Fuel starvation at high speed

[sprint_9]

I had a 93 GS with 150 main jets and I never had any starvation problems, and I maxxed my GS out atleast a few times.  I had the stock lines and the what not, just something to go off of, maybe try to rejet, really makes the bike run better.  You might also find some unexpected things too when you in there.

PS. dont go to 150's though, thats with a K&N and V&H exhaust.

[Queso]

I have noticed that when it is time to switch to reserve it seems as though I still have a few gallons (it looks like alot is still in there). I can only get 2.5 to 3 in it though.

I have that exact same problem... I'll try cleaning the petcock today, though. I'm going in to put in 135 mains, 37.5 pilots (woulda got 40, but those were out of stock, it was 37.5 or 45). I also got 127.5 mains in case 135 leaves it too rich. UNI filter in the stock box with a V&H exhaust, BTW...

I'll see if this fixes my problem. I had my bike up to 90 today for a short stretch, and was holding 80 for a good while with no problems. It only acts fuel starved when I've got about 100 miles from a full tank of gas.

I also noticed my tank mounted petcock shutoff valve is facing the left side of the tank, whereas everyone elses faces the right... Kinda convenient, I think, but obviously the PO messed with it a bit...

[Kerry]

I also noticed my tank mounted petcock shutoff valve is facing the left side of the tank, whereas everyone elses faces the right...

Interesting.

Do the fuel hoses "cross over" each other between  the tank and the ON/RES/PRI switch?  If not, then the ON position will drain all the way through your REServe fuel, at which point the RES and PRI positions will do nothing (because they're  incorrectly attached to the ON hose).

[Queso]

I also noticed my tank mounted petcock shutoff valve is facing the left side of the tank, whereas everyone elses faces the right...

Interesting.

Do the fuel hoses "cross over" each other between  the tank and the ON/RES/PRI switch?  If not, then the ON position will drain all the way through your REServe fuel, at which point the RES and PRI positions will do nothing (because they're  incorrectly attached to the ON hose).

Yeah, according to the clymers diagram, I got all the tubes going to the right places.

[Kerry]

Are you talking about THIS diagram?



When the tank-mounted petcock is mounted correctly -- long tube in FRONT -- the hoses should NOT cross over each other between the fuel tank and the ON/RES/PRI switch.  If the tank-mounted petcock is mounted backwards, the hoses SHOULD cross over each other.

So I ask again:

Do the fuel hoses "cross over" each other between  the tank and the ON/RES/PRI switch?  If not, then the ON position will drain all the way through your REServe fuel, at which point the RES and PRI positions will do nothing (because they're  incorrectly attached to the ON hose).

[Kerry]

Here's an old post which may explain things a tiny bit better. 

[NiceGuysFinishLast]

I had my bike on the interstate for a total of about 4 hours today, all over 80 MPH and none of the dreaded fuel starvation.. guess I'm just lucky! yay!

[NiceGuysFinishLast]

*knocks on wood*

[Queso]

My bike did fine even at 100+ mph until I got to 116 miles from a full tank, then it acted like it was out of gas... This after running perfectly fine for over an hour at 85-90. As I slowed I downshifted, kinda bump starting before stopping, and it got me to a gas station - for another 2.5 gallon refill  I'm going to do some extensive fuel system work over the next couple weekends. I want to get at least 200 miles out of a tank before I have to refill. I can make it that far if I don't use freeways, but I go on freeways all the time, so... Gotta fx itl

In regards to that post - I'm not actually running out of fuel. There's plenty left, I just can't use it at high speeds for some reason. I can't ride to even near actually needing reserve because it cuts out with over a gallon left, and switching to RES doesn't help... Yet I can start it back up with no problem to get to a gas station.

[Jenya]

I had cured the problem completelly.

Had it exactly as the rest describe, which I couldn't live with. It is plain dangerouse for the bike dying while going 80 mph on the fast lane. So I redid the fuel system.

1) Modifyed petcock. Probably the most important step.

Petcock on our bikes is a disaster. It is poorly designed and is notorious for failing.
Here is the picutre of a gunk I pulled out from the vacuum line of the petcock:

http://culbertfamily.com/jenya/gs500/petck_dirty.jpg

Having that gunk in there would mean not enough pull on the diaphragm, hence not enough fuel flow.
However, simply removing the gunk would fix the problem only temporarely. One ought to wonder as to where all that gunk came from. I can't think of anything, but from the fuel. The diaphragm in my petcock was undmaged, no holes, nothing, which can only mean that the diaphragm leaked the vacuum around its edges.
I tried to assemble the petcock together many times, even making new gaskets for it and checking it for a vacuum leak with a pump. I could never get it hold the vacuum.

All that lead me to a decision to convert petcock from automatic vacuum operated shutdown to a manual fuel cut off, the way it is described in details in the following post with few pictures of mine:

   http://gstwins.com/gsboard/index.php?topic=19363.0

2) Drained the tank and checked tank petcock. In my case, tank petcock's fuel filter was in the absolutelly clean condition, so doing all that was just for the peace of mind.

3) Replaced all fuel lines. I went on the large size of the inner diameter of the line by using 5/16 hose.

   http://culbertfamily.com/jenya/gs500/petck_connected.jpg

4) Completelly disassembled carbs, soaked everything for a day in the carb cleaner, blew with an air compressor, assembled and tuned both idle mixture and synchronization.

I haven't had a slightest sign of the fuel starvation since. And I normally go 80mph all the time. Occasianally, bringing it up to over 90.

Jenya

[Queso]

I took out my petcock and took the screws off the back, but it was stuck together som'm fierce, and I didn't want to ruin the gasket without a spare. I'm going to try all that to a slightly lesser extent... I like those fuel lines you got, I'm going to be replacing mine soon. Do you have an additional fuel filter in any of those lines? I noticed an extra one on my bike...

[Jenya]

I took out my petcock and took the screws off the back, but it was stuck together som'm fierce, and I didn't want to ruin the gasket without a spare.

Keep in mind that it isn't possible to get any replacement parts for the petcock, including the gasket. I ended up cutting a new one myself.

Stock configuration doesn't use any fuel filters other than the one inside of the fuel tank. However, I installed a fuel filter into the line going from the petcock to carbs. I kinda felt that it may be a good idea, considering that I used JB Weld to modify the petcock and thought it would be good to prevent some of the dust  that may still may be loose from traveling into carbs. I went to the auto parts store and looked at their non-paper filters. Actually, I didn't just look at them, I opened every package very quietly, so that store employees won't notice, and blew through all of them to find the one with the lowest resistance.

Jenya

[VSG]

Exactly.  In the "ON" and "RESERVE" positions gas can only flow if the engine provides a vacuum to a rubber diaphragm in the petcock.  This pulls the diaphragm back (OK it is really atmospheric pressure on the other side that pushes the diaphragm) opening a passage for fuel to flow.  If the vacuum hose or diphragm have a leak, or there is crap inside the valve area of the petcock then it will not open fully and you have gas starvation at high speed.  Check by turning the petcock to "PRIME" which does not use the diaphragm - if that fixes your problem, then you know it is the vacuum line or diaphragm or crap in the frame mounted petcock.

If it does not solve your problem, then check the tank mounted valve - it is fully open when the slot is inline with the outlet and totally off when it is turned 90 degrees.  If that is OK it may be crap on the screen inside the tank - empty the tank and take the tank mounted valve off - clean the screen and re-install.

I was getting the typical fuel starvation.  On the highway at 70 MPH the bike felt like I was running out of gas.  Sometimes I got around it by downshifting and giving it some more gas.  A couple of times I pulled over, turned off the bike and started back up again.  In addition to that, it just felt sluggish in general.

I'm glad I found this thread.  I tried putting it on PRIME today and went on the highway again.  I had absolutely no problems.  I could even accelerate in 6th gear - obviously not quickly, but it was possible.  So my next step will be to take out the frame-mounted petcock, try and clean it up and at least replace the fuel lines.

It's definitely good to know that it's not the carbs and something fairly simple.