GS550T running lean, what else to check?

[sacruickshank]

Resurrecting a GS550T with Mikuni carbs and still getting an apparent lean condition. I'm using the throttle cable adjustment to easily test idle settings since the idle screw is not easily accessible with my big hands. I can get it to idle at ~1.5k rpm, but the slightest throttle touch makes the revs go up to 3-4k rpm and hang there until I back off the throttle cable.

Current status of work is:
- Carbs have been off, jets removed, ultrasonically cleaned in pinesol&water mix, passages blow through with compressed air, WD40 soak, etc. This is not my first carb cleaning.
- diaphragms in place and I didn't see any rips or pinholes
- rubber stoppers over the pilot jets are in place, but they are a little old/hardened
- float height verified in spec, ~22mm
- air mixture screws (top, engine side of carb) were removed, cleaned, and have new orings. They are currently set at 1 turn out
- carbs were bench synced, but not yet synced while running. In my experience, bench syncing usually gets it close enough, then running sync will fine tune.
- stock airbox, no pods for me. Boots not cracked and all connections seem solid
- Intake manifolds in decent shape, but some paint/coating has peeled.
- Orings on intake manifolds still intact and still round enough to offer some sealing to the cyls. I can feel the bump of the oring when running a fingertip over the flat surface of the manifold
- valve clearances are mostly good, but 1E, 3I, and 4I are still a little tight
- new spark plugs
- original air filter in place for previous tests, but will replace with new oiled foam element today.

Am I missing something else to check?
Should I try setting the mixture screws to 1/2 or even 0 turns? Or will that make the system even more sensitive?
Can the peeled coating on the intake manifolds cause air leaks? I have not yet tried squirting WD40 or other substance on the manifolds while running to see if that causes a rev surge

Thanks
carb.jpg

[Redman]

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- Orings on intake manifolds still intact and still round enough to offer some sealing to the cyls. I can feel the bump of the oring when running a fingertip over the flat surface of the manifold.
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Intake boot o-rings was my first suspect based on your description, especially if it get worse when engine hot.
Your description of the orings seems good. But next time have carbs off, try a new set.
See Cycle ORing web site (a fine member here) for a set of orings and maybe the S.S. allen bolt set.

I had occasion of hanging idle, like you describe but worse, and went through all the common suspects, boot Orings, new boots. Eventually I determined it was the orings around the carb enricher. Which required I break the carbs apart from the rack to replace those orings. (Previously I replaced all the other orings but those.)
I understand the carbs on 550 are different, so I dont know if have the same situation there.

[Redman]

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Can the peeled coating on the intake manifolds cause air leaks? I have not yet tried squirting WD40 or other substance on the manifolds while running to see if that causes a rev surge

///////EndQUOTE

My experience has been that the intake boots (against the head) may seem okay, dont notice an cracking or such.
But then get a new set (yah, not cheap) and see how soft and rubbery a new set is. Then realize how hard and stiff the old ones are and probably not sealing well, especially if the clamps are stretched out from being tightened too much - because was suspect air leak.
Get new clamps too.

And with new boots, reinstall the carbs was so, so much easier.

[Nessism]

New intake boot O-rings for sure, and it sound like you need new boots too.
Pilot screw should be open further. Try 3 turns for a start and go to 3.5 if necessary.
Vacuum sync affects idle smoothness and the hanging idle factor.

The carb O-rings should be fresh and pliable. They should be replaced when cleaning the carbs.

[sacruickshank]

Intake boot o-rings was my first suspect based on your description, especially if it get worse when engine hot.
Your description of the orings seems good. But next time have carbs off, try a new set.
See Cycle ORing web site (a fine member here) for a set of orings and maybe the S.S. allen bolt set.

I had occasion of hanging idle, like you describe but worse, and went through all the common suspects, boot Orings, new boots. Eventually I determined it was the orings around the carb enricher. Which required I break the carbs apart from the rack to replace those orings. (Previously I replaced all the other orings but those.)
I understand the carbs on 550 are different, so I dont know if have the same situation there.

I did buy the oring kit, intakes, and ss bolts from that source, but there was a mix up in either shipping or ordering, so i re-used the originals . Will replace the intake orings/gaskets next time I take the carbs off.

I did not replace the orings in the choke (carb enricher) since the brass nuts were being stubborn, but also not sure how that could introduce more air. The choke mechanism seems to be functioning fine, at least by feel.

[sacruickshank]

Thanks all for the quick response. Some answers:
- Yeah, those intake manifolds were hard. I gave them a squeeze when the carbs were first off and they felt like they still had some pliability left, but getting the carbs back in was a major pain. After far too long wrestling with them the only solution was a heat gun. In hindsight I should have softened them up with wintergreen. I'll look for replacements, and hopefully can find ones with fresh rubber.

- I'll try three turns out on the mixture screws, but I thought that would increase the air since these are on top of the carb. Do they control idle fuel as well?

[sacruickshank]

Well, the mixture screw seems to have almost no effect. 3 turns out, revs at ~4k rpm. 0 turns out, still revs at ~4k rpm. Haven't tried 4+ turns out, but done testing for the night.

I've seen similar behavior on other bikes when the floats were set way too high, but they seemed correct on this bike when I checked them.

Looks like I'll have to find the vacuum leak somewhere else.

re: intake manifolds, does anyone have experience with these aftermarket options ... https://www.amazon.com/MOTOKU-Carbur...7986604&sr=8-3

The OEM ones are still available, but $120+ for all four.

[timebombprod]

You're going to need to have the throttle cable adjusted properly, it will make a HUGE difference running at idle off the idle turn knob strictly. Boots should be a hard fit, if they weren't, they'd leak! If you have some long arm pliers you can reach the adjustment knob no problem, I actually keep a pair on my bike but soon I shouldn't have to.

Get the idle adjustment set properly and make sure the throttle cable routing is smooth. Also try a water test, spray header pipes with water when bike is warm to see that all 4 are hot and steaming. If any aren't steaming, boom you got a lead on seeing if the spark is weak, or if fuel isnt getting to the cylinder, but the spark will let you know if you pull it out.

[sacruickshank]

You're going to need to have the throttle cable adjusted properly, it will make a HUGE difference running at idle off the idle turn knob strictly. Boots should be a hard fit, if they weren't, they'd leak! If you have some long arm pliers you can reach the adjustment knob no problem, I actually keep a pair on my bike but soon I shouldn't have to.

Get the idle adjustment set properly and make sure the throttle cable routing is smooth. Also try a water test, spray header pipes with water when bike is warm to see that all 4 are hot and steaming. If any aren't steaming, boom you got a lead on seeing if the spark is weak, or if fuel isnt getting to the cylinder, but the spark will let you know if you pull it out.

Thx for the feedback. I've likely already eliminated those possible causes.
- Since the original post I've set the throttle cable and idle screw adjustment to completely "off", so the butterfly valves should be completely closed. Still getting the high idle
- Agreed that the intake boots should be a tight fit, but I've done enough other projects to known how much effort it normally takes and these were way beyond that.
- I use ice cubes rather than water spray and the pipes are all ~equally hot, so all 4 cyls seems to be firing.

[Nessism]

- Since the original post I've set the throttle cable and idle screw adjustment to completely "off", so the butterfly valves should be completely closed. Still getting the high idle

Clear indication of air leak.

[sacruickshank]

Clear indication of air leak.

Unless I screwed up royally when measuring the float height (which I doubt since I've screwed it up on other projects and am super careful now ... famous last words), then I agree it's an air leak. Now I just need to find it and fix it.

I just thought of one more variable. The valve cover breather tube has shortened due to age. It is connected to the valve cover, but not the airbox so I have duct tape covering airbox's mount to prevent excess air intake.

When running, the breather tube is pushing air out. I'll get a longer tube and don't believe the breather to open air should cause the high idle since many people just run breather filters.

Does that info help at all?

[Nessism]

Fuel level or the breather can't cause a high idle where the carb butterflies are all the way closed.

[timebombprod]

Thx for the feedback. I've likely already eliminated those possible causes.
- Since the original post I've set the throttle cable and idle screw adjustment to completely "off", so the butterfly valves should be completely closed. Still getting the high idle
- Agreed that the intake boots should be a tight fit, but I've done enough other projects to known how much effort it normally takes and these were way beyond that.
- I use ice cubes rather than water spray and the pipes are all ~equally hot, so all 4 cyls seems to be firing.

How's the airbox sealing if you have one, I'd just connect the breather filter to the airbox.


How's your idle sound when it actually stays at idle? If it's anything other than a constant drone something would most likely need work, does it go to high idle after being on a certain amount of time, like when it's all warm?

[sacruickshank]

Short story - Nothing worked. Time to take the carbs off again and see what's what.

Long story -
Replaced 3 stretched band clamps for the intake manifolds with new hose clamps. the previous ones could be rotated on the manifold
Created a gasket to seal the airbox cover
Positioned the breather hose correctly
Looked for visual airleaks/gaps in the connection between the metal and plastic airbox.
Started OK the first time, but went straight to 4k rpm
Shot some WD40 on the intake manifolds and it then shot up to 6k rpm. Looks like a leak!
Completely closed off the air mixture screws ... still 6k rpms. I didn't let it run long enough to see if it would come down
Put the old air filter element back in, thinking maybe the material was denser than my new aftermarket (and oiled) foam filter
Stuffed paper towels in the airbox intake. No change.
Accepted that the carbs have to come off again and shuffled the garage around to put this one back on the lift. Bummer.

Thanks for all the suggestions.

[steveb922]

Throttle cable binding? Wouldn't explain the increase in revs with WD40, but may be worth checking anyway.