gs500 only running on prime & hanging idle issues

ziemniaki · May 09, 2015, 07:39:46 PM

Hey everyone. I'm new to the forum and motorcycles in general. I just purchased a 96 gs500 with 6300 miles. The past few weeks and test rides have revealed a few issues. I cannot start the bike unless it is on prime with at least half choke. Furthermore, if I set the petcock to on or res, the rpms will slowly dip until it dies out even after it is fully warmed up. However, when its on prime it runs decent and does not die out. From my research I've read that the petcock diaphram may not be opening up completely? Can someone provide instructions on how to check this?

Also, the throttle response in general is laggy. If I slowly roll on the throttle it is somewhat ok but it does not respond well to a quick blip of the throttle. Also, when the rpms do climb up to 3-6k they will hang for a bit and are slow to return down.

The spark plugs look good. The oil is clean. The previous owner said it had old gas and was sitting a while. I drained the gas and it was super dirty. Today I had some friends help me pull off the carbs and we cleaned them. We pulled of the main jets and cleaned them but could not get the pilots/secondaries off because I didn't have a screw driver small enough to fit. My friend will bring over his tools tomorrow so hopefully we will get the pilots off and clean those. After this initial cleaning we started it up and the engine seemed to run smoother/steadier, and the throttle was slightly more responsive but the rpm hanging was still evident.

Any further suggestions will be appreciated.

bmf · May 10, 2015, 03:40:36 AM

Petcock not opening and the hanging idle makes me think the vacuum system used to open the petcock is not working/holed, sucking air into carb and preventing petcock opening.

ziemniaki · May 10, 2015, 07:59:32 AM

Quote from: bmf on May 10, 2015, 03:40:36 AM
Petcock not opening and the hanging idle makes me think the vacuum system used to open the petcock is not working/holed, sucking air into carb and preventing petcock opening.

Would cleaning the petcock help or should I just get a new one? If so which one would you suggest?

bmf · May 10, 2015, 08:04:11 AM

First check that the vacuum hose from carb to petcock is present and not perished, the other vacuum point on the other carb must be plugged up to prevent sucking air = hanging idle.

ziemniaki · May 10, 2015, 08:34:47 AM

Quote from: bmf on May 10, 2015, 08:04:11 AM
First check that the vacuum hose from carb to petcock is present and not perished, the other vacuum point on the other carb must be plugged up to prevent sucking air = hanging idle.

Plugged into what? I thought the other vacuum point hose on the carb is just draped over the airbox and connects to nothing?

Janx101 · May 10, 2015, 11:09:06 AM

Plugged into a t piece to other hose I think? .. open end tube means vacuum not there!

bmf · May 10, 2015, 12:10:01 PM

You should get hold of a manual, it helps ALOT! There is a central breather from the carburettor that goes back to the airbox, but each carberator has a vacuum port, in earlier bikes only one port is used to activate petock while the other is blanked off. I think the newer bikes have t piece with both vacuums connected to the pet chicken

ziemniaki · May 10, 2015, 05:46:44 PM

Quote from: bmf on May 10, 2015, 12:10:01 PM
You should get hold of a manual, it helps ALOT! There is a central breather from the carburettor that goes back to the airbox, but each carberator has a vacuum port, in earlier bikes only one port is used to activate petock while the other is blanked off. I think the newer bikes have t piece with both vacuums connected to the pet chicken

Haha definitely. I am picking up the manual from the previous owner soon. Thanks for the advice, I will re-check every hose and replace the plugs just to be safe. What type of plugs do you guys typically run?

Janx101 · May 10, 2015, 08:41:02 PM

Oem plugs, iridium plugs, fancy plugs.. so long as they meet the correct/required numbers! ... which is stamped on the plugs!

You may have just started another plug war! Lol!

ziemniaki · May 10, 2015, 10:17:29 PM

Quote from: Janx101 on May 10, 2015, 08:41:02 PM
Oem plugs, iridium plugs, fancy plugs.. so long as they meet the correct/required numbers! ... which is stamped on the plugs!

You may have just started another plug war! Lol!

Lol oh snap.....

W201028 · May 11, 2015, 03:25:54 PM

The petcock has four lines running to and from it, two from the tank, one to the carbs to provide fuel, and one to the carbs for vacuum. The vacuum is the smallest diameter line, the other three are the same size.
Easy test is pull the line that sends fuel to the carb, and pull the vacuum line as well. Set the petcock to on or reserve and suck on the vacuum line. If fuel comes out the line that sends fuel to the carb, the vacuum system works. If none comes out the vacuum does not work. If you suck gasoline through the vacuum line, it really doesnt work and is torn.
I am not to be held responsible if you swallow gasoline, it may be best to have a less intelligent or litigious friend perform this test.

ziemniaki · May 12, 2015, 02:24:56 PM

Quote from: W201028 on May 11, 2015, 03:25:54 PM
The petcock has four lines running to and from it, two from the tank, one to the carbs to provide fuel, and one to the carbs for vacuum. The vacuum is the smallest diameter line, the other three are the same size.
Easy test is pull the line that sends fuel to the carb, and pull the vacuum line as well. Set the petcock to on or reserve and suck on the vacuum line. If fuel comes out the line that sends fuel to the carb, the vacuum system works. If none comes out the vacuum does not work. If you suck gasoline through the vacuum line, it really doesnt work and is torn.
I am not to be held responsible if you swallow gasoline, it may be best to have a less intelligent or litigious friend perform this test.

So the vacuum line runs from the frame mounted petcock and is supposed to go to which carb? The left?

W201028 · May 14, 2015, 02:02:23 PM

I think on my 09 all vacuum is fed from the right carb, but at the end of the day it doesnt matter. Either cylinder should provide enough vacuum to operate the petcock.

There is a real good vacuum diagram somewhere on the gs500 wiki site, for both the old and new bikes. Try a search and see what you find.

ziemniaki · May 16, 2015, 03:22:36 PM

Well I changed the plugs, backed off 2.5 turns on the mixture screws and put fuel system cleaner in it. It now starts and runs on "on" and the idle doesnt hang as much. Before it wouldnt even start on ON or when switched from prime to on it would die in a minute or two. Thanks for all the suggestions, I learned a lot. I went for a longer ride today and it felt great except it died on me once when downshifting to a stop light towards the end of my ride. Though now that I think about it, its possible that I bumped the stand when shifting.

W201028 · May 16, 2015, 03:39:57 PM

Your bike has pretty low miles, but if it continues to stall occasionally when you stop, check the valves. Mine did that when it when I had no lash, as well as having a low idle when cold, and a higher, hanging idle when hot. Bad valve lash shows symptoms that make everyone tear into their carbs, just like I did.

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gs500 only running on prime & hanging idle issues