Bike running.... irregularly. Help?

[Eisenfaust]

Okay... my GS is behaving in a rather unusual manner. This issue started when I first got the bike, but has only rarely reared its head.

For reference, the bike is a 91 GS with about 21k miles on it.

Occasionally, always at speed (IE, 6th gear, 60-75mph... sustained open throttle), even when full on gas, the bike acts like its running out of fuel.

The first time this happened was just after my first-ever fill up. I was cruising home across the SF Bay Bridge, all was going well.... then... putt putt.. the motor starts stumbling and being laggy. Wouldnt respond to calls for throttle, started losing large amounts of speed due to aero drag... ended up cruising stable at about 40mph... got off the bridge, and the bike was fine.

I attributed it to some gunk in the fuel system somewhere, and wrote it off. I noticed slight hesitation a few more times, but it was nothing bad until the other day.

Once again, after a fill up, I was coming home across the bridge. Same drill as before, only this time the engine actually acted as if it ran out of gas. Stalled out and I coasted to a stop. I put it in neutral, hit the starter, and the engine fired up again. Didnt want to rev at first, but after a few seconds of running (say, 15), it started responding smoothly as if nothing had happened. It did it again, today (not after a fillup), exact same thing. Today was also my first long-distance trip on the bike (4 hours), and it acted up only once.

When its not having this problem, the engine runs perfectly. Starts just about instantly, warms up quickly, then revs smoothly and responds quickly. It never does this during city driving (and I've done nearly 1000 miles of that, I'm only now venturing onto the freeways). My mileage is fairly normal, I think. I get about 30-40mpg in the city (lots of short trips, cold starts, warm ups, and cold weather), and ive noticed 60+ on the freeway. My bike only seems to need about 2.6 gallons before it hits reserve... (measuring how much gas it takes to fill up after I switch to reserve, I thought I was supposed to have a bigger tank o_O(, but yeah.

I dont think its an engine problem, is what I'm geting at... I think its something in the fuel delivery system. Its rather disconcerting... especially given that the engine works pretty much perfectly otherwise.

Any ideas?

[pantablo]

it may be a fuel delivery problem along he lines of a small particle clogging a passageway somewhere-particularly if theres some rust in the tank. That might have gotten loose and clogged something up.

However, there is one common issue with some older models (I say older because I rarely heard it for the 2001 I had). Problem is at steady constant throttle the vacuum is not sufficient to pull fuel through the vacuum activated stock petcock causing the exact symptoms you describe. Solution escapes me but some I remember being a bit exaggerated include swapping the petcock for a racing style fuel valve.

Try doing a search for vacuum or petcock and see what you come up with...

[Kerry]

Yep - you've got the classic symptoms of "fuel starvation".  It usually occurs at fairly high, sustained speeds.  Our best guess is that the carbs suck the fuel out of the float bowls faster than the bowls get refilled.

There are several possible causes:

* The fuel tank-mounted petcock may not be fully turned on.  The slot should be VERTICAL.  (See

this photo.)

* A dirty or clogged screen inside the fuel tank.

* A partially clogged ON hose (attached to the shorter, rear outlet tube on the fuel tank-mounted petcock) or RES hose (attached to the longer, front outlet tube).  If the symptom appears in both the ON and RES settings on the fuel selector switch, then this is less likely to be the cause.

* A problem inside the fuel selector (ON/RES/PRI) switch.  There could be a clogged fuel passage, or a torn diaphragm, or a vacuum leak where the hose from the left carb attaches on the inward side of the selector switch.

* A vacuum leak along the vacuum hose between the fuel selector and the left carb, or at the carb end of the hose.

* A clogged fuel hose between the fuel selector and the lower T-connection between the carbs.  (Somewhat unlikely.)

* Clogged fuel passage(s) between the lower T-connector between the carbs and the float valve that allows fuel into the float bowls.

* A dirty float valve seat (or two).

* Maladjusted float height(s).[/list:u]What can you do about it?  Here are some things that have been tried in the past, with varying degrees of success:

1) Ride with the fuel selector set to PRIme all the time.  

Advantages: It's FREE, and the fuel flow is gravity-fed, not vacuum-actuated.  Disadvantages: IF the carbs are dirty a stuck float valve can empty the gas tank onto the floor by overflowing the carburetors.  You will have no REServe; when the bike runs out of gas it is truly OUT.

2) Check your float heights.  This is non-invasive and practically free - all you need is a short length of hose with a suitable diameter.  (See my Float Height Check page.)  Advantage: You will know whether the (static) float height is reasonable.  Disadvantages: This is a static check rather than a real-time one.  It may find a definite problem, but a lack of evidence while the bike is sitting still is not sufficient to certify that there are NO problems.

3) Replace the fuel hoses with 5/16" ones.  Advantages: Brand new hoses will be clean, larger inner diameter will let more fuel flow.  Disadvantages: Hard to work with, especially at the fuel tank-mounted petcock.  May not be able to clamp it down enough to get a good seal at certain connections.

4) Replace the ON/RES/PRI fuel selector.  You could try another stock unit (same part number from 1989-2002 so far) or a Pingel petcock ($$$ ) or a cheapie, plastic riding lawnmower petcock.  Advantages and disadvantages vary according to budget and taste.  

5) Entirely BYPASS the ON/RES/PRI fuel selector.  You would do this by capping the shorter outlet tube at the fuel tank-mounted petcock and running a fuel hose directly from the longer outlet tube to the lower T-connector between the carbs.  You would also need to cap the vacuum takeoff on the left carb.  Advantage: You no longer need to worry about rips in diaphragms or leaky vacuum connections.  Disadvantages: You no longer have a REServe, and the previous warning about the PRIme setting applies.

6) Clean the carburetors, including a good soaking of all metal parts in carb cleaner.  Advantage: If there IS dirt or varnish in the system, the problem will never completely go away until the carbs are cleaned out.  Disadvantage: If you're inexperienced at pulling the bike apart and putting it back together, you may hesitate to tackle it the first time around.[/list:u]

[Eisenfaust]

So that might explain why, if I go to WOT when it starts hesitating, the problem goes away (for a few seconds anyway, until I find myself doing 85+ and climbing).

I wasnt aware that the fuel petcock was vacuum operated. I figured the prime function was operated by vacuum, but I assumed the rest was just a plain old valve.

My first thought was that debris in the tank was blocking the line somehow... but that notion was dispelled when it happened after the bike sat overnight and I rode for 35-odd miles. If I'd just gotten gas... makes sense. After things have settled... debris doesnt seem as likley (or if it was debris, it would be a constant, not intermitent problem).

[Eisenfaust]

Hey, Kerry, thanks for all the info! I really appreciate the help.

I'm going to spend wednesday trying out all the free options in your list. I'm sure my carbs are dirty. The bike is in pretty sound mechanical condition, but I doubt its carbs ahve ever been overhauled (judging from the chain I just replaced, stuff like that wasnt high on the previous owners' to-do lists). My curent plan of action is to buy a spare set of carbs on ebay, rebuild / clean them,  swap them for my current carbs, and then sell my current carbs.

I have pretty good inate mechanical skill, and I'm used to working with lots of small parts that go together a specific way (my main hobby other than bikes is scale modeling).. so I think I can do it. Looking forward to the challenge, actually.

How long will it take for fuel to start leaking on PRI is the floats are dirty? Will a quick ride around the neighborhood let me know?

[Ed89]

OK.  I have that problem all the time on the freeway.  Almost 50% of the time.  Anyway, I am in the process of trying to finally figure out what is going on by putting on clear fuel hoses (not recommended since I don't think they are made for delivering fuel, but it is just temporary).  Below are (i) my set up, (ii) my observation, and (iii) my theory so far.

SETUP:
- Clear hoses from gas tank (both primary and reserve) to petcock.
- Short clear hose from petcock to transparent plastic filter.
- Very long (almost 1.5 feet)  black rubber hose from filter to carb.

OBSERVATION:
During starvation, the fuel filter is pretty much empty, however, both primary and reserve lines from the tank is filled with fuel.  The starvation happens whether I run the petcock in ON/RES/PRI.  When starvation happens, I stop by the road side on idle and can see that fuel is going into the filter.  (The filter is normally almost totally full, but during starvation, it is pretty much empty.)

THEORY SO FAR:
There are two problems that work together to cause this: (a) air getting into the line between petcock and carb, and (b) the stupid design of the petcock location and geometry.

The design of the petcock is dumb.  If you look at the hose routing, it makes a W shape: from the tank down and the up into the petcock, then from petcock down again and up a little to the carb.  The problem is that the petcock is on a "high" point in the middle peak of the W.  Let's have fun with alphabets:

tank V petcock V carb
What happens is that air gets trapped in the second V, between the petcock and carb.  Let me break the second V into three segments before comtinuing:

tank V petcock \_/ carb

At first, the air is only in "\".   This air has no way of escaping into the gas tank because it is already at a high point (it rises/floats).  It cannot escape into the carb because, well, it rises.  So it stays there.  It slowly gains in size/volume until it also pretty much fills the "_".  This is close to the critical point.

Now, at this critical point, you are at high throttle, the carb float is not really full, so the float needle is raised to allow more fuel to come in.  And guess what, woop, it sucks in a big fat gulp of air as the air in "_" rushes into the "/" section and the fuel in the "/" section flows backwards into the "_" section.  You suddenly need a lot of fuel but the upstream system cannot deliver fast enough.  Boom.  Starvation.

Pull to side of road, idle or put into prime.  Bikes starts right up after a few seconds and you continue with your journey while scratching your head wondering what the hell just happened.

Well, that's my theory anyway.  :mrgreen:  If that is the correct theory, then the solution is to either (i) eliminate the air in the system, or (ii) fix the geometry of the fuel delivery system.

Those who bypass the petcock and route directly into the carb (pingle or direct) are basically taking the (ii) solution.  The geometry in this case is more like a U, which is OK.  Since even if air bubble were to find its way in, it would immediately float to either the tank or the carb, and never stay stuck in the hose to develop into a critical "woop".

I am trying the solution to (i), which is to fix the air going into the fuel system in the first place.  In order of decreasing likely hood: (a) petcock leak, (b) carb side leak, (c) vaporization of the fuel.  I am suspecting petcock, of course.  I don't feel that it is the vacuum side leak, so it is most likely the selector knob leaking.  I just put a glob of vasaline around the o-ring there.  We'll see how that holds up.  Once I get my intake valve fixed, that is.  :x

Also, I think it happens so often for me is probably because I have such a loooong route from the petcock to the carb.  That translate to a \__________/, lots of air to be trapped in the "__________".  So I think if the hose length between the petcock and carb is reduced, the starvation will also not happen that often.  Still, that is not excusing the brain damage design of the petcock geometry that could allow innocuous changes to hose length/routing to develop into a potentially hazardous problem.  And freaking mysterious, too.

Cheers,
e.

Rats.  Maybe I should have started a new thread. :dunno:

[Kerry]

I am trying the solution to (i), which is to fix the air going into the fuel system in the first place.
[...]
I just put a glob of vasaline around the o-ring there.  We'll see how that holds up.  Once I get my intake valve fixed, that is.  :x

I'll be very interested in your eventual findings.  I hope the valve rebuild is coming along OK.  (<-- That IS a different thread!  :mrgreen: )

[Ed89]

I am trying the solution to (i), which is to fix the air going into the fuel system in the first place.
[...]
I just put a glob of vasaline around the o-ring there.  We'll see how that holds up.  Once I get my intake valve fixed, that is.  :x

I'll be very interested in your eventual findings.  I hope the valve rebuild is coming along OK.  (<-- That IS a different thread!  :mrgreen: )

Sorry to dig up old thread, but I think it might be helpful to others in the future if I added my findings.

Anyway, after shortening the fuel hose (petcock to carb) and eliminating the fuel filter, and making sure that as little as possible of the hose is below the level of the petcock, I no longer have fuel starvation problem.  Sorry, but I don't remember whether the vaseline trick worked.  My guess is that it didn't, which made me go to the next option of shortening the hose, etc.

Yes, Kerry, the valve was a different thread.  But that went swimmingly well also with my home-rigged valve compressor.     It was the valve and I was an idiot for bending it.  But I got to clean the top end and piston head real well.  I also get to inhale gasoline and other bike related fumes for a week while the bike is in pieces in my bedroom.  I am very certain I am now more stupid as a result of that. :lol:

Cheers,
e.