Totally lost with my GS550

[Guest]

First of all, I want to thank everyone for all of the help this forum has been thus far on this journey. Now for the details

1981 GS550T, all stock except for a K&N in the airbox. When I got it it ran not so good
-Shimmed the valves
-New o-rings in all four carbs. Totally disassembled, dipped, etc.
-New intake boots and o-rings
-New plugs
-Changed the oil

After all of that, it was running quite well. It seemed lean, but nothing dangerous. With this newfound confidence I decided to try out a Kerker 4-1 I acquired awhile back before I even had a GS. Install went fine, and it even ran pretty good, but broke up a little bit in the mid range. Instead of just putting the stock pipes back on, I decided I was going to try jetting up a hair for this new exhaust. I thought it might even be the answer to it running lean on the stock pipes. When I had the carbs apart previously I noted that it had all stock jetting, despite a PO previously stating that it was jetted. So 92.5 in the mains, and 40 for the pilots.

I drove all over Texas trying to find 97.5/100 mains, and 42.5 pilots. I finally found the last of them this afternoon, and came home to put everything together. Put the carbs on the bench, installed new 97.5 main jets and 42.5 pilots, double checked the float height, and everything seemed to be in order. Put the carbs back on the bike, got it fired up, let it warm up awhile, set the idle mixture with a colortune, and decided she was ready for a road test.

That's where it all went to crap. Idle was fine, any throttle at all made it break up like mad. WOT with the choke pulled was somewhat decent. Got it up in the revs, killed it, pulled the plugs and it appears to be WAY lean, which seems very counterintuitive having added a decent amount of fuel to the equation.

Tomorrow I'm putting it all back to stock except for the pilots, one of the stock 40s was broken, so I don't have a full set to go back in.

Can any of you think of something I might be totally overlooking?

[Nessism]

I'd try stock pilots and make sure the pilot plugs are in place. Your larger mains should be fine. Reducing the plastic spacer height above the needle is also a good idea. Check the carb rebuild tutorial for specific info about that mod. Do these things and good chance it will run better.

[05Astro]

Could it be possible that the oe had drilled out the stock jets bigger than marked? And When You went bigger, you really went backwards?

[Steve]

Could it be possible that the oe had drilled out the stock jets bigger than marked? And When You went bigger, you really went backwards?

That would be my guess, as well.

And why drive all over Texas, when you can have jets delivered to your mailbox?

Jets R Us even offers a choice of Genuine Mikuni or aftermarket jets.
Although they cost a bit more, Mikuni jets are preferred.

.

[leecatd8209]

Could it be possible that the oe had drilled out the stock jets bigger than marked? And When You went bigger, you really went backwards?

This is my thought, but they'd have to be pretty huge. Seems too big for it to still be running lean with everything stock. Does the K&N make that big of a difference.

The pilot plugs are in, and new.

I drove all over because I'm kind of on a time crunch, and wanted them faster than the mail can deliver. I have an order in with jetsrus, but some stuff came up.

[Nessism]

I had a 550T and it ran perfectly with stock jetting. Don't know why anyone would drill the jets. The K&N could make it run a little lean but nothing crazy. You could always try taping up the filter some to restrict flow and check mixture.

[roeme]

(...)Got it up in the revs, killed it, pulled the plugs and it appears to be WAY lean, which seems very counterintuitive having added a decent amount of fuel to the equation.

Can any of you think of something I might be totally overlooking?

Uhm...this sounds familiar.

If it's really lean: Needle valves or fuel system obstructed. I suppose you've already eliminated air leak as a cause, and since you've had the carbs on the bench, surely they're clean enough.

If it's rich: Binding floats maybe...or needle valves also obstructed.

Encountered the similar symptoms on my bike on the highway, weak power, hesitating. Add choke, power at WOT is a little bit better...but the overall power envelope was wrecked nonetheless.
In the end, the problem was debris in the fuel system. That prevented the needle valves from closing properly, so I got fuel spillage at red lights (nervous moments!), yet caused insufficient fuel flow under other circumstances.

It was hard to debug for a newbie, since symptoms were all over the place. I had the carbs off more than a few times until I traced the root cause: a dislodged filter element of the petcock. This allowed dried out gasoline flakes and various other debris to get into the carbs.

[smilinbrad]

you only took the mains up one size. with 4 into one and a K & N you might need more like maybe 3 steps up which I recall would be about 107's. My 550 came with a K & N filter and that alone made it so lean it couldn't make the mid throttle transition off the needle.
a suggested jetting guideline for Mikuni's can be found on-line if you look. I found it once

[leecatd8209]

you only took the mains up one size. with 4 into one and a K & N you might need more like maybe 3 steps up which I recall would be about 107's. My 550 came with a K & N filter and that alone made it so lean it couldn't make the mid throttle transition off the needle.
a suggested jetting guideline for Mikuni's can be found on-line if you look. I found it once

3 steps would be a 100, they go by 2.5

[leecatd8209]

Uhm...this sounds familiar.

If it's really lean: Needle valves or fuel system obstructed. I suppose you've already eliminated air leak as a cause, and since you've had the carbs on the bench, surely they're clean enough.

If it's rich: Binding floats maybe...or needle valves also obstructed.

Encountered the similar symptoms on my bike on the highway, weak power, hesitating. Add choke, power at WOT is a little bit better...but the overall power envelope was wrecked nonetheless.
In the end, the problem was debris in the fuel system. That prevented the needle valves from closing properly, so I got fuel spillage at red lights (nervous moments!), yet caused insufficient fuel flow under other circumstances.

It was hard to debug for a newbie, since symptoms were all over the place. I had the carbs off more than a few times until I traced the root cause: a dislodged filter element of the petcock. This allowed dried out gasoline flakes and various other debris to get into the carbs.

I'll definitely take a look at them again while they're off. I did remove the in line fuel filter I had in there due to reading that they can cause problems as well.

[Steve]

I did remove the in line fuel filter I had in there due to reading that they can cause problems as well.

Good move there.

Some filters will work OK, as they are designed for lawn tractors, which also have gravity-fed fuel systems.

The main problem is in sizing the fittings. Most filters are available with 1/4" or 5/16" fittings (6.25 or 8mm), while the bike's fuel fittings are 7mm.

.

[smilinbrad]

read up on it. 2.5 is a half step

[leecatd8209]

It seems that the stock jets have been drilled out substantially. In stock form with the K&N installed the bike runs pretty good, but has a rich bog and virtually no acceleration at WOT.

They've been drilled out to larger than any jets I have on hand. I might order some 105s or larger to try and make the Kerker work. For now it's ridable, but my local riding needs seem to warrant a larger bike.

[Chuck78]

You also need to raise the slide needle height as mentioned above. I thought this was done primarily by shimming underneath the needles, but the mention above said to reduce the thickness of the plastic spacers above the needles. They have springs under them, so this makes sense. I only have one bike with CV carbs (GS425), so I'm not quite a tuning expert on CV's, but that is fundamental carb knowledge there. Almost all of your riding, aside from wide open throttle, is metered by the slide needle or also it is called the jet needle. The main and pilot jets only come into play at the lowest and at the higher throttle opening positions (75% throttle or more, and just off idle).

The GS550 shouldn't be too small of a bike for your riding unless you are of substantial stature or carry a passenger. They are great bikes and pods/pipe really wakes them up. Dynoman performance makes a 630cc big bore custom order piston kit for them, and Megacycle makes performance cam grinds for them, both of which will really wake up that bike substantially if you decide that you need more acceleration.

Best of luck

Chuck on Ohio

[Steve]

You also need to raise the slide needle height as mentioned above. I thought this was done primarily by shimming underneath the needles, but the mention above said to reduce the thickness of the plastic spacers above the needles. They have springs under them, so this makes sense.

This is very true. The springs under the clip just push the needle into place, the spacer above the clip determine the height. To raise the needle, you need to reduce the thickness of the spacer, which is usually done by replacing the plastic spacer with multiple thinner washers. It is common to need 5 washers to equal the thickness of the plastic spacer, so installing just 2 or 3 washers will raise it about the same as one notch on an adjustable needle.

.