Carb Questions:I really have tried to find answers but still wondering!

[supertech411]

OK, I've had my '81 GS550T for a little less than two months now; I've replaced the stator, fixed a broken clutch tension spring and adjusted the clutch (amazing how good it works when you follow the directions in the manual!), replaced the front tire and repaired (flat) the back, cleaned the horn contacts, and a bunch of other stuff. The biggie has been the carbs. I removed them and cleaned them per the .pdf available from bikecliff's web site. I replaced all of the o-rings, rubber plugs, manifold o-rings, and manifold boots. I sync'd the carbs with my home made manometer so the manometer levels look like they do in the manual (the two outer cylinders the same level with the two inner cylinders the same level as each other, but a little lower than the outer cylinders). I have not adjusted the valves.
Anyway, here is my delima. The bike starts when cold without any problems (using the choke), and settles down to a nice 1000-1100 idle. It pulls just fine and runs really well, but once it gets hot, it idles at 2000-3000 rpm, or it will stall. The fast idle usually happens earlier in the ride, and the stalling at the end of a ride. I pulled the plugs and found plugs 1,2,& 3 black with dry carbon deposits, and #4 perfect; no carbon deposits and a slight grey on the tip. As part of the carb cleaning I removed the jet needle on the upper front of each carb per the instructions in the pdf. This is the one that comes capped and has the needle, spring, washer, and o-ring. In the pdf it if referred to as the air screw (page 17 of 53). I have tried to find answers in the forum for the following questions, but it still isn't clicking. Perhaps if someone would have pity on me and answer these questions I can get this thing straight.
1. True or False: Outside of the adjustment to sync the opening of the butterfly valves/idle level of the carbs, this "air jet" is the only other adjustment necessary on a CV carb.

2. What are you adjusting when you adjust the air jet screw? Fuel, air, or emulsion flow?

3. Is the air jet what determines whether a carb flow is "lean" or "rich"?

4. If yes, does screwing the air jet clock-wise (in) cause the carb to have a more lean setting or a more rich setting.

5. In the Highest RPM method instructions, it give directions to adjust the "idle mixture screws". Is this refering to the air jet screws? (Just trying to get my venacular straight)

I tried the Highest RPM method, but could not hear any change in engine speed when I adjusted the air jet setting, so for now they are all 2 turns out. I did not touch #4 because it looked good already.
This has been a great learning experience getting this thing running. I could not have got this far without the help of the friendly and knowledgable folks on this website and the bikecliff site. If I am asking questions that have already been answered elsewhere, please forgive me.


Thanks in advance for any help provided. Happy riding to all...

 

[Guest]

That screw is properly called the pilot fuel screw. Screwing it out allows more fuel (makes it richer). It only effects the carbs at idle and slightly above idle, up to about 1/4 throttle. And yes, it is the only adjustment you can do besides the synch and idle. Sounds like you may be rich on those three cylinders, but you may also have an air leak somewhere. Check and make sure you have no air leaks first. Airbox sealed up tight? If so, do some plug chops. Run you bike down a straight level road at 1/8 to 1/4 throttle for at least 1 mile. Pull in the clutch and kill the motor. Pull over and ready your plugs. Adjust you pilot fuel screws accordingly.

 

[waterman]

Did you check your float levels and therefore the fuel level when you cleaned your carbs? I agree with Bruce it maybe running rich on the 3 cylinders.

Did you replace the o rings that connect the carb boots to the cylinder head. These dry up and crack over time and will cause the idle to race. They can be a little tricky to remove, use an impact driver on the philips screws.

Finally, don't neglect the valves, their clearance also needs checked. Your are doing the necessary things that will reward you with a smooth running reliable machine. You are on the right track.

 

[Steve]

I will have to respectfully argue with Renobruce here and say that the even more correct term is "idle mixture adjustment screw."

One the earlier VM carbs, there are, indeed, a pilot fuel screw and a pilot air screw. On the BS carbs on your bike, there is a fixed pilot air jet in the carb intake and a fixed pilot fuel jet in the float bowl. These create a mixture of a given ratio that can be changed by replacing either or both jets. That mixture is then allowed to enter the engine through a series of ports in the top of the carb throat that are sequentially uncovered with increasing throttle opening. ONE tiny port is uncovered at all times, and the amount of mixture that is allowed to flow through it is controlled by the "idle mixture adjustment screw".

Now let me address your questions:
1. True or False: Outside of the adjustment to sync the opening of the butterfly valves/idle level of the carbs, this "air jet" is the only other adjustment necessary on a CV carb.
That is correct. By the way, "CV" is a type of carb. The model name for your carbs is "BS".

2. What are you adjusting when you adjust the air jet screw? Fuel, air, or emulsion flow?
Emulsion flow. (see explaination above)

3. Is the air jet what determines whether a carb flow is "lean" or "rich"?
Only at, and just above, idle. There are also the needle and main circuits that control increasingly larger throttle openings.

4. If yes, does screwing the air jet clock-wise (in) cause the carb to have a more lean setting or a more rich setting.
Generally speaking, turning a screw clockwise decreases the amount of what it's controlling. Since that adjustment screw is controlling a mixture, turning it clockwise will make the idle circuit leaner.

5. In the Highest RPM method instructions, it give directions to adjust the "idle mixture screws". Is this refering to the air jet screws? (Just trying to get my venacular straight)
It is actually referring to the "idle mixture screws", but you have been calling them "air jet screws" so far. Yeah, same screws.

I tried the Highest RPM method, but could not hear any change in engine speed when I adjusted the air jet setting, so for now they are all 2 turns out. I did not touch #4 because it looked good already.
I have found that you don't always get much of an increase in speed, so I turn them slowly clockwise until I get a decrease in speed, then back off about 1/4 turn. This basically gives it the leanest mixture that still runs decently.

 

[Nessism]

My now departed 550T needed to have the pilot mixture screws set at 3.5 turns before it would run right - had a hanging idle.

 

[Guest]

I will have to respectfully argue with Renobruce here and say that the even more correct term is "idle mixture adjustment screw."

One the earlier VM carbs, there are, indeed, a pilot fuel screw and a pilot air screw.

I stand corrected....

 

[supertech411]

Did you check your float levels and therefore the fuel level when you cleaned your carbs? I agree with Bruce it maybe running rich on the 3 cylinders.

I did check and adjust the float levels per specs when I rebuilt the carbs.

Did you replace the o rings that connect the carb boots to the cylinder head. These dry up and crack over time and will cause the idle to race. They can be a little tricky to remove, use an impact driver on the philips screws.

Yes, I ordered a complete O-ring set including for the manifolds and installed them with the new manifold boots.

Finally, don't neglect the valves, their clearance also needs checked. Your are doing the necessary things that will reward you with a smooth running reliable machine. You are on the right track.

Thanks for your reply and encouragment! Adjusting the valves is a little outside my comfort zone right now, but if it needs to be done it needs to be done

 

[supertech411]

That screw is properly called the pilot fuel screw. Screwing it out allows more fuel (makes it richer). It only effects the carbs at idle and slightly above idle, up to about 1/4 throttle. And yes, it is the only adjustment you can do besides the synch and idle. Sounds like you may be rich on those three cylinders, but you may also have an air leak somewhere. Check and make sure you have no air leaks first. Airbox sealed up tight? If so, do some plug chops. Run you bike down a straight level road at 1/8 to 1/4 throttle for at least 1 mile. Pull in the clutch and kill the motor. Pull over and ready your plugs. Adjust you pilot fuel screws accordingly.

I'll double check on the air leak and try the chop routine. Thanks the reply!!

 

[supertech411]

My now departed 550T needed to have the pilot mixture screws set at 3.5 turns before it would run right - had a hanging idle.

What is a "hanging idle". Never heard that term before. Thanks for the reply!!

 

[supertech411]

I will have to respectfully argue with Renobruce here and say that the even more correct term is "idle mixture adjustment screw."

One the earlier VM carbs, there are, indeed, a pilot fuel screw and a pilot air screw. On the BS carbs on your bike, there is a fixed pilot air jet in the carb intake and a fixed pilot fuel jet in the float bowl. These create a mixture of a given ratio that can be changed by replacing either or both jets. That mixture is then allowed to enter the engine through a series of ports in the top of the carb throat that are sequentially uncovered with increasing throttle opening. ONE tiny port is uncovered at all times, and the amount of mixture that is allowed to flow through it is controlled by the "idle mixture adjustment screw".

Now let me address your questions:
1. True or False: Outside of the adjustment to sync the opening of the butterfly valves/idle level of the carbs, this "air jet" is the only other adjustment necessary on a CV carb.
That is correct. By the way, "CV" is a type of carb. The model name for your carbs is "BS".

2. What are you adjusting when you adjust the air jet screw? Fuel, air, or emulsion flow?
Emulsion flow. (see explaination above)

3. Is the air jet what determines whether a carb flow is "lean" or "rich"?
Only at, and just above, idle. There are also the needle and main circuits that control increasingly larger throttle openings.

4. If yes, does screwing the air jet clock-wise (in) cause the carb to have a more lean setting or a more rich setting.
Generally speaking, turning a screw clockwise decreases the amount of what it's controlling. Since that adjustment screw is controlling a mixture, turning it clockwise will make the idle circuit leaner.

5. In the Highest RPM method instructions, it give directions to adjust the "idle mixture screws". Is this refering to the air jet screws? (Just trying to get my venacular straight)
It is actually referring to the "idle mixture screws", but you have been calling them "air jet screws" so far. Yeah, same screws.

I tried the Highest RPM method, but could not hear any change in engine speed when I adjusted the air jet setting, so for now they are all 2 turns out. I did not touch #4 because it looked good already.
I have found that you don't always get much of an increase in speed, so I turn them slowly clockwise until I get a decrease in speed, then back off about 1/4 turn. This basically gives it the leanest mixture that still runs decently.

Thank you, thank you, thank you! You have answered my questions to the "T" (pardon the pun). Now that I have a better idea what I am dealing with I believe this thing will finally get set right. I will check for any air leak while the engine is hot and go from there.

thanks again!!

 

[Nessism]

Set the mixture screws to 3 turns out and give it a try. A hanging idle is when the idle is fast when hot and doesn't like to settle down very fast...idle slowly goes down, not quickly like it should after you blip the throttle. Basically the idle mixture is too lean when this happens so the symptoms are the same as an air leak.

 

[supertech411]

It's working!

It's working!

Thanks to all for your help in educating me sufficiently to resolve T's idle issues. The idle mixture screws were set too far out, causing the plugs to foul at idle, causing me to think the idle was set too low (and turning up the idle), causing a fast idle after riding for a while (burning off the carbon?) and then stalling when at the stop light when I turned the idle back down. Anyway, after cranking in the idle mixture screws, checking the plugs, and then setting the idle correctly T runs great. No hanging idle, no stalling! I had a great ride yesterday. Today I went out for a run and guess what...My head gasket has had an anurism. Oh well, time for another project!