1 carb rich, 1 carb lean, low mpg, tricky idle

[gratinox]

Hi, I'm new. I've trolled the forums but can't seem to find the solution to my problem.

I bought my '96 GS500E a few months ago, accepting the fact that it was going to come with a few minor issues. A few of them I fixed simply by cleaning the carbs, but I have a few left. Overall, the bike gets good smooth power.

1. Low mpg. I get about 35 mpg.
2. Strong gas smell most noticeable at startup even without the choke on.
3. Carb1 runs very rich. The spark plug is pretty black. Carb2 runs very slightly lean.
4. The idle is finicky. Even after a good 40-60 minutes of riding, the idle will still be below 800 RPM. I'll turn up the idle to 1500, and get the hang. After a while, the bike stays consistently at 3K RPM until I turn it back down. It stays consistent at this point, until I turn it off, let it cool down a bit, and then start riding again, where the cycle starts all over.

What I've done:
1. Cleaned the carbs. Rejetted pilots with the recommended 40. Turned screws out by three turns on the lean, 2.5 turns on the rich.
2. Checked for vacuum leaks with carb cleaner. Sounded good.
3. Checked float heights.
4. Checked airbox filter-- it's clean.
5. Cleaned petcock filter.
6. Checked and replaced missing o-rings.
7. Checked carb springs.

Things I might need to do:
1. Timing?
2. Balancing?

Any suggestions would be awesome.

Thanks,
Rachel

[romulux]

Dirty side isn't firing well.  Diagnosis:  Electrical or spark plugs.

[The Buddha]

You didn't do mains ? Just pilots ?
If you've really checked all these out and are confident ... especially vacuum leaks - left being lean is definetly 99% vacuum leak ... and float level - both need to be equal, measured by U tube method, and to up the ante, do it with it idling ... And startup gas smell = overflowing floats - look in the airbox for liquid gas before start up. Find any and floats are leaking.

OK set air screws back to 3 before start up test ...
That leaves just 2-3 possiblities. In the order of likelyhood - Oh, synched the carbs ? Eyeball synch is fine, vacuum is OK too.

1. One carb has an air screw that has been driven deeper into the body and hence has opened it out and they need to be set to where they will burn right, so set em unequally to get the same burn color in the plug and dont worry about it.

2. Exhaust leak. Yea find this no matter what.
3. Valves need to be adjusted on 1 cyl. How many miles since last check.
4. Rings dont seal on 1 cyl. How many on the bike ? you try a compression test ? how is oil useage ? Your 35 mpg sounds like it could have 30K+ miles.

Take your pick ... but I'd revisit everything. Float level especially.

Cool.
Buddha.

[gratinox]

Thanks for your help.
  Yeah, I only switched out the pilots, the main jets are stock. I checked the floats with the utube (neat method, thanks) and they were spot on. Tomorrow I'll check again for vacuum leaks and I'll try adjusting the richer pilot screw.  What do you suggest squirting around the carbs to check for leaks?  wd40 or something else?  I showed the "lean" plug to a friend and he said it looked about right, not lean as I had thought, just tan in color.

Definitely no exhaust leak. I don't remember the #'s offhand but I did a compression test ~1 month ago and it was within spec. Oil usage seems normal but I've only had the bike for three months. This bike only has ~8K miles on it.

[romulux]

You can use WD40, starter fluid, etc. for leak checks, but I've personally never had any luck with that at all.

Professionals hook carbs up to a smoke machine and pump smoke through the system at low positive pressure.  Smoke is visible from any leaks.

I would get a few new spark plugs to have on hand.  Replace the fouled one with a new one.

Take the plug out of the hole and rest it against the metal cylinder head.  Unhook the other plug so it doesn't start.

Use the starter button and see if you get consistent, good sparks.  You may need to hold the rubber boot and push the threads against the cylinder lightly.

I'd be interested to see the results of the spark check.

[black and silver twin]

might be time to do valve shims, if those are out of whack it can cause funky idle symptoms and bad gas mileage.

[aero_b]

Hey there,

I know this is an old thread now, but in case you haven't found the problem yet, it might be a leak in the petcock. If the one that is running rich is the one that the vacuum hose (the smallest hose connecting the petcock to the top of one of the carbs) connects to, then your problem is most likely a leak in the petcock. If you've ever opened it before, it's the rubber diaphragm that is leaking, either through a small hole or around the edge.

Easy way to check: ride it until it starts going stupid, then pull over, turn it off, and pull the vacuum hose at the petcock; if you have a naked GS500 this will be easy, I'm not sure how hard it might be on a dressed up one. If the hose has fuel in it, there's your problem.

Solution 1: buy a new petcock (diaphragms are NOT sold separately, which is VERY frustrating).
Solution 2: modify it by punching a bigger hole into the diaphragm, cutting the vacuum hose and plugging both ends. What this does is allow fuel through the petcock ALL THE TIME, even when the engine isn't running (the vacuum hose usually causes the diaphragm to pull open and allow fuel through only when the carbs are creating a vacuum, i.e. when the engine is running). When fuel is allowed through all the time, the only thing stopping it from overflowing the carbs and dumping fuel into the cylinders are the float needles, but that's what they're for anyway. Just turn off the fuel at the tank when storing the bike for a long time, just to be safe.

[gratinox]

Hot damn dude, I think you're right. I had to stop riding the bike for about a year/ year and half due to illness. I had completely re-built the carbs and I was STILL having the issue. It got worse from what it was a few years ago, so it was no longer really fun to ride. Checked the vacuum tube out of the petcock and sure enough, it is wet inside and smells like gas. I'm buying a new one tonight. I can't believe it!! I never would have guessed it was the petcock leaking!!!!!! I thought for SURE it was a vacuum leak.

[yamahonkawazuki]

Thanks for your help.
  Yeah, I only switched out the pilots, the main jets are stock. I checked the floats with the utube (neat method, thanks) and they were spot on. Tomorrow I'll check again for vacuum leaks and I'll try adjusting the richer pilot screw.  What do you suggest squirting around the carbs to check for leaks?  wd40 or something else?  I showed the "lean" plug to a friend and he said it looked about right, not lean as I had thought, just tan in color.

Definitely no exhaust leak. I don't remember the #'s offhand but I did a compression test ~1 month ago and it was within spec. Oil usage seems normal but I've only had the bike for three months. This bike only has ~8K miles on it.

When you cleaned carbs, was it an overnight soak?,  or a spray can?

[gratinox]

I've cleaned the carbs since then... They've honestly always been absolutely spotless inside, so I've never done a full soak. I'm also careful not to get any cleaner on the o-rings or gaskets. The carbs are clean, the needles are new, ect ect.

I put in new plugs, checked the spark - we're doing great on both sides. OK, it's not the spark.

Valves are in spec. Compression hasn't changed.

The issue that I had was getting worse so I finally had taken it apart this past weekend. I figured it was some vacuum leak I'd never be able to locate, but even a vacuum leak didn't really explain what was happening. I noticed that at stoplights, the idle would surge. I finally connected this to the fact that I lean the bike to the left (the rich side and petcock side) when at a light because I rest my right foot on the brake. I'm short, 5'3" on a good day, so flat footing with both feet isn't an option for me. As soon as I righted the bike or leaned it to the right, the idle would go back down to normal.

At this point I figured it could be the float getting stuck open (weird angle making something slip? who knows...), so I ran the bike until it was doing this again, leaned it, shut it off, then checked the float height. Still perfect. I took it apart, leaned it around at all angles, and it moved as intended. Damn. I was ready to convince myself that it was a sticky float.

I checked the carb balance... at idle, it was perfect. At throttle, however, my left carb just went nuts.

Turning in the mixture screw on that carb did near to nothing, which was really weird. The right carb works as expected.

Anyway, I finally came back to this post and saw the petcock thing. There's gas (not a ton, but it's wet) in the vacuum line. Tonight I'm going to throw the petcock in prime, and block off the vacuum ports on both the carb and the petcock and see what happens.