Dumb carb sync question

Well, I tried to clean out my carbs today. All the pilot passages and stuff seemed to be clear, but there was a bunch of crap under the diaphragm on two of them. I can't even imagine what it was.

Anyway, i'm trying to get them synced now using the two-bottles-and-some-hose tool. But I'm having a little conceptual problem.... Will opening the butterfly further increase or decrease vacuum? Obviously it will increase vacuum before the butterfly, but what about after?

Right now I can't tell if I've made things better or worse. I guess I will know tomorrow.

 

Right now ... WE ... can't tell; either. Why don't you take a little break, read a few pages of a manual, maybe chat with another bike-guy or two about what some of the parts are called or what they do ...

I thought I was up-to-date on all the makeshift and unorthodox methods for setting-up mixtures and doing syncing ...

But, ... well ... er ... ah, ... I guess I'll just have to confess that I ain't never heard of that-there "Two bottles and some hose" method.

How's that get hooked-up ... and, how do you know you got it just right?

 

Well, just right will come later. I may do the 4-hose manometer thing that people have discussed here. Right now they are so far out of sync that a rough tool is better, and this is rough. I found it on a ninja board. It's two bottles. Each bottle has a hose from the top to a carb. Then there's a hose that goes to the bottom of each bottle. Fill the bottles partway, and if vacuum is unequal, water will travel from one bottle to the other. So, like a manometer, but no chance of sucking liquid into the carbs no matter how far off they are... because the hose from the carb is not in the liquid.

Most importantly, it could be made from what I had around the house in a hurry. And it told me some valuable information, which was that my sync could hardly have been any worse.

And I did find my answer on another site... Opening the butterfly will reduce vacuum. I thought that that made sense, but I just kept second guessing myself... But yeah... A closed butterfly will have a lot of vacuum behind it. Open it, air rushes in to fill the vacuum. Right?

Whereas, if you are measuring vacuum before the butterfly, you'll see more vacuum if you open the throttle.

So I'll give it another shot tomorrow. I think it's better than it was, but I won't know until I try it cold.

 

Is this a YICS bike? If so, you'll need a YICS tool before synching the carbs. You can buy one or make one yourself quite easily.

 

No YICS. Despite the timing cover that says "YICS"

Must mean someone smashed the previous cover. Along with every other thing on the bike.

 

You'll get there adrian. In know time you'll work out which way to turn the screws to raise or lower the vacuum and it will become second nature.

Remember to try and sync carb 1 to 2 and then 3 to 4 before trying to sync left to right.

You will find that as soon as you adjust one screw all the others will change slightly and you have to fine tune as you go along.

Just beware not to work at it for too long or the engine will overheat. Give it a break if it is taking too long. Did you set the mixture screws all at the same setting before you started?

Nothing wrong with the bottle method if built correctly and it can be a good starting method but you will never get it very accurate with it.

Definately go the four tube manometer when you get closer as it will give you a near perfect sync. Better than vacuum guages IMO but can't compare it to mercury sticks as I've never used them.

Patience will get you there.

 

Ok for starters I recomend that you look at this page
http://members.tripod.com/dave_jack/id38.htm
No, it wont teach you everything, but it's a good starting place.

You really do need a good manual for this kind of work, not a clymer or haynes. I guess they'll do but there are better in my opinion. check out the XJ cd.

& Yes, vacuum does drop off when you open the throttle plates.

The best way to start out is the method I've sen it this websites threads, by using a fine drill bit for a guage to set all throttle plates equally for a referance point to start.

Good luck

Persistance!!!! That is the only way to work on these bikes. It's easy to give up, but the reward for persistance is well worth the hassle.

I promise!!

now GOOD LUCK

Mike

 

Seems to me that one large bottle with 2 hoses into it would be easier to set up.

 

Attention: Clarification related to Vacuum!!!

Vacuum INCREASES as the throttle plates are OPENED. The engine draws HIGHER vacuum as the throttles are OPENED. That's why there's the caution to be especially careful NOT to "Blip" the throttle OPEN when connected to MERCURY Carbs Sticks.

The INCREASED Vacuum from OPENED throttles might DRAW the MERCURY into the engine under INCREASED Vacuum and an extremely toxic Mercury Vapor gas will result.

That sweet, deep, "Whooh-aah" sound, you hear when rolling-on the power ... is produced by heavy VACUUM at the intakes, pulling atmosphere through the Airbox vent and intakes.

As throttle butterfly's are OPENED >> VACUUM is INCREASED.

Higher RPM = HIGHER (stronger) VACUUM.

OPENING Throttle Butterflys = INCREASED VACUUM.

 

The way I understood it, it depends upon whether you are measuring the vaccuum at the point where air enters the carb or at the point where the air enters the chambers.

As throttle opens you have increased air flow (air volume) but acually less of a vaccuum.

As throttle decreases you have less airflow with more vaccuum.

Symantics- are we really walking about air flow or vacuum because they are not technically the same thing however they are related.

 

As the piston pulls air into the chamber it creates a vacuum between the intake port and the carb. Thus high vacuum because the piston is trying to move more air than the carb can provide.
There is a differential of pressure between the intake port and the butterfly valve.

As the butterfly valve is opened - air rushes thru the carb body to the vacuum area and into the intake port.
As you introduce more air into the area that has high vacuum that vacuum level will decrease because you are filling it with more air volume.
Actually decreasing the differental air pressure

 

Vacuum: an enclosed space from which matter, esp. air, has been partially removed so that the matter or gas remaining in the space exerts less pressure than the atmosphere.

Pressure: the exertion of force upon a surface by an object, fluid, etc.,

Volume: the amount of space, measured in cubic units, that an object or substance occupies.

 

This is what I was looking at:
http://www.surenet.net/~verhey/colourtune/Intro.htm

I think there are two machanisms at work here. At idle, with the butterfly nearly closed, the idle near the vacuum ports seems like it would be die to rarefaction as the piston goes down. Near the bypass and pilot holes, there would be enough velocity to generate local vacuum by the bernoulli effect, but there probably would not be enough flow to generate a pressure drop near the vacuum port.

When you open the throttle, vacuum definitely goes up-- no argument there! But at that point it would be bernoulli.

At least that's how I explain this guy's advice... I don't have any more time to mess with the bike until the weekend, so at that point I will hook up a proper manometer and see which is right. And it won't matter so much which is right, because with the right tool, I can just watch the result! I have to break down and get a colortune too.

In the meantime, it gets me to school, though not gracefully. But it's all about parking that I can afford on a grad school budget. I can't remember why more school ever seemed like a good idea.

 

It's a Vacuum Operated Petcock.

There is Vacuum at the Nipples where the hoses for the Carb Sticks are attached.

The demand for Intake Air ... caused by the Piston on its Intake Stroke with the Exhaust Valves Closed -- CREATES A VACUUM -- into which there is a rush of air and fuel (mixed) -- to fill the >> VACUUM << present!

If there weren't a VACUUM present ... the air-fuel mixture would NOT rush to fill the VACUUM.

The VACUUM created is strong enough to draw particulate in with the rush of air. Therefore, ... Air Filters are used to prevent such particulate from >> BEING DRAWN INTO << the intakes when the engine is running.

 

Ok Rick think about it.
The reason they dont want you to bip the throttle when the manometer is connected is this; as the engine speed increases, the volume of air consumed increases, then when you let go of the throttle suddenly, the throttle plates go shut and the air flow path is cut off, that's when the vacuum goes through the roof because it can't suck air through the throttle plates so it will suck it from where it can, the manometer hose attached before the carb is the path of least resistance thus sucking the mercury out.

I will concede that there is an increase in vacuum as rpm increases.... but, if you hook your vacuum guage to your car and drive it, you'll see that as the throttle plates go open, the vacuum decreases but recovers with rpm increase. Then when you start to slow down and let off the throttle the vacuum goes up.

That's how I know it.
If need be we can agree to disagree

 

I'm sorry ... but that is absolutely incorrect.

The reason they don't want you to blip the throttle is that when the rpm's are increased rapidly ... the vacuum created might draw the mercury into the engine.

When the engine is raced ... that's when there is a danger. Not when the throttles are closed and there is NO appreciable vacuum created to be a danger.

The vacuum port is on the atmsophere side of the butterfly. When the butterfly is closed ... there is minumum vacuum at the intake manifold.

The danger is when the butterfly is opened exposing the atmosphere side to the vacuum; NOT the opposite ... which would defy laws of physics.

I agree to disagree with you.

I can't allow others to agree with you if their health and safety would be placed in jeapordy.

 

The Vacuum stroke is illustrated here:

http://www.keveney.com/otto.html

 

Ok take a look at this link. Time for school.

http://roadstarclinic.com/content/view/56/96/

Read the part under "Theory of Operation"

Note when vacuum is highest

No arguing just facts... but I agree to disagree but want those reading to read for themselves too.

 

Also that link you showed , showed the 4 cycle principle nicely

 

one more

http://www.answers.com/topic/manifold-vacuum

If the engine is operating under light or no load and intermediate throttle, the throttle is closed and the engine pumps the air out of the intake manifold as fast as it can leak in through the throttle. The engine speed is limited only by the amount of fuel/air mixture that is available in the manifold. Under full throttle and light load, other effects (such as valve float, turbulence in the cylinders, or ignition timing) limit engine speed so that the manifold pressure can increase -- but in practice, parasitic drag on the internal walls of the manifold, plus the restrictive nature of the venturi at the heart of the carburetor, means that a low pressure will always be set up as the engine's internal volume exceeds the amount of the air the manifold is capable of delivering.
If the engine is operating under heavy load at wide throttle openings (such as accelerating from a stop or pulling the car up a hill) then engine speed is limited by the load and minimal vacuum will be created. Engine speed is low but the butterfly valve is fully open. Since the pistons are descending more slowly than under no load, the pressure differences are less marked and parasitic drag in the induction system is negligible. The engine pulls air into the cylinders at the full ambient pressure.
Vacuum is created in some situations. On deceleration or when descending a hill, the throttle will be closed and a low gear selected to control speed. The engine will be rotating fast due to the fact that the road wheels and transmission are moving quickly, but the butterfly valve will be fully closed. The flow of air through the engine is strongly restricted by the throttle, producing a strong vacuum on the engine side of the butterfly valve which will tend to limit the speed of the engine. This phenomenon, known as compression braking, is often used in engine braking to prevent acceleration or even to slow down with minimal or no brake usage (as when descending a long or steep hill). Note that although "compression braking" and "engine braking" are sometimes used to describe the same thing, "compression braking" here refers to the phenomenon itself while "engine braking" refers to the driver's usage of the phenomenon. Compression braking can be greatly increased by closing the exhaust with a valve on the over-run, which is often done on large trucks.

 

HVBND s description is accurate.

When you "Blip" the throttle manifold pressure will momentarily increase, then decrease again as the revs go up.

The problem with the mercury is after the "Blip" when the throttle butterfly is closed but the engine is still spinning at higher speed, pumping out the intake manifold faster than the carb will let fuel/air in. The pressure will drop radically, risking pulling the fluid out of the manometers.

As you close the butterfly on an individual carb, the pressure in that manifold will drop causing the fluid in the manometer to rise. Since the other three cylinders will be carrying more load the pressure in their manifolds will drop (just like HVBNDs description of a whole engine under load).

So, when you hook up the manometer, the cylinders with higher manifold pressures (less vacuum) are doing more than their share of the work. Closing their butterflies or opening the butterflies of the carbs with lower intake manifold pressure (more vacuum) will balance hte load.

 

Bernoulli's principle here folks. Google Wikipedia, its got some nice details.
Rick, your on the right track.
Hvnbnd, your argument has merit. I'm still trying to envision the process as you've discribed it. I just reasoned it out but I cannot explain the momentary build up of pressure immediately following the throttle plate opening. I'm no engineer so I'm going to remain blissfully ignorant for now and just accept it.

 

Does the bike being anchored to a lift or placed upon it's work stand alter anyone's position to any degree?

The engine's not under load while it's being tuned.

 

Venturi's Law ... where a volume of air increases velocity as it becomes constricted is more applicable.

 

Sorry if my post seems a little out of date. Apparently by the time I posted it there were several responses in the que and I ended up out in the boonies. It should have been on the first page right after Hvnbnd's first disagreement with Rick.
I've not heard of Venturi's law my friend. Bernoulli developed the principles that describe the dynamics inside a venturi but I've not run across anyone named Venturi to date. Excellent discussion here fellas, looks like a great opportunity for some learnin'!

 

Wow, interesting points of view from everyone. Some I agree with and some I disagree with but rather than going into detail, I will first draw your attention to the following analogy.

Go fetch you vacuum cleaner.
Plug it in and turn it on.
Now place your hand over the hose.
Now your vacuum is screaming and your hand is stuck to the hose.

Your hand is stuck there due to high vacuum. increased motor revs makes it higher.

But there is no "atmosphere" entering the hose so is the vacuum higher or lower than when you take your hand off. Just because there is no air movement does not mean there is no vacuum.

Take this analogy and apply the same principle to your carbs where hose=inlet manifold and hand=butterflies

Does the vacuum increase or decrease when you take your hand off/open butterfly.?

Of course opening the butterfly adds fuel, therefore revs increase increasing airflow but just like taking your hand off the hose, vacuum should decrease per revolution.

Now would someone like to decipher this and tell me who I am agreeing with??? :lol:

 

vacum at the buterfly decreases as the buterfly opens the more air that comes in increases vacum at the main jet by the nature of the venturi,,,,Dan

 

The vacuum initially drops considerably when the butterly opens (hand comes off) but as the engine speed picks up the vacuum starts to recover or increase.
The vacuum is created by the inability of air to flow past the butterfly, when the throttle is fully open the air flows freely causing less vacuum (but lots of air flow) as the speed picks up and the throttle starts to close vacuum starts to increase due to the restriction of air flow.
I'm refering to the manifold vacuum on the engine side of the carb.

When you have cracks in your intake boots your idle increases as there is air flow where there shouldnt be.

That's all I'm going to say about this unless anyone asks, I feel like I've been a bit overbearing, but this is a fundamental principle that really needs to be grasped if one is to troubleshoot/understand their carbureation system.

That's all folks

 

great discussion guys, and a good read! But I think I have to agree with hvnbounds explanation. Way to make something that seems so complex understandibly simple hired goon!