter

Do this Now 

If it's an automotive filter, then chances are it's doing more harm than good, if it's a filter for a motorcycle, then I doubt it's the issue. Either way this is a good place to start.

[Janx101]

Not adding anything to your fuel to 'pep it up' ?

[Nipedley]

Nope, nothing added to the fuel, it's regular 95RON that we have here in the UK (5-10% Ethanol). The filter is a motorcycle one, but given that the issue is not enough fuel flow to the carbs it's the only thing I can think of that could be impeding it. Also worth mentioning that I thoroughly checked the vacuum hose on one of the previous times this happened and that was completely fine.

I think step #1 is to remove the fuel filter and check the fuel line lengths, if I still have a problem then it's probably going to be fuel pump time, if I can find something with a very low pressure. I used a facet solid state fuel pump on my CBR600 which has been amazing. I just can't think of any other reason the fuel flow could be low.

Thanks for your suggestions guys

[Janx101]

Sooner or later every poster on here with an added filter gets rid of it I think? .. causes more dramas than it solves. ..

E10 eh? .. I've been using that on purpose for a couple years with no big problems. ... aside from I'm reasonably sure it made my oem fuel lines stiff and a tad brittle. ... replaced them earlier this year with some new fuel hose that is "supposed to be" ethanol resistant. ... got it from a BMW bike dealership, just took some of the old hose in for size matching

[Old Mechanic]

Check the purge valve in the filler cap, or just release the cap when it starves for fuel and see if that helps.

regards
mech

[tegboi]

Try switching the frame petcock to prime when it happens? I had an issue similar to this, but was a mixture of old soft fuel lines pinching lightly, plus O ring in frame petcock being bad, and found when switching to prime it was able to let the bike run normally. The soft fuel line pinching was probably the main line, and then when switching to prime uses the reserve line without vacuum to let it flow.