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'77 GS-550 carb screw settings

  • Thread starter Thread starter braindead0
  • Start date Start date
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braindead0

Guest
I was looking for specs for '77 GS-550 posted here in the 'Carb OEM spec per factory manual' thread, and I suspect the specs here aren't quite right.. or.. I'm mixing up the screws.

GS550L (77-79) the specs are for the VM's

air screw 2.0 turns back
pilot screw 1.0
Click to expand...
I think that's telling me to set the pilot air (screw on side top of carb, the one that's easy to get to) to 2.0 turns out, and the 'mysterious emissions screw' under the carb to 1.0. I set my carbs there, they are totally clean, floats are perfect, all new o-rings..etc... bench synced..etc.. And the bike will not idle at all without choke on. With the choke on, I could probably ride it (albeit a little rich) the mid-range and above circuits seem to be operating fine. I'm not running any in-line fuel filter, at this point the tank is prepped for paint so I'm using a fuel can that I've attached a fitting to for supply.. so I'm fairly certain I'm getting plenty of fuel (oh yeah, and I double checked..all carbs getting fuel).

Could it be as simple as..whoops, got them reversed? I thought I'd check here, perhaps someone knows.. before I go about spending a lot of time tweaking these screws. it's always so nice to have a good baseline first.

Of course, it's my own darned fault, should have checked them before I rebuilt the carbs and noted the settings..
 
Duh, nevermind... figures.. The bike ran pretty good when we got it, idle just fine..starting easy..etc.. So when it didn't, and I knew the compression was good..figured fuel...

Turns out a condensor died on me, and the points are a wee bit burnt anyway..time to replace. It should run a lot better on 4 cylinders ;-)
 
No, you didn't mix the screws up. Those settings should get you in the ballpark, then fine tune.
The side air screws should be adjusted using the highest rpm method. Adjust them after any pilot fuel screw adjustments.
If the bike is warming up in a reasonable amount of time, idles good (about 1,100 rpm) both cold and hot, pulls away without a stumble, rpm's return to correct idle after blipping the throttle, no dark exhaust while blipping, etc, then the pilot fuel screws (underneath) are probably set well.
 
Just so I know for sure.. I'm presuming the pilot screws underneath, unscrew for more fuel. The top screws I'm guessing are pilot air, ie: un-screw leans the mixture.

I got it running sorta okay today, good enough to sync the carbs and button things up for the winter. Still doesn't idle very well, and blipping the throttle hangs the r's which if memory serves me indicates lean mix. Perhaps I need to back out the pilot fuel screws a bit..
 
braindead0 said:
Just so I know for sure.. I'm presuming the pilot screws underneath, unscrew for more fuel. The top screws I'm guessing are pilot air, ie: un-screw leans the mixture.

I got it running sorta okay today, good enough to sync the carbs and button things up for the winter. Still doesn't idle very well, and blipping the throttle hangs the r's which if memory serves me indicates lean mix. Perhaps I need to back out the pilot fuel screws a bit..
Click to expand...
Yes, turning the pilot fuel screws out, richens the mixture.
Turning the side air screws out, leans the mixture.
Hanging rpm's usually indicate a lean condition or intake leak, as long as you know the throttle cables are adjusted correctly and the slides move freely.
 
Yeah, throttle cables are adjusted correctly, slides move freely, I sprayed down the intake boots with WD-40 to no effect, same on the boots to the air filter box. Freshly cleaned and oiled the air-filter..etc..already installed new o-rings on them, and their in really good shape, no cracks,etc.

I'm pretty sure it's just a wee bit lean, seems to run fairly well but will not idle too well below about 1400r's.

Thanks for the info.
 
Well, a poor idle could be the pilots and air screws not adjusted correctly, among other things.
Is this bike stock? Stock air box with the lid still on? Stock exhaust? If stock, the pilots may work anywhere from 1/2 turn out to 1 1/4 turns. If pipe/filter mods, up to 2 turns out is common. Then you have to adjust the air screws for highest rpm after adjusting the pilot fuel screws. Takes some tinkering.
You say the carbs are clean and I'll have to take your word for that.
The floats must be set correctly too.
The two floatbowl vent lines need to be clear and no kinks if stock. If pods/pipe, remove them and leave the ports open.
Also, a poor idle can still be synch related. A bench synch is no substitute for a vacuum tool synch. Just one carbs vacuum level off enough will screw up the idle quality. This bike should idle well at 1,000/1,100 rpm's.
Correct jets are important too. Have you verified the jetting?
Don't know what to think about the slow to return idle. Since you know the slides move freely and the cables are slacked right, that would narrow it down to intake/lean mixture problems. But you say there's no way you can have an intake leak. So hopefully the jetting can be figured out and fix this problem.
There is one other possibility. Be sure the ignition mechanical advancer is operating smoothly. Put a timing gun on it and read it for advance action and return action. Lube it if any sticking is evident.
If all looks good and you try tinkering with the screws, give them a large adjustment of about a 1/2 turn out more and test. Just keep records of all adjustments so you can put them back if the results are worse than expected. Let us know how it goes.
 
KEITH KRAUSE said:
Well, a poor idle could be the pilots and air screws not adjusted correctly, among other things.
Is this bike stock? Stock air box with the lid still on? Stock exhaust? If stock, the pilots may work anywhere from 1/2 turn out to 1 1/4 turns. If pipe/filter mods, up to 2 turns out is common. Then you have to adjust the air screws for highest rpm after adjusting the pilot fuel screws. Takes some tinkering.
You say the carbs are clean and I'll have to take your word for that.
The floats must be set correctly too.
Click to expand...
100% stock and the air-cleaner is fully assembled, I know how vacuum works ;-)..

I did a full rebuild on the carbs, new float valves/seats, all new o-rings (from cycleorings, that was handy). Float set withing spec.
KEITH KRAUSE said:
The two floatbowl vent lines need to be clear and no kinks if stock. If pods/pipe, remove them and leave the ports open.
Click to expand...
I left the float bowl vent lines off, as far as I've even experienced that shouldn't make a bit of difference (unless you leave the lines on and their kinked of course). Of course I could be wrong? Can't imagine an atmosphere vent requiring a hose, but perhaps there's something I'm missing.
KEITH KRAUSE said:
Also, a poor idle can still be synch related. A bench synch is no substitute for a vacuum tool synch. Just one carbs vacuum level off enough will screw up the idle quality. This bike should idle well at 1,000/1,100 rpm's
Click to expand...
I never trust a bench sync, did a vacuum sync and they synced extremely well.
KEITH KRAUSE said:
Correct jets are important too. Have you verified the jetting?
Don't know what to think about the slow to return idle. Since you know the slides move freely and the cables are slacked right, that would narrow it down to intake/lean mixture problems. But you say there's no way you can have an intake leak. So hopefully the jetting can be figured out and fix this problem.
Click to expand...
While rebuilding I verified all the jets and examined for any damage, everything seemed fine in that respect. As far as vacuum leaks, I hosed everything down really well with WD-40 and it had no affect on the rpm's, but I'll certainly not discount the possibility that there still is a leak somewhere.
KEITH KRAUSE said:
There is one other possibility. Be sure the ignition mechanical advancer is operating smoothly. Put a timing gun on it and read it for advance action and return action. Lube it if any sticking is evident.
Click to expand...
Now there's something I didn't think of, I did check the timing via the light-bulb method, but of course I could always use a timing light...
KEITH KRAUSE said:
If all looks good and you try tinkering with the screws, give them a large adjustment of about a 1/2 turn out more and test. Just keep records of all adjustments so you can put them back if the results are worse than expected. Let us know how it goes.
Click to expand...
Thanks, like I said I winterized it after getting it running sorta ok. As the tank is prepped for paint, I used a gas can with a fitting to supply fuel and I know at least at one point I had it up high enough with enough fuel to force fuel past the float valve on #1 (and yes, I had the vacuum line for the petcock plugged ;-). I lowered it to around where the stock tank would be and that went away, but I figured for optimal results I might as well wait for the tank to be painted over the winter. Plus with the tank on, I'll be able to take it out and get a better overall feel for the rest of the jetting, if it's running lean at WOT then I think that'll tell me there's something more wrong that just the idle circuit.

Perhaps though I'll double check the mechanical advance before I really put it away (where it's hard to get at)..

Thanks for the run-down of causes, it's nice to get a double check of all the things that are so easy to miss sometimes.
 
Did you make sure it was firing on all cylinders when you synced the carbs the first time?

My adventures in carb land (78-79 GS550's) have had all my pilot screws about 3/4 or so, never above 1 turn out. I'd have to look where my air screws were, I forgot. I have the original pilot jet size still, only increased mains for the pods and pipe.

I found I needed to mess with the air screws (close them in, I believe) to get my bike to idle off the choke, from what I recall doing in the past.

Have you looked at the plugs at all? Are you running really rich?

~Adam
 
Yup, all cylinders are firing (easy to check, just make sure all the headers pipes are getting similar temp).

As I'm still stock, the screw settings should be different than yours anyway..
 
OK. Just let us know if you need help when you get the bike back out.
At this point, when you bring the bike back out I would set those pilot fuel screws to about 3/4 to 1 turn out, then adjust the side air screws for highest rpm. You know how to adjust for highest rpm? The bike should idle well at 1,100 rpm's after these are set...correctly.
But first, be sure there's no intake leaks at the manifolds. The water mist or WD40 test doesn't always expose a leak. If it's been awhile, I suggest replacing the manifold o-rings (cheap). Give them a coat of high temp bearing grease and replace the stock Phillips screws with Allens and torque to about 6 ft/lb. Inspect the manifolds too. The o-rings should be replaced because they're probably flat/hardened/cracking. As long as the stock Phillips screws come out OK, this job is too easy to ignore. Your hanging idle may very well be the advancer or the pilot fuel/air screws out of adjustment, but I would automatically replace the o-rings too.
Test the pilot circuit at about 35 mph in 4th gear, just steady cruising. You want the bike running at minimal throttle position. Chop off and get plug reads. Do what the plugs/performance say. The pilot fuel screws are sensitive to adjustments so be aware. Always check the side air screws adjustment after adjusting the pilot fuel screws. May take a while, keep a record. If you find the pilot fuel screw for each cylinder is set a little differently, that's normal because of small differences in each cylinder. They should come out fairly close, if not even. Once jetting is correct, the bike should idle at around 1,100 hot or cold, warm up well, take off without any stuttering, no decel' pop, rpm's should return to set idle quickly without hanging (lean) or dropping below set idle/stalling when you lightly blip the throttle(rich). There should be no dark exhaust when you blip the throttle after idling.
Any questions, just ask. :)
 
Thanks for the tips, already replaced the manifold o-rings and installed hex cap screws (did that do all the carb screws as well). I'm pretty sure I torqued around 5-6ftlb..
 
AOD said:
I couldn't get an impact driver in on my carb boots last I tried... :(
Click to expand...
Yikes, why would you need to? I don't remember my original screws being that tight, although I did have the motor completly disassembled..
 
It's much easier if the engine is out of the frame...trust me.

Every screw that has not been loosened ever is ridiculously tight after years of thermal cycles.

~Adam
 
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