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Carb tuning can't get the idle under 4000 without choking

  • Thread starter Thread starter dnsmith
  • Start date Start date
D

dnsmith

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I'm new to this forum and to my 78 GS550. I took it as a salvage for 20 bucks. I rebuilt the carbs replaced the points and coils and got it running. It has good compression but the carbs are driving me nuts. I am trying to get them dialed in as best as I can. Currently to try and fix mixture issues I made some adjustments to try and get to basic factory specs. My new problem is the idle won't go down under 3000 to 4000 rpm. I have tried putting everything back to before I tried to correct the inconsistent idle and hesitation. It seemed as if one or two cylinders would stop running well and it would almost die. I would adjust the ilde speed and it would come back up and then rev up high again and I was bouncing back and forth. Can someone shed some light on what my problem (besides my ignorance) could be.
 
Welcome to the forum ! It's hard when you don't know the history of the bike. I would start by replacing the O rings on the intake boots and take a good look at the boots themselfs while you have them off, replace if needed. Then go back & start from basics Bottom fuel screw out about 3/4 turn, side air screw out about 2 turns. Then do a mechanical carb sync with the carbs off the bike. Start with carb #3, back the idle screw out (not touching) Then adj the #3 carb (linkage under the top cover) until it just closes. (use a light) Then adj the idle screw up until a small 1/16 drill bit can be insered from the intake side. Adj the other three carbs to match..
 
You have a lean condition. Check everywhere for air leaks. Spray WD40 around all intake parts, boots, carbs, etc. If you have side air screws out too far (more than 2 1/2 or 3 turns) this can give similar symptoms, too high an idle speed). Follow SqDancer1's advice.
 
Thanks for the advice. The gasket on the top cover over the slides was torn so it was probably leaking. I am waiting for a new one to come in. As for the mixture screws, I had them way too far out about 2 turns. I'll check the O rings on the boots next.
 
I am trying to adjust the slides but on the cut away side it does not close all the way down. It closes off completely on the engine side but not on the incoming air side. Is there something wrong that can be adjusted or is the cutaway size wrong?
 
dnsmith said:
I am trying to adjust the slides but on the cut away side it does not close all the way down. It closes off completely on the engine side but not on the incoming air side. Is there something wrong that can be adjusted or is the cutaway size wrong?
Click to expand...

If you are talking about the throttle slide then that is the way the slide is designed and built. The engine side has the throttle slide closing all the way down and on the air filter side there is a cutaway (usually about 1.5) that lets a small amount of air into the mixing chamber (venturi) to mix with fuel from the pilot (idling) circuit. that is normal.
 
dnsmith said:
Thanks for the advice. The gasket on the top cover over the slides was torn so it was probably leaking. I am waiting for a new one to come in. As for the mixture screws, I had them way too far out about 2 turns. I'll check the O rings on the boots next.
Click to expand...

Wait, you're saying 2 turns is way too far out? Aren't normal GS's supposed to be 1.5 - 2 turns out? (Mine's 3.5 and could prolly use more but there's some jetting issues with mine)
 
The top cover gasket shouldn't change the idle.
You have start by eliminate things. The # of turns depends on which carbs you are talking about they are different VM22's on your 78/550 side (air) screw should be 1.5-2 turns, bottom fuel screw 5/8th to 3/4 turns
INTAKE boot O rings should be changed !
Throdle cable adjustment
Carb mech sync
Air filter assembly
valve adjustment
 
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I have found several problems thanks to all of your advice. I have gotten some important info in other threads as well. Turns out the o rings were toast and the previous owner tried to some sort of orange RTV on them which wasn't working. Secondly I busted off the tips of the mixture screws but luckily all the tips came out. I have replacements on the way for both the screws and the o-rings. Third the valve clearances were way way too tight. couldn't even get the .03mm feeler in there. Since it was running ok and I didn't run the engine much I am taking a chance that the valves are not burnt up since i don't want to tear into it that much. The compression is still decent about 100 on all 4. Chances are it might improve when I get the valves adjusted. Not going to bother with rings at this point. When its all back together it should run great.
 
good deal. where did you get the mixture screws?

i can recommend realgaskets for the valve cover gasket...dont know if they have one for your engine, but worth a look.

i danced around when i got my (good) compression readings....

keep us informed

greg
 
greg78gs750 said:
good deal. where did you get the mixture screws?

i can recommend realgaskets for the valve cover gasket...dont know if they have one for your engine, but worth a look.

i danced around when i got my (good) compression readings....

keep us informed

greg
Click to expand...

Mixture screws available from Z1 as part of a carb kit (screws are not available from the dealer as near as I can tell). http://www.z1enterprises.com/detail.aspx?ID=487

Real Gaskets for the 550 is not recommended since the tach gear is in the cover and the gasket is too thick (reduces tach gear engagement).
 
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