float level vs idle mixture

[Crazcnuk]

I replaced all the idle misture screws with thier assorted springs, o-rings and washers.

When I went to set the carbs, hot, of course, 3 of the carbs wated the screws closed (seated) and the 4th is open about 1/4 turn?

The bike is idling smoothly, runs good off idle, no stumbling or anything.

I am thinking that this indicates float level still too high?

I am assuming, on these, that the idles screws control fuel not air?

[randy]

Yes, our idle screws control fuel, not air. An easy way to remember this is, if the needle is on the engine side of the carb, it controls fuel. If the needle is on the air intake side of the carb,it controls air.

Have you played with the main jet needle position? Somehow the fuel is getting into the airstream, are the needles up too high and the floats too high? If you have raised the needles, try setting them to the middle slot after you re-adjust the floats.

[Crazcnuk]

I haven't changed the needles, and my floats are set to 23mm.

I was thinking I may have to go to 25mm on the floats?

I can never remember which way is lower!

[John]

Higher is lower.

The higher the setting, the lower the float.

[79XS11F]

Johns got it. Keep in mind that the float height is set with the carbs upside down. So then your set the floats higher your actually lowering the fuel level. Set the floats to factory along with the needles if they were changed by a PO
Rob

[prometheus578]

"Uhmmm..."

Yes, our idle screws control fuel, not air. An easy way to remember this is, if the needle is on the engine side of the carb, it controls fuel. If the needle is on the air intake side of the carb,it controls air.

On our system... the pilot screw controls the amount of a premixed fuel/air combo that gets introduced into the carb throat.
On air screw systems, the air screw controls how much air gets mixed with fuel, this mixture then goes into the carb throat uncontrolled by a pilot screw, it's volume of flow controlled by a throttle plate or a slide system.

What may be happening, Crazcnuk is that you're idle setting is set too high.
At idle... your throttle plates should be almost totally closed. Fuel would be coming from the pilot fuel circuit... being metered by the pilot screw.
As your throttle plates open, fuel then also flows out from the pilot circuit discharge holes behind the throttle plate.
Now then.... if the bike is running at idle and you have the pilot screws all the way screwed down, the the engine is getting it's fuel from those discharge holes behind the throttle plate in order to run.(meaning the throttle plates are open a little bit.)
Remedy: Back off your idle adjustment wheel so that the throttle plates are all the way closed. Open the pilot screws to the standard 1 1/2 turns. Crank the engine.
The bike may not start... as the throttle plates are totally closed, providing insufficient airflow past the pilot screw orifice to draw up fuel.
Slowly, while cranking, turn the idle adjustment wheel to open the throttle plates.
Once they're open enough to draw fuel from the pilot screw hole, the bike will start to sputter and try to run. (too low RPM)
Turn the idle wheel more...Opening them further, will provide more air and will also partially uncover the pilot discharge holes behind the throttle plate and it'll start to run better, etc.
Then just set the idle where it needs to be.

[Crazcnuk]

Ok, will give that a shot.

I have it idling at 1100, right now.

I know that when I set them (the 3 that I could) last year, they were turned out quite a bit further than they are now.

[prometheus578]

Yeah... 1 1/2 turns is the standard starting point for pilot screws. then, depending on engine variables, they get adjusted slightly to make it run better.

[Crazcnuk]

Last fall when I did the initial clean/rebuild, I started at 1.5 turns and fromthere I had to go out 1/2 turn to 1 more turn depending on the carb. Then I set the idle.

This time, I replaced all the mixture needle assy's and now 3 of them want to be all the way in, with one about 1/2 turn out.

I didn't change the initial setting of the idle screw, but I did change my float level slightly (2mm).

It is idling nice, no stumble off idle, runs smooth. Only thing I haven't checked yet is mileage.

[prometheus578]

"The parable of the straw."

As you know, there's a lot of variables involved here.
Float height does affect a lot of things.
Air flow past a port "siphons' up liquid...like blowing across the top of a straw.
The faster you blow, the more diet soda you get spraying out. The slower the flow, the less diet soda you get.
Now.. with a steady blown across the straw... the higher the initial level of soda in the straw, the easier it is for it to be drawn(more soda).
The lower the level, takes more effort to bring soda up the straw, hence less soda.

This is true of all sodas, not just diet.

How carbs work... I haven't a clue.

[TADracer]

Alot of things need to be in balance on these carbs for them to work properly. First, if you are set at 23 mm on your floats your fuel level is 2.7MM to high and that could give you some syphoning affect. Factory is 25.7 mm +/- 1mm. Second, if you are using the bikes tach to set your idle you may be off by a few hundred RPM since that tack is not accurate enough for tuning work. If you have another tach, use that. Your idle could be too high and you not even know it. If that is the case then your idle mixture screw might not even be in play anymore and will show no effect.

My first shot at this would be, raise your float level to 25.5MM, sync your carbs, if using your bikes tach, lower your RPM to under 1100 and adjust your mixture screws in until they are 1 1/4 turns out from VERY LIGHTLY SEATED. Adjust in 1/4 turn at a time and allow about 15 seconds for the carb to level out. When the engine starts to stumble, unscrew 1/4 turn at a time until you reach the highest RPM. Do one carb at a time and after all are set, adjust your idle speed screw until you get the right idle RPM.

Works for me.

[mro]

Works for me.

Hi Mike,
that should work just fine on your 79...
78/79 carbs = BS34 II

80 + carbs happen to be BS34 III


mro

[Crazcnuk]

Heh, Tad. I've been setting carbs for about 30yrs now, that is why this is a bit odd.

I did it just as you describe, only, instead of turning out, as this did nothing, I turned them IN 1/4 turn at a time. The idle increased steadily, and the bike got smoother and smoother as I turned the screws in until they hit the 'lightly' seated spot....

That is when i knew I had an issue.


The main issue right now, I think, is that I got my float heights backwards, I have them too high atm.

[TopCatGr58]

Hey Crazcnuk,

What you did makes perfect sense when the floats are too high(right side up)=too rich. The pilot screw controls FUEL, and since you had too much already, you had to turn them down=lean them out before the engine would idle properly and not act like it was choking down!
T.C.

[mro]

Hi TC...

originally posted by TC
The pilot screw controls FUEL

originally posted by da Prom
On our system... the pilot screw controls the amount of a premixed fuel/air combo that gets introduced into the carb throat.

Float level affects all RPM ranges.


mro