Fuel starvation - A simple petcock fix

[NortwestRider]

Ya thats I was thinking,just exploring all posible causes.


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[yamahonkawazuki]

pingle FTW

[DoD#i]

Cruising down the freeway at 65+mph on my wifes 94 ( trey ).Full tank,80 degrees or so,5500rpm's bike starts acting like it's running out of gas and then dies.Pulled over,removed tank bag and opened fuel cap.After closing the cap it started right up and ran fine!!.
Could this be a petcock issue .

No. Tank vent, in the gas cap.

How to clean the gas cap tank vent (from the FAQ section)

http://gstwins.com/gsboard/index.php?topic=41061.0

[ggg]

I'd say the same thing.

Full disclosure on the petcock fix: I've had to disassemble and reassemble the petcock twice since I rejetted the restrictor plate because the fuel starvation symptoms were coming back. Both times, I found fuel on the vacuum side. The diaphragm is not punctured, so it's possible that the extra suction force is pulling a little too hard on the diaphragm and creating a fuel leak at the edge. When I rejetted, I rebuilt without sealant, the fix lasted 50 miles. I tried a small amount of multi-purpose grease in the diaphragm seat groove, that lasted 230 miles. Last night, I put gasket sealant in the groove (on the fuel side) and around the rim of the diaphragm (on the vacuum side). We'll see how this works.

[Badot]

If fuel is getting into the vacuum side and the diaphragm isn't punctured, air is getting in too. The leak is more likely than not the original cause of your problem.

Again, there is no extra suction since you widened the hole, unless doing so offset a leak -- that would be like saying when a tire gauge reads the pressure through a valve stem that the pressure in the tire is higher than the pressure in the gauge, or that in a compression test the pressure in the cylinder is higher than the gauge reads since it has to go through the hose (neglecting the volume of the hose).

[ggg]

That may very well be... Today, the bike ran great for 70 miles, then the problem came back. As before, switching to PRI got rid of the issue. More interestingly, I switched to RES after a few minutes on PRI and the problem did not come back after a few minutes like it does when I go to ON. Maybe the diaphragm is porous after all, maybe the fuel on the vacuum side comes up the vacuum line--at any rate, I have reached the limit of my patience on this issue. I just ordered a new petcock on eBay and we'll call the fix unproven until someone else replicates it and reports results.

[ggg]

One more data point: the bike ran flawlessly today with the petcock on ON, and I didn't change anything since last night. Also, I got 160 miles on a tank (same commute route, same traffic, same everything as usual) without needing to switch to RES. The other time I knew the petcock was leak-free after rejetting it, I had to switch to RES after 157 miles. Before the fix, it was 135-140 reliably. More questions than answers at this point:

• Could the problem be linked to ambient temperature? Since I did the fix, starvation has always happened on my way back from work in the afternoon with temps in the 70s, not in the morning with temps in the 50s
• Does the fix help get more gas out of the tank? It can't improve fuel economy--that's controlled by jet size and fuel pressure, and neither has changed. My theory is that with a larger fuel passage under the diaphragm, there is less resistance to fuel flow, so the fuel can keep flowing at a lower fuel pressure, i.e., with less fuel in the tank (fuel pressure = atmospheric + weight of the fuel column in the tank).

Thoughts?

[Badot]

Just a theory, but it may be air bubbles working their way out of the system. I had odd symptoms like you described with the bike becoming fuel starved and switching to reserve and back fixed it, this was after converting my petcock to manual flowing. Happened 2 or 3 times on my 320 mile ride home, but didn't ever happen again.

[ggg]

I was just thinking along similar lines before reading your post in view of today's ride home. 15 miles into it, the bike bogged down again. I switched to PRI for one mile, then back to ON, and everything was normal for the rest of my 50-mile commute. Switching to PRI bypasses the diaphragm but not the petcock and would indeed work around any air left in the diaphragm chamber.

Your theory makes even more sense considering that I did not re-prime the fuel circuit after resealing the petcock two nights ago, because I wanted the sealant to cure overnight and I wasn't sure that exposing the uncured sealant to fuel was a good idea. I'll keep an eye out for 1-2 tankfuls--and maybe I'll return that new petcock I bought on eBay after all. (I can reassign that $50 to something more useful such as Progressive springs.)

[ggg]

Another day, another data point. No starvation issues whatsoever, still on ON after 158 miles since filling up. Looks more and more like Badot was right and my recent issues were caused by air bubbles--and also like the fix is working.

[mimikeni]

Just for fun, I took the GS out for a highway run to test for fuel starvation.  As expected, after a minute or two at 70-75 mph, the bike began to sputter.  I suddenly remembered to flip the petcock switch to PRIME and the bike roared back to life. I think I'll stick with this solution rather than start widening holes in my petcock's baffles.  Although I do applaud ggg's ingenuity.

[ggg]

Same symptoms indeed - mine took 4 miles at steady 7000 rpm in 6th. The verdict for today's tank: 163 miles before switching to RES. Definitely on to something.

[adidasguy]

Just for fun, I took the GS out for a highway run to test for fuel starvation.  As expected, after a minute or two at 70-75 mph, the bike began to sputter.  I suddenly remembered to flip the petcock switch to PRIME and the bike roared back to life. I think I'll stick with this solution rather than start widening holes in my petcock's baffles.  Although I do applaud ggg's ingenuity.

Could be the petcock is wearing out. Any part on the bike with rubber can wear out. So replace it. Might be a leaky, pinched or plugged up vacuum line from the carb. The space between airbox and frame isn't much. I find it easy to get the hoses pinched a little unless I'm careful and shove them where they fit best.

[ggg]

600 miles on the petcock fix and no signs of fuel starvation, even after a washer-under-needle rejet which should trigger it a little earlier. I'd say we're moving from possible to probable validation.