Fuel starvation at high speed

[pandy]

Basically, for all of us, it doesn't seem as though there's a golden rule that Solves The Problem. We each just try different things until we find what works for us!

pandy  

[gerharddvs]

O well Lets hope I find the problem cause I'd rather be riding it each weekend then trieng to fix another problem. This is only supposed to happen with the old bikes. Thats why I paid more and got a GS500E and not a GS550 but anyway somebody has to get the dud bike else somebody else gets it. I should just do a rebuild and get done with it cause oil consumption is a Buddha Loves You. Unfortunately it's my only transport so I can't afford to have it standing for months. Plus Suzuki spares like pistons and stuff are quite expensive around here but the saddest thing of all is that nobody on this forum lives in South Africa so I'm pretty much on my own but I have no intention of selling my bike. I'll fix her even if it takes forever to solve all the problems

[Faxxxy]

O wel Lets hope I find the problem cause I'd rather be riding it each weekend then trieng to fix another problem.

Buy a Honda...  :lol:  :mrgreen:      

[pandy]

Ban him!!!    

pandy  :mrgreen:

Buy a Honda...  :lol:  :mrgreen:      

[Mountaineer]

So did you very carefully check to make sure that the tank shutoff valve is fully open? That means the slot in the valve is vertical.

[mjm]

.... have to put the bike on prime at high speeds cause the little vacuum pump can't supply fuel fast enough. ...

there is no pump - gravity feed only

[gavin]

.... have to put the bike on prime at high speeds cause the little vacuum pump can't supply fuel fast enough. ...

there is no pump - gravity feed only

This is a question I've had for some time.  So the vacuum to the petcock just, what, opens a valve?

-Gavin

[mjm]

.... have to put the bike on prime at high speeds cause the little vacuum pump can't supply fuel fast enough. ...

there is no pump - gravity feed only

This is a question I've had for some time.  So the vacuum to the petcock just, what, opens a valve?

-Gavin

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.

[gerharddvs]

This morning I tried driving with the fuel cap open but it still does the same.
I'm in the process of reassembling the bike cleaned all the hoses including the tap on the tank and the one with the vacuum but it seems they were all fairly clean. It did seem that the float levels were very low which by the way wasn't detected with the pipe method which I'm never using again in my life, (waste of time) I set the float as fairly high but so it doesn't overflow thru when I tilt the bike. If I still have fuel starvation I don't know.
Interesting it seems I've still got 120 main jets in my carbs and 40 pilots.
What is a reasonable size I can make the main jets without having to change the stock exhaust and where can I buy them maybe on the web (remember I'm in South Africa so they have to be able to ship to me)
Anyway I'll be testing it tonight and I'll let you guys know whats the verdict on the original problem.

[gerharddvs]

I tested it his morning and it's working. I can average at about 160-170kmh for long period without any problems and on long stretches if the road isn't uphill I can drive 180+ kmh, no problem.

I have developed another problem and that is that my clutch is going. When the bike is hot it starts slipping but from what I hear it's not diffecult to replace and you can do it in the frame but that's another project for another day.

[calamari]

...
early this morning (2:30am) I warmed up the bike (gs500f 2005) for about 2 minutes, then I started riding normally in 1,2,3rd gear until I got to the freeway (empty at the time).

I had 163 miles on the tank, so I figured I still had some 20 more to go before refueling (I average 188mpg), so I get on the freeway and start accelerating.

Some fool tries to pass me while im still accelerating and I let him know that I mean business by leaving him behind and reaching 90mph.

30 seconds later, when the guy is just a speck in my rearview mirror, the bike starts to sputter, I figure it's the fuel and switch to reserve and the engine dies  
I'm like, "what in the name of lucifer!"

I pulled over, try to start the bike, switch to 'normal' (not reserve) and it just wont go.
Take key out, wait a second, put key back in, push starter, and it works.

I start to accelerate my way out, and after reaching some 75mph, the engine dies again  :dunno:

Pulled over, repeat procedure, no go. repeat, nothing.

Finally it works, I go to my destination slowly (as in 45mph), get there, leave bike sitting for about 4 1/2 hours, go back to bike, switch to reserve, turn bike on, warm up, go to gas station, fill her up, accelerate to 80mph on freeway, and everything seems ok  

How can I go from getting 188 miles per tank to 166 miles? is it because before I was only doing 61mph and now I go well beyond that? (75-90mph)

[mjm]

Mileage can easily vary by 30~50% based on how you are riding/driving any vehicle - and speed is one of the big variables in the equation.

[Emmajoe]

im on a '93 and getting the same problem at just about any speed. All it takes is the bike to run for a bit. 20 minutes or so. I had it in 2 diffrent shops, both spent a number of hours but didn't charge me cuz they couldn't figure it out. they got it running better though. Then I had to go and play with it, i didn't do anything other than take the carbs apart to see for myself. Everything seemed fine so i put it back together and now i'm back to where i started. So when anyone figures it out, please post what you find.

[roguegeek]

This is now starting to happen with my '97 GS. Fuel starvation of some sort at 85 mph. Yeah, it's annoying. I think, more importantly, it's down right dangerous. Driving these SoCal freeways at 85 with extremely dense traffic around you and all of a sudden you drop 20 mph for no reason. Now you need to dodge traffic (which wont slow down of course) from in back of you. It's nuts and it's at the point right now where I'm actually driving my car to work this week. I'm going to be going through all of these previous posts tonight to see what I can take care of this coming weekend.

[boosdad]

My commute is 27 miles each way (most of it on the freeway).  I make sure I am on prime for my commute and I have no problems (most of the way running 70mph +).  I to have noticed that when it is time to switch to reserve it seems as thought I still have a few gallons or gallon (it looks like alot is still in there). I can only get 2.5 to 3 in it though.

[Mandres]

.... have to put the bike on prime at high speeds cause the little vacuum pump can't supply fuel fast enough. ...

there is no pump - gravity feed only

This is a question I've had for some time.  So the vacuum to the petcock just, what, opens a valve?

-Gavin

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 think mjm is on the right track here.  In the fuel system circuit there are two items that seem like the obvious bottlenecks; the carbs and the petcocks.  If the carbs have been recently cleaned / rejetted then they're probably not the problem.  It seems much more likely to me that one or both of the petcocks have accumulated some sludge in their passages that would restrict the flow of fuel at high speeds.  Try removing, disassembling, and cleaning the cocks and see if you still have problems. 

[gerharddvs]

Hi,
You are all wasting your time the fuel starvation is on all the GS500s I suspect it was designed like that as some kind of speed or ref limiting mechanism. The only solution without modifying your bike is to put it on prime before you get on the freeway.
In the racers corner (http://www.gstwin.com/racers_corner.htm) they show a way of replacing the whole system with just a fuel cut of valve because apparently this bike has too much plumbing and I agree. Down side to both of these methods is that you loose reserve.
I usually keep an eye on my trip counter because I know more or less after how many miles I run into reserve. I was thinking of trying to modify the stock petcock in some way. Maybe drilling the whole behind the diaphragm to let more fuel pass but I'd rather find an extra one and play around with that.
I've found that if you set your floats a bit higher you can go a bit faster before fuel starvation  One of my friend removed the floats completely when they were dyno tuning it. In this case the petcock regulates the flow. Just never put it on prime!(it tends to dilute you're oil:)) It then runs like hell but it's very heavy on fuel. Wouldn't recommend it unless you're into drag racing:)
Somebody mentioned to me that the old Suzuki 650 Katana had the same problem. Apparently they solved it by using the petcock from some Kawasaki (don't know which model). This petcock apparently had a larger diaphragm. I don't know if there's any truth to this maybe somebody knows?

[Cal Amari]

I suspect most of these problems are related to water in the gas, caused by normal condensation; here are some suggestions on how to deal with this problem...

Drain your gas tank COMPLETELY into a suitable container; try to get every drop out of it. Then, remove the fuel chicken assembly (number 1 in the drawing) and fuel filter assembly (number 9 in the drawing):
http://www.psndealer.com/fiche/images/Suzuki/1997/Motorcycles/2104_34.gif

Next, flush the tank with some fresh gas (filter it through cheesecloth or a CLEAN disposable rag if you want to try to re-use it, to filter out the CRUD). Try to flush all the nasty junk out of the crap pockets while you're at it...

While you're doing all that, the gas you recently drained from the tank will be floating on top of the WATER that was sitting in the bottom of your fuel tank. Condensation causes water to form on the inside of your gas tank; the more air space in the tank, the more chance for condensation to cause problems such as many of you are experiencing. I don't have the time to explain the science behind this, but read this thread by V8Pinto (Topic: water in the gas) for more information:

http://gstwins.com/gsboard/index.php?&topic=20114.0

Then, read this one by red_phil:

http://gstwins.com/gsboard/index.php?&topic=13730.0

Read the entire thread, including the reply by scratch...

You have nothing to lose, and you just might solve most of your fuel-related problems by flushing your gas tank, and cleaning the fuel filter; also, make sure to check the HIDDEN "on/off" valve at the base of the fuel filter unit (number 9 in the drawing). You normally can't see the on/off switch unless you lift (or remove) the gas tank; check to be sure the valve is OPEN ("ON"). If the slot is VERTICAL, fuel will flow; if the slot is HORIZONTAL, fuel flow will be blocked. Use a large, long-shanked flat-blade screwdriver to turn the valve to the full-open (vertical) position, if it isn't already in that position.

Lastly, you can put a few ounces of rubbing alcohol, Heet, Dri-Gas, or any similar alcohol-based product into the gas tank, which will help you compensate for the effects of the condensation, but I don't think you should make a habit out of it. Gasoline is formulated to burn at a certain rate, and alcohol burns much faster; too much alcohol will probably lead to other problems down the road, since it will attack the various rubber parts and other components it will come into contact with on the way to the cylinders to be burned... you'll be much better off in the long run if you drain the tank, flush the crap pockets, and get everything set up properly BEFORE you develop holes in your gas tank (caused by the collection of condensation in the crap pockets, which are the lowest part of the fuel tank).

[Trwhouse]

Hi all,
This is a very common GS500 problem -- and is caused by fuel starvation due to poorly designed or undersized vacuum petcock components.
Cal Amari, this is probably not condensation in the fuel, this is a GS500 weak point, but your suggestions are certainly relevant in other cases.

Here is a previous thread:
http://gstwins.com/gsboard/index.php?topic=20287.0
Here was my input from that thread:\
Hi there,
My 1991 GS500E had the same problem a few years ago and after much trial and error, the only thing that fixed it was buying a new replacement Suzuki petcock (the frame mounted petcock on the left side that has the lever for ON, PRIME and RESERVE). Apparently the rubber diaphragm inside the petcock degrades over time and causes the problem, which doesn't allow the vacuum petcock to flow fuel as it should.
I bought my petcock from a mail-order dealer in Pittsburgh, PA, for about $50, which was $25 or so less than the local dealer.
I'd go with a new one like I did and not try for a used part, which at some point will likely have the same problem as yours does now. 
I also recommend replacing your old fuel lines at the same time to get rid of one more variable and potential trouble spot. Get 1/4" black rubber FUEL line at a local auto parts place. Be sure it is marked for FUEL and NOT for VACUUM. Fuel line is made of special materials with nylon webbing to add strength. The fuel hoses do deteriorate over the years and they are cheap and easy to fix, as well as an important safety issue. Use small hose clamps and all connections on the new hoses and you will be in good shape.
There is also a second petcock on the GS500 as you probably know, but that one is a simple fuel shut-off under the tank. There is no vacuum mechanism on that petcock, just on and off, but you have to be sure it is fully ON.

Read through the boards and do a search on fuel starvation and you will see this issue come up often for our beloved GS500s.
If you need more help or advice, just ask!


Good luck and I hope you get it sorted out.
Yours,
Todd

[Mandres]

I would suggest cleaning the petcocks before spending $50 on a replacement part.  When I bought my GS it had been sitting for over a year.  The gas left so much sludge in the tank petcock (shut-off valve) that it would no longer flow until I cleaned it out.  Here's a picture:



The yellow plastic piece is what actually flows or cuts off the gas.  On my bike both passages in this piece were completely clogged.  On your bike maybe they're only partially restricted, which would result in fuel delivery problems.