Cyclinder balance

[erk]

Been lurking for a few months and decided to join after purchasing my first bike, a 97 gs500 

Been having an issue with cold starts, I know these bikes are cold blooded and require alot of choke, but after 10min yesterday at full choke, my zuki was still acting up, turned around and drove my metal cage to work (it was ~55degrees)

This morning, same situation but let it warm up for 20min full choke, still was idling at 2k as opposed to the normal 3500 or so during the day, almost as if the choke was barely even working, next to no response in engine speed as the choke was twisted to "full" So I got to work finally and pulled the plugs and the left side plug was dark black whereas the right side was a nice whitish color, pulled the wire off the left cylinder while bike was running and has very little idle drop, did the same to the right side (nice looking plug) and the bike promptly dies (as I assume it should)

Took my coworkers advice and checked the spark with a spark tester, both sides gave a nice orange color and had the same intensity thru all rpm ranges.

So my question to you is what do you think I should check next? After the cylinder balance test and based on the darkness of the one side, I am assuming the left hand cylinder has an issue, but what all could it be? I am a newer mechanic by trade and specialize in fuel injection, but am still wet behind the ears and know jack about carbs.

[Dizzledan]

both sides gave a nice orange color and had the same intensity thru all rpm ranges.

When you spark test (ground plug to frame, crank engine) you should be seeing a bright blue spark (in a darkened room).

When one cylinder has trouble and the other doesnt, it could mean a few things.

Look on the bottom of your carbs, you should see 2 recessed flat head screws. These control the fuel to air mixture that goes into the engine. They should be set at the same number of turns out from all the way in (usually 2-3 depending on upgrades).

Another solution may be that one of your carb jets is dirty and needs to be cleaned. It's a pretty painless process depending on how you go about it. Depending on which model GS you have (89-02, or 04+) your carbs have 2 or 3 jets to get the fuel to to the engine. It's possible that given your bike is a 97 all you need to do is run some seafoam or other injector/carb cleaning agent through your gas. If that doesn't work, you may need to take off the carbs and give the jets a once over with a piece of wire.

[erk]

Sorry, the spark test was done while running, with the tester inline between the coil wire & the spark plug, at 9am (pretty damn sunny) I am unable to bring the bike into the shop for testing and must be done outside, ive tried googling the mixture screws and am unable to distinguish the difference between screws 

[salamander]

The screws you're looking for are recessed and near the bowl drains.  The recessed area may be blocked with a brass plug, so you might not actually see the screw heads.

[erk]

So it appears it is definitely running rich in the problem cylinder, put a new spark plug in and after just the one ride it was pretty blackened and fouled.

the thing I dont understand is, the bike ran great for about a month, then it ran out of gas once and had to switch to reserve for a little bit, and since then its been getting worse. I would check the mixture screws but I assume they dont just turn magically by themselves causing the rich condition. So a few questions:

1. when unplugging the spark plug (for the problem cylinder, or left side) the idle barely changes but performance was def. affected. When unplugging the wire for the other (right hand) carb the bike dies, should it not die when unplugging either one?

2. What could cause one cylinder to run rich? Someone suggested the float binding, but the old owner specifically referenced replacing them once, and all about how they are problematic, yadda yadda

3. What else should I test before just ripping hte tank off and staring at these carbs?

[RossLH]

First, I'd check the petcock. If the diaphragm is damaged, the left carb will pull fuel in through the vacuum line causing that side to run very rich. If that checks out, give the carbs the good old clear tube test to check the float heights. If that also checks out, its time to pull the carbs off and dig in.

[salamander]

the thing I dont understand is, the bike ran great for about a month, then it ran out of gas once and had to switch to reserve for a little bit, and since then its been getting worse.

Like Dizzledan mentioned, you may have something plugging up your carburetor.  Since the problem started the first time you used the reserve, maybe there was some trash in the reserve hose that got swept into the carbs.

1. when unplugging the spark plug (for the problem cylinder, or left side) the idle barely changes but performance was def. affected. When unplugging the wire for the other (right hand) carb the bike dies, should it not die when unplugging either one?

If your current idle is the result of only one cylinder functioning well, then pulling the plug wire on the bad cylinder wouldn't really make much difference.  Pulling the wire on the good cylinder doing all/most of the work ... different story.  I've seen a number of posts on the forum that these bikes can run on one cylinder, just not all that well.

[mister]

This morning, same situation but let it warm up for 20min full choke, still was idling at 2k as opposed to the normal 3500 or so during the day, almost as if the choke was barely even working, next to no response in engine speed as the choke was twisted to "full".

Why are you letting the bike warm up for 20 minutes or whatever before riding it?
While you let it idle in your driveway for this length of time did you give any input to the throttle?

Here is my warm up routine:

1 - Put choke on full
2 - Start bike, give Small throttle blip as soon as it starts and let rev to whatever it wants to rev to
3 - put bag on bike put on jacket
4 - reduce choke until it revs at 2000 rpm
5 - leisurely put on helmet and gloves
6 - sedately ride off
7 - a couple of clicks up the road turn off choke entirely.

Note: when the bike starts getting hard to start when the engine is cold, check the valve clearances.

Michael

[erk]

And how would I go about checking the petcock? It moves with vacuum applied, correct?

[RossLH]

Remove the petcock, put it in the on position, and blow into the main fuel line. If air passes through the petcock, its bad. Mine was bad to the point in which fuel poured out of the vacuum line when I removed it.

[erk]

i am leaning more towards the petcock now because when I ran out of fuel, I switched over to RES but it only got me like 2 blocks of spitting and sputtering in the city before it completely died, def not the range it should of.

And you are talking about the frame mounted petcock correct? Not the one under the tank? And when taking the petcock on, I should ideally pinch off the fuel supply hose or it will just dump fuel all over right?

Sorry for the noob questions

[RossLH]

Shut off the tank petcock first. The fuel lines will pour out whatever fuel is left in them; I usually just collect it in a bottle and dump it back into the tank.

And yes, the frame mounted petcock is what I'm suggesting has gone bad.

[erk]

So I tested the petcock, and got the following results:

Main fuel line from tank petcock to frame chicken has full flow in prime position (duh)

No flow in the on position

but what was odd, with the frame petcock 1/2 way between res & on, there was flow between the 2 lines coming from the tank (ON & res)

So I am guess the petcock is not my issue, what what could those finding mean? Is this normal?