GS550 Carb Fun!

[drcoopster]

Good evening fellow GSers,

My recently-acquired GS550 starts and runs nicely, to a point. If I'm riding along, and I quickly open the throttle fully--in any gear--the bike will misfire and bog at about 4500 RPM. If I slowly open the throttle, I can work it to 5500 RPM before it bogs down and misfires. It doesn't cut out completely, just refuses to rev any higher.

In neutral, the bike will rev fine unless I snap the throttle to full, in which case it will bog and misfire and go kind of wonky.

A member of the GS_Bikes Yahoo group suggested that I ride with it bogging down for a few seconds and then cut the engine, pull in the clutch, coast to a stop, and check the plugs. I did that, and *all* of the plugs were black and sooty. The plugs weren't noticably wet. But all four were equally black and sooty (after riding around without bogging, the plugs come out fairly light-colored; it looks like I'm a tad on the lean side). I am hesitant to think it's not an ignition problem because it seems to happen at a specific manifold pressure, not RPM. If I ease on the throttle, I can consistently get it up to 5500 RPM, but if I just suddenly apply full throttle, it will cut out at around 4500 RPM. And it does not exhibit the behavior when there's no load -- but note that manifold pressure will not get very high when there is no load.

Am I just running really rich? Should I go to a smaller main jet? I was thinking about trying to run it without the airbox and see if it still acts in the same way; perhaps it was jetted for pods before I received it. Right now it has the stock airbox. The jets that are in there currently (all 4 are the same size) are labeled 92.5 -- I assume this is the jet size.

Are these the right size main jets for a stock '81 GS550L? Am I running really rich due to incorrect jets or needle settings, or is this an ignition problem?

Thanks in advance!

-Ian

[chef1366]

The stock main jets are 92.5, pilot jets 40 and pilot air 150. If not low battery or ignition problems it sounds like a dirty air filter or an obstruction of some kind not allowing air to flow properly. Try cleaning your pilt air jets. If this all checks out try dropping your needle jet one notch if adjustable. Did you check your diaphragms? What is your adjustment on your idle adjustment screws?

[drcoopster]

The stock main jets are 92.5, pilot jets 40 and pilot air 150. If not low battery or ignition problems it sounds like a dirty air filter or an obstruction of some kind not allowing air to flow properly. Try cleaning your pilt air jets. If this all checks out try dropping your needle jet one notch if adjustable. Did you check your diaphragms? What is your adjustment on your idle adjustment screws?

Okay, so it looks like I have stock jets. The carb is clean. The diaphrams are good. How do I adjust the needles? Looking in the top of the CV slides, I can see a little circlip, does that mean it is adjustable? I'm not sure where the idle adjustment screws are, but the main idle adjustment screw in the middle of the rack is not far in at all; the throttle plates seem to close about fully at idle. In other words, it's not holding the butterfly valves open to artificially correct for a blockage.

The airfilter looks good but I'll try without it (still with the stock airbox) and see if it may be causing problems.

-Ian

[chef1366]

The screws are on top of the carbs right where they enter the intake manifold. They might have a cover on them which looks like a circle with a small hole in the center. These can be removed to get to the screws with a drill bit and a wood screw. Be careful when drilling and don't go to far and damage the screw head. If the covers are still on they probably are still set from the factory. To remove the circlip to get to your jet needle you will need long sliim needle nose pliers that the ends fit into the holes of the circlip. The needle itself will have notches on the end with the clip. To lower the needle you will move the clip up one notch. (this is if you have an adjustable needle)

[drcoopster]

Okay, cool. I've seen those screws, there's no cover or anything. I'll see what the setting is and let you know. Does anybody have a picture of what the emulsion tube *should* look like?

-Ian

[drcoopster]

I lowered the needles in the CV slides and it still bogged down at around 5000, though I could work it up to nearly 7000. The bogging down must have to do with the amount of air/fuel in the cylinder. Maybe the spark plugs are not gapped correctly? I did not change the gap when I took them out of the box, they are approximately at 0.030". The type of spark plug is NGK B8ES. Is this reasonable for a 550? Also, can someone with a 550 with the Mikuni CV carbs (80-82, I believe) measure the length of the needle as it protrudes from the bottom of the slide? Thanks.

-Ian

[drcoopster]

Is there a way to test the ignition for a good spark? The plugs are brand new.

[chef1366]

I adjust my plugs at 0.25 inches. I do have an 1100, but I use the same plug. You can check spark by removing the plug and attach it to your spark plug boot and groud it to your head and hit the starter button. You should see a spark jump in the spark plug gap. I'm sure some electrical gurus will jump in with the correct readings.

[stanski68]

carbs

I have an 81 550t with very similar problems, please let me know if and when you find a solution. mine does the same thing with no filters on the carbs, sooty plugs,..... acts like it is starving for gas..., revs in neutral,
I have cleaned the carbs twice, cleaned the tank and petcock ect.
rejetted to 102.5.
Good luck, I hate these carbs.....at least until we figure this out

Thanks, Steve

[drcoopster]

I adjust my plugs at 0.25 inches. I do have an 1100, but I use the same plug.

I assume you mean 0.025"; a quarter-inch gap is on the far side of "gaping". I'm going to try and regap my plugs to 25 mils and see how it turns out. The fact that it's happening in all four cylinders is really bugging me.

You can check spark by removing the plug and attach it to your spark plug boot and groud it to your head and hit the starter button. You should see a spark jump in the spark plug gap. I'm sure some electrical gurus will jump in with the correct readings.

I know that it is getting a spark at some point since it does run smoothly and pull hard up until 4500-ish RPM (in any gear). Testing for a spark in the manner that you describe will confirm that you're getting a spark at virtually no load, but still leaves the condition undefined under load. We have a spark plug tester here in the lab but that will only test the plug (which is virtually new).

-Ian

[Roadwarrior]

My 78 GS550 never ran right until I installed a set of coils off of a 2001 Katana on it. It would bog, get poor milage, occasionally foul plugs, and feel gutless. I always had a spark, but it must have been a week one because the new coils woke the bike up.

[drcoopster]

Do you know if the spark was uneven, i.e. one coil bad? Or do the coils generally deteriorate evenly?

My 78 GS550 never ran right until I installed a set of coils off of a 2001 Katana on it. It would bog, get poor milage, occasionally foul plugs, and feel gutless. I always had a spark, but it must have been a week one because the new coils woke the bike up.

[Nerobro]

Sounds like you removed the airbox.. are you running pod air filters?

[drcoopster]

Sounds like you removed the airbox.. are you running pod air filters?

Stock airbox, stock filter, like I said in my original message.

[drcoopster]

I tried gapping the plugs *wider* to about 40 mils but that did not change anything.