Bike only runs on prime !?

[Sutty.89]

First of all Hi!, I'm new here, Aussie, owner of 2005 gs500f.

I have an interesting and perplexing issue. I have read a bunch of posts with similar problems, but the solution for them has not worked for myself.

The bike only runs on prime - try running on 'on' then it conks out once the float bowls are too low. The fuel tap with the vacume valve is working. Took it off, dismantled, cleaned and checked diaphragms, no issues, reassembled, suck on vacume valve and seal it with finger, and valve is open. Checked if the bike is making vacume, stuck my finger over the hose and a pulsating vacume is felt.

Checked the in tank filter/tap - clean and working.
Checked vacume hoses for holes - none
One way valve on vacume hose is working
Looked for vacume leaks with wd40 while engine running - nothing.

Some more background for those mechanical detectives:
Bike was running fine, but would tend to backfire and not rev when getting a quick blip from the throttle when rev-matching on downshifts. I guessed this was because it was running a little lean. So I gave it a dynojet stage one kit.

When synching the carbs (both before and after dynojet) my cheap Chinese vacume gauges read 20 (cm hg vac) which on the gauge reads "late valve timing, or leak at intake manifold or heat rise". The vacuume gauge takes vacume from the same place as the fuel tap. I assumed it was nothing because my gauge set is for cars and couldn't even make a good seal on the tiny nipple on the carb.

As a frame of reference, I can open the valve on the tap by sucking on the hose, but if I suck on the carb synch gauge I can't get it much past 30. I reckon 20 should be enough to open the valve. (Yeah I'm a bit weird)


Bike was running really well after dynojet - no more backfire. Nice brown plugs. But then developed an intermittent flat spot at 5000rpm. Would just die down and run like it was fuel starved for a while and then it would get over it. My philosophy on intermittent problems is to just leave them until they don't go away so they can be diagnosed - got my wish.

A week or so later it completely died on me - Worked fine all day in town, but when it was copping a thrashing on the twisties it just lost all power and died. Left it for a bit and it would run for a while and die again - regardless how it was being ridden. I nursed it home in short bursts - ride a bit / sit a bit. It would die on either prime or on or reserve.

So I went searching, dismantled and checked the entire fuel system, from tank to carbs. Cleaned carbs, checked float levels, hoses, both taps. Could not find a problem, everything is fine. Reassembled and now the bike runs fine, just on prime mode - won't run on 'on' or 'reserve'.

What have I done!? Has anyone else had this problem? Is there something I'm missing or something I should check?

Any help would be appreciated and I will let you know if I find a solution.

John.

Ps. not long before any of these issues bike had a valve clearance check (at the shop) and was in spec.

[ajensen]

Wow! A lot to digest. 1. I had the run-on-prime only problem until I bypassed all the extra vacuum stuff and ran a line straight from the intake to the frame petcock. Now, it works fine unless I am at high throttle openings for an extended period. 2. I had a similar problem with the engine dying at times. It turned out that I had reversed the fuel lines coming from the tank to the frame petcock. By the way, I recommend checking your own valves--set them a little loose. Best wishes.

[pliskin]

Not on my GS but another bike I had a similar problem. Wound up being the gas cap vent was stuck. This caused a vacuum in the tank thus starving the motor of fuel. As soon as I'd pull over and pop open the cap it would start right up and run. Not sure about the GS vent but my other bike had a BB valve in the cap that was sticking. Cleaned it with some de-greaser spray and never had the problem again.

[Sutty.89]

Thanks for your replies.
Ajensen, not sure exactly what you meant, by my interpretation of your idea seems to have worked. Ran the fuel petcock it's own private vacume hose from the other carb and blocked it off on the other side. Now the right carb sends vacume to all the computer doodads and that little vacume chamber attached to the frame. And left carb goes direct to petcock (instead of being blocked off). I don't intend to leave it this way, as the fact that it stopped working means there are problems elsewhere - this is just a bandaid.
Couldn't find a blockage in the fuel cap but couldn't find the hole either. :S will explore this as I know it needs to get air from somewhere. But in my mind it wouldn't run on any setting if a vacume was created in the tank.

Thanks again.

If anyone whats a more indepth explanation of my dodge fix let me know.