2004 GS500F rev hanging and possible causes

[mentalshark]

Hi all, first just want to say thank you for all the great information here on this forum. With this forum, GStwin wiki, and Haynes manual, I feel like I can do almost anything to the bike myself.

I have a 2004 GS500F, the bike has a problem with rev hanging, where the engine would just rev to 5k rpm by itself, and sometimes it will slow down to 1.2k rpm. I have found few possible causes, but since I have already taken the carbs out, I am looking for few more places to check before I put it back together.

From what I have found, the few possibilities are:
1) vacuum leak
2) not synchronized carbs
3) clogged-up jets.

I sprayed w40 around the air booth between carbs and the engine when the engine is running, no changes. I have tried to play with the idle screw and I was able to get idle down to about 800rpm(when warm) and the rev hangs go away. Any higher the rev would hang, any lower the bike would stall at every red light. Since I just got the bike, I thought I would just clean the carbs anyway, the carbs are very clean. I found one of the hoses from the carb has cracked (picture below) which may cause a vacuum leak. I have shortened the tube atm and reusing the middle section as a temporary fix. I don't know where that tube goes and didn't know how that crack effect the engine. If that didn't help I would try to sync the carbs.



Some background: I am new to motorcycles and picked up a 04 GS500F for about 500 bucks (Canadian dollar). I am located in Surrey, B.C, Canada. the bike battery has died and the bike would run once jump-started. the owner already moved to a different city with the bike sitting in a shop for about a week and she just wants it gone. I e-transferred her money without seeing the bike or the owner in person, the whole thing felt like a scam from the beginning to the end, but I am glad I took this opportunity and the bike did show up after 5 hours. Her mechanic told her it is not just the battery and something else must have died. I found out that it was the regulator rectifier that broke, fixed with a new r/r and a new battery. So far I am loving how easy it is to work on the bike. The bike is in decent shape, but I decided to do full maintenance since the oil looks like mud water. So the bike may cost $500 itself ($624 including towing and taxes), I think I am looking at another thousand dollars on parts and tools haha.








pictures of the bike (I have to, I am way too excited) :

[mr72]

You are on the hunt, but just get the theory out of the way. In order for the revs to climb by themselves, one of two things has to happen: a) some way air has to get into the carbs intermittently prior to where the jets are exposed, or b) some way more fuel has to be added intermittently without throttle input. In particular, this has to be the pilot jet, since in this case I am sure you are talking about under no load. The pilot jet is close to the exit of the main venturi, so basically any "vacuum leak" is likely to result in the "more air" condition, and a leaking pilot needle o-ring can cause the "more fuel" condition. Nice thing is, the fix for both is basically the same, rebuild the carbs, replacing ALL of the o-rings, eliminates air/vacuum leaks and internal fuel leaks.

Normally I'd call this a "surging" or "hunting" idle. When you say "hanging" idle, a lot of shade tree motorcycle mechanics are going to think that means after you rev it and it is meant to return to idle, it is slow to return to idle, instead the revs "hang". That's a distinctly different condition than your surging idle.

Look in my signature, there's a link to a blog post that can walk you through straightening out a lot of stuff on these carbs.

BTW, the fact that you can set the idle speed to 800 rpm and this problem goes away is a good sign. That means you are very close to getting it running right, and that the problem is likely the surging due to air leaking past the throttle plate because you've set the throttle stop ("idle speed") too high. So go ahead and set it to that 800 rpm, then adjust the pilot mixture (probably richer) until it speeds the idle up to 1100. Good to go.

But I'd still pull the carbs and rebuild them anyway if they haven't been done in the past couple of years that you can verify. Seems like I have to do mine once a year just due to not riding it frequently even though I use ethanol free gas.

[mentalshark]

Thank you for the detailed reply! and this really helped me clear a few things up.

I had open the carbs and they look super clean, most of the o-ring feels fine. The carburetor was rebuilt last year when the owner forgot to drain the gas and left it over the winter. you mentioned a "problem is likely the surging due to air leaking past the throttle plate", so the cracked vacuum hose on the carbs may be the cause of the issue? I am having some difficulty finding what exactly that hose does since the other end is just plugged by a screw.

Also, having a problem understanding why a lean condition causing by the leak would rev up the engine, didn't lean means less fuel, therefore, producing less power, which should result in rpm dropping? anyway, I am going to proceed with reading the link in your signature, maybe I will find the answer there.

[mr72]

idk about any cracked vacuum hose or plugged but that is likely a major cause of the problems. And the o rings I am talking about in particular are the float needle seat o ring, the pilot needle o ring, and any o ring in a vacuum port. You don't get to these until you pull the carbs all the way apart and the pilot needle o ring is basically impossible to remove without damaging it.

Also I forgot to mention a common place for air leak is at the intake boot o ring.

Anyway, reason any air leaking in will make it rev is if the air leak is upstream of the pilot jet, which means more air pulls more fuel with it from the pilot jet. It's exactly the same as blocking the throttle plate open or opening the throttle. Let more air in, the jet provides fuel.

[mentalshark]

forgot to reply, dove super deep into researching while reading the article in your signature. Thank you sir.