Bike constantly bogging down around 1/2 throttle

[newbie1993]

Recently started working on my 1997 gs500e again. I completed basic maintenance (oil/oil filter change) carbs cleaned out, new gas etc, etc. Finally got her started and running seemingly fine. But then I go to take her for a test ride and all of a sudden around 1/2 throttle she struggles to maintain rpms or even gain rpms. Does anyone have any advice or has anyone experienced the same issue? Thanks in advance

[the_63]

What have you ruled out? When I put my airbox back on the rubber boots never hook under the carbs right which would cause airflow problems.

How long is the bike running before it drops revs?

Chris

[Watcher]

Especially if it happens only above a certain speed, there could be a bad seal between the airbox and carbs, or even something as simple as the breather hose routed incorrectly.

You could also have an old or fouled filter element causing a lack of airflow when you really open the throttle.

How's your jetting?

[The Buddha]

You have correct fuel hose routing ? Sounds like the floats aren't filling up.
Or a rip in a diaphragm.
Cool.
Buddha.

[newbie1993]

What have you ruled out? When I put my airbox back on the rubber boots never hook under the carbs right which would cause airflow problems.

How long is the bike running before it drops revs?

Chris

That was my first guess but when I first put everything back together after I cleaned the carbs I noticed when I turned the gas tank flow back on i was losing fuel between the airbox boots and the carbs. I got the leak to stop but do you think I still don't have a good seal?

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[newbie1993]

Breather hose goes from the engine to the airbox right? Or is that the top hose on the carbs?
Jetting is completely stock. I will try cleaning the air filter out. I might have skipped that step when doing everything.

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[newbie1993]

You have correct fuel hose routing ? Sounds like the floats aren't filling up.
Or a rip in a diaphragm.
Cool.
Buddha.

Fuel hoses are routed correctly. Double checked that because I thought that might be an issue. Where would possible rips be? Im guessing between the carbs and airbox or between carbs and the engine?

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[newbie1993]

What have you ruled out? When I put my airbox back on the rubber boots never hook under the carbs right which would cause airflow problems.

How long is the bike running before it drops revs?

Chris

That was my first guess but when I first put everything back together after I cleaned the carbs I noticed when I turned the gas tank flow back on i was losing fuel between the airbox boots and the carbs. I got the leak to stop but do you think I still don't have a good seal?

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Also, the bike idles perfectly until I mess with the throttle. Rpms either jump drastically and stay around 4k or rise to around 6-8k and bounce around until the bike cuts off

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[Watcher]

when I first put everything back together after I cleaned the carbs I noticed when I turned the gas tank flow back on i was losing fuel between the airbox boots and the carbs. I got the leak to stop but do you think I still don't have a good seal?

How did you solve this issue?  Because it sounds like all you did was make sure the airbox was on the carbs tight so that the fuel didn't drip out of the boots.

Fuel should be leaving the carbs in one direction and in one state.  Towards the engine, and as a vapor.
If it is leaking out the back as liquid the carbs are overflowing, even if the fuel is collecting in the air box it's still overflowing.

In which case the floats in the carbs are set improperly or the needle and seat is bad.

[The Buddha]

Diaphragms are in the carb, they're under that plastic top.
Tiny tiny rip you cant even see will do this.
Cool.
Buddha.

[newbie1993]

when I first put everything back together after I cleaned the carbs I noticed when I turned the gas tank flow back on i was losing fuel between the airbox boots and the carbs. I got the leak to stop but do you think I still don't have a good seal?

How did you solve this issue?  Because it sounds like all you did was make sure the airbox was on the carbs tight so that the fuel didn't drip out of the boots.

Fuel should be leaving the carbs in one direction and in one state.  Towards the engine, and as a vapor.
If it is leaking out the back as liquid the carbs are overflowing, even if the fuel is collecting in the air box it's still overflowing.

In which case the floats in the carbs are set improperly or the needle and seat is bad.

So as the carbs are filling with fuel while the petcock is is the prime position. I should see no fuel on the airbox side unless i have a flooding issue. And yes, all i did is get rid of the leaking issue. And I didnt bother with the needle adjustment or anything because I didnt think it needed it because they were last adjusted when i last rode and i had no flooding issues or leaking issues. And flooding would mean the bike would be running rich right?

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[Watcher]

Think of the carbs as like tiny versions of your toilet, and the fuel line from the gas tank is like the water line coming from the wall.

You flush and the water from the tank drains, as the water level in the tank drops a float also drops, which opens a valve to let more water in.
As the tank fills and the float rises to a certain point it shuts off the valve, otherwise it would just overflow.

The carbs behave the same way, so if your carbs are leaking, or more specifically overflowing, the floats are not shutting off the fuel flow.
This can be from a float out of adjustment, which is something than can happen with age, or a bad valve.  In a carb this would be the needle and seat.
Often the needle and seat are just dirty, often just the needle is faulty, and sometime an O-ring that seals the whole valve can go bad.  It's hard to say what the culprit is.
But something here is to blame.

And a carb that is flowing too much fuel like this is essentially drowning itself.  It might work fine on idle when vacuum is low but when you open it up it's just sucking way too much fuel and bogs the whole engine down.

[newbie1993]

Think of the carbs as like tiny versions of your toilet, and the fuel line from the gas tank is like the water line coming from the wall.

You flush and the water from the tank drains, as the water level in the tank drops a float also drops, which opens a valve to let more water in.
As the tank fills and the float rises to a certain point it shuts off the valve, otherwise it would just overflow.

The carbs behave the same way, so if your carbs are leaking, or more specifically overflowing, the floats are not shutting off the fuel flow.
This can be from a float out of adjustment, which is something than can happen with age, or a bad valve.  In a carb this would be the needle and seat.
Often the needle and seat are just dirty, often just the needle is faulty, and sometime an O-ring that seals the whole valve can go bad.  It's hard to say what the culprit is.
But something here is to blame.

And a carb that is flowing too much fuel like this is essentially drowning itself.  It might work fine on idle when vacuum is low but when you open it up it's just sucking way too much fuel and bogs the whole engine down.

About how often do those needles in the float assembly need to be replaced? I replaced the jets recently and when i cleaned out the carbs the o rings werent bad. Everything still fit snug. And also at one point everything was fine. Throttle was good and responsive. Idle was perfect. Would the wd40 method help me out at all in this situation?

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[Watcher]

Hard to say how long they last.  Sometimes you can get away with cleaning them, but not with WD40.  WD40 is a solvent and will destroy rubber.

Use something that isn't destructive, like alcohol.

Or sometimes just opening the drain screws on the bowls is enough to flush off any dirt that is clinging to the needles.  You might get lucky.

[newbie1993]

I meant as far as making sure it isnt a vacuum leak. But i doubt it is. Would telling you i get quite a bit of backfiring help at all? I'll try and flush out anything that might be stuck in the there. But just in case the airbox isnt sealed quite right would that possibly solve my issue. And also a little bit more info. When i had the bike running earlier it would run fine and idle perfectly and all of a sudden rpms rise and then they fall and then the bike cuts off. But the needles are pretty new. My bike was my daily for pretty much every season.

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[Watcher]

Best advice I ever got for rooting out vacuum leaks was a spray bottle of water.

Carb cleaner and WD-40 are both bad for rubber.  Water isn't.  Just instead of a rise in idle you're looking for a drop.


From here, though, I'd be taking the carbs off, opening the bowls, inspecting the needles for wear or debris, and verifying proper float height.
If all that checks out, then I'd go hunting for vacuum and other causes.

[newbie1993]

I will say that the bike would only start if i sprayed the silicone wd40 at the boots of the airbox at first. But that was before i took of the small fuel leak i had. What would worn out needles look like? And also if it float height had been previously adjusted would i have to bother with it this time?

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[Watcher]

The GS uses rubber tipped needles, but you're looking for basically the same thing as this image.
Rings that imply wear, discoloration, or any sort of obvious debris.



No matter how recently the carbs were adjusted, it never hurts to check again.
Especially with how malleable the tabs are for setting float height they could easily have been bumped with the bowl off and been pushed out of spec.

[mr72]

Definitely also check that one of the diaphragms in the carburetors is not torn, pin-hole, misaligned, or deformed from contact with carb cleaner. That mid-throttle, low-rpm bogging/sputter is exactly the symptom of the slide and main needle not moving up and opening the main jet.

A severe vacuum leak could also cause this. Not having the caps on the vacuum ports, for example, or missing the o-rings in the carb tops that are under the vacuum ports could do it. It'd take a serious problem of the intake boot to do it, like the rubber part completely separated from the mounting flange for example (happened to me).

Vacuum moves the slide and main needle up by causing the rubber diaphragm to move. If the rubber diaphragm doesn't seal, won't flex, is torn, or there is a vacuum leak so it doesn't have enough vacuum on the top side to move, then the bike won't get off of the pilot jet and it'll top out at like 4K rpm and likely stall at more than about 1/4 throttle.

[the_63]

So if it was me, I'd pop the tank off, then take the black tops off the carbs and check the diaphragms inside. I had to replace one of mine recently because it was wrinkled.

Then I'd check that the carbs are firmly situated in the boots between engine and carbs, then I'd check the rubber boots between the carbs and airbox, make sure that there's nothing wrong there.

Breather hose goes from the breather cover to the airbox, and there's a hose that comes from the carbs that lays over the top of the airbox.

Chris