Gs650 carbs on a gs550

[BassCliff]

Hi,

It sounds like you are rich when cold but then as the engine warms up an air leak starts to occur, making you run lean (high idle, white plugs).

Mikuni BS(CV) Carburetor Rebuild Tutorial
(Mr. Nessism)

CV Carburetor Rebuild Guide
(Courtesy of GSR and John Bloemer)
(Click here to see the CV Carb Notes)

Where is the fuel screw? Is it the idle screw that's inbetween the carbs?

Note: Perhaps "Main Idle KNOB" would be a better description.

Bench Sync Your CV Carbs

Synchronize Carburetors
(With the Morgan Carbtune)

Morgan Colortune/Carbtune Tips
​

Above and below: Carb Tool in use during carb sync procedure.

Float Height Illustrations

***********Quoted from Mr. bwringer************
Every GS850 [and most other models] has (or had) a set of well-known issues that MUST be addressed before you have a solid baseline for further troubleshooting. It's a vintage bike, and it's quite common (as in, every single GS850 I have had contact with) that there are multiple problems that have crept up and slowly gotten worse over the years. It's not like a newer vehicle, where there's generally one problem at a time.

These common issues are:

1. Intake O-rings (install NEW OEM or Viton only - common nitrile O-rings will quickly deteriorate from heat)
2. Intake Boots (install NEW -- these cannot be repaired)
3. Valve clearances (more important than most people think)
4. Carb/airbox boots
5. Airbox sealing
6. Air filter sealing
7. Petcock (install a NEW one)
8. On '79 models, install new points or Dyna electronic ignition (or at least verify that the old points are working correctly)
9. On all models, it's fairly common to have problems with the spark plug caps. These are $3 or $4 each, and often worth replacing if you're keeping the stock coils/wires.
10. Stock exhaust with NO leaks or holes -- good seals at the head and at the junctions underneath.

What I have noticed at the rallies is that very, very few 850Gs are actually running right. Make VERY sure it's actually running the way it's supposed to before busting out the modifications.

Brian's E-Z and fun plan for GS850 happiness:

1) Seal the airbox and air filter with weatherstripping.

2) Ensure no intake leaks. Spraying WD-40 or water doesn't tell you much, since very small air leaks can cause problems even though they won't suck in enough WD-40 to make a difference. Replace your intake boot o-rings and boots if needed, and seriously consider spending the lousy $28 for new airbox/carb boots.

3) Ensure clean carbs with correct settings, new o-rings, and original OEM jets. No, not just squirted with something. I mean completely disassembled.

4) Check/adjust valve clearances (Manual calls for every 4,000 miles. This is not optional.)

5) Ensure healthy electrical system.

6) Seriously consider upgrading coils and plug wires.

7) Install new, stock NGK B8-ES plugs gapped to .031".

8 ) Fine-tune float height and idle mixture screw to ensure best off-idle transition.

9) Clean air filter and reinstall with only the lightest oil mist -- over-oiling and/or letting the filter get dirty is a common and critical mistake, and will make the bike run funny at low speeds and run rich. This may take a few tries.

10) Make sure the exhaust seals are sealing.

11) Ooh, much better now, huh? You're gonna need upgraded suspension - Progressive or better fork springs and shocks. Set suspension sag appropriately.

12) Upgrade brakes with new pads and stainless lines to deal with all that extra speed.

13) Install new petcock, since I'm going to head to the roof with a rifle if I have to read about yet another #2 plug fouling and failed hillbilly attempts to rebuild the petcock and/or deny there's a problem.

14) Oh yeah -- check compression somewhere in there to ensure the valves and rings are reasonably healthy.

15) You'll probably need new OEM clutch springs -- the clutches last forever, but the springs get tired after 20 years or so under pressure. E-Z and cheap.
=======================

It's just a machine. When all of the issues have been properly addressed it will run right. Be meticulous.

Have you replaced intake boots and O-rings along with the air box boots?

Air Intake Repair:
Airbox removal, intake boots and O-ring replacemen

Airbox Sealing
(by Mr. Roostabunny)

After the carbs are clean they must be bench sync'd. Set the mixture screws (or pilot screws, sometimes incorrectly referred to as "fuel screws") to three turns out to begin. Follow the vacuum balance procedures.

Are the valves adjusted?

What is the voltage at the coils?

Have you replaced the spark plug caps?

Have you over-oiled the air filter? Have you oiled it at all?

To be continued....




Thank you for your indulgence,

BassCliff

 

[Steve]

Maybe, this pic will show you what you are fighting.... courtesy of Steve

That's actually from the factory service manual, but thanks for the credit.

.

 

[SILENTxIRIE]

Thanks for the info guys, I upped the jets by two increments and turned out just fine, now the only problem left is that it only feeds fuel to the carbs when its on prime and not on run. Any ideas?

 

[cowboyup3371]

Is your vacuum line connected to the carbs at the correct spot and is it working?

To test, put the petcock in run and try to apply suction to the vacuum line. Fuel should flow from the petcock; if not it's bad

 

[Chuck78]

You said something about it running really horribly at 40mph and up? Have you made absolutely sure that there are no tiny tiny tiny pinholes in the rubber vacuum diaphragms on the tops of the carbs, when you pull the top cover off? That will cause that carb to run really lean on it's own from the others with good diaphragms, because the needle will not raise as much as the others. Also a high flowing filter and 4->1 exhaust will cause it to run lean and you won't have nearly enough power at higher speeds to even overcome wind resistance (go up at least two sizes in jets and play with the needle height settings by raising to a higher clip notch to lower the needle or more likely for you if at stock settings, lowering the clip a notch or two to raise the needle to richen the midrange).

The factory installed caps that cover the fuel screw are a "preset, do not tamper" setup and you have to really work to pop out those tamper proof caps.

Also, when you say you replaced the O-rings... Did you pull the intake boots off of the cylinder head and replace the 4 huge viton o-rings that seal to the head? big vacuum leak area there. The intake boots themselves need to be very thoroughly inspected for cracks and probably just replaced if there is any doubt. THEN... you have the complete o-ring kit that you need for internal carb parts, for all of the mixture screws and more. AND when you install these, they really need lubed up well, as rubber is very grippy against metal, and the o-rings will get damaged if they are too dry going in, and you will have a vacuum leak all over again even with "new" o-rings.

I have just gotten in the habit of dipping carbs and blowing them out twice with high pressure air, chased with aerosol carb cleaner spray, and blowing them out again and again and again, as well as using a high-E guitar string to gently clean out the tiny passages. Also, be sure the tips of the mixture screws are intact, if they get overtightened, the tips on some carbs are a fine needle point and will break off in the carb passage, totally restricting low speed flow.

Another thing, those fuel bowl drains at the bottom of each carb... those are more for checking your float height settings and needle and seat condition. You do this by slipping a proper sized clear piece of poly tubing onto the hose barb sticking out, and bending it in a U-shape pointing back upwards, and then opening the drain screw with the bike running and sitting level side to side (holding bike upright is probably better than on center stand) with the bike running, and then look through the clear tube to check where the fuel level is sitting at in relation to the carb body and fuel bowl. This is very crucial to check!!!! On the last set of KZ650 BS32 CV carbs I looked at, 2 or 3 had VERY HIGH fuel levels and one very low, no wonder he kept asking me about bigger bikes or big bore kits, the bike was in terrible tune!!!! Turned out that at least one of his needle and seats were grooved out a bit and leaking fuel even when the bike was of. AND they were stainless steel, not like all of the brass needle and seat sets on most of the bikes I usually work on that mostly have Mikuni VM carbs. can't lightly sand the stainless to clean them up for a better seal like you can the brass.

 

[Chuck78]

AND your petcock is most likely bad unless you have low engine vacuum, cracked vacuum hose, or hose hooked incorrectly to the fuel bowl vent port, and possibly then even having the vacuum port open or with a vent hose attached to it and just sucking in un-metered air and totally screwing up the running condition.