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TBS question???


ahirsch

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In the past two weeks I have synched two throttle boddies. The first was on my bike (1100rt) where a friend I met on this board showed me how using a tube and water manometer. Went well, however I never really understood whether to tighten or loosen cables/BBS. Just moved stuff to balance until water ws equalized.

This evening I did my dad's 1200Rt for the 600 mile service. Balanced just fine at idle and Rpm, however I again just moved the adjusters until they were balanced. Trial and error approach.

The long of the short of it is that I am looking for the physics behind the balance. If cylinder one is pulling more water do I loosen or tighten cable and on which cylinder?

Thank you in advance!!!

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In the past two weeks I have synched two throttle boddies. The first was on my bike (1100rt) where a friend I met on this board showed me how using a tube and water manometer. Went well, however I never really understood whether to tighten or loosen cables/BBS. Just moved stuff to balance until water ws equalized.

This evening I did my dad's 1200Rt for the 600 mile service. Balanced just fine at idle and Rpm, however I again just moved the adjusters until they were balanced. Trial and error approach.

The long of the short of it is that I am looking for the physics behind the balance. If cylinder one is pulling more water do I loosen or tighten cable and on which cylinder?

Thank you in advance!!!

 

Ahirsch, easy to understand if you think of the engine pistons as a large individual vacuum pumps, with both the throttle plates & brass screws as air adjusting valves that allow more air to enter the vacuum chamber..

 

When you hook your manometer to each cylinder you are hooking the hose to the TOP of the water column & to the intake manifold BETWEEN the piston & closed throttle plate.. So the higher the vacuum in the manifold the higher the water will be pulled in the manometer tube.. Or to put it another way HIGH manifold vacuum = higher water level in the manometer..

 

In a simple form each piston can only displace a fixed amount of air on each stroke so that is a constant.. When you open a throttle plate slightly or open a brass screw you are letting MORE air in to the manifold so the vacuum (negative pressure) in the manifold raises (closer to atmospheric pressure) that in turn doesn’t pull as hard on the top of the water column in your manometer so the level drops in the tube..

 

OPEN BRASS SCREW MORE = lower water column ON THAT SIDE

OPEN THROTTLE PLATE MORE = lower water column ON THAT SIDE

 

CLOSE BRASS SCREW MORE = higher water column ON THAT SIDE

CLOSE THROTTLE PLATE MORE = higher water column ON THAT SIDE

 

Twisty

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When you open a throttle plate slightly or open a brass screw you are letting MORE air in to the manifold so the vacuum (negative pressure) in the manifold raises (closer to atmospheric pressure) that in turn doesn’t pull as hard on the top of the water column in your manometer so the level drops in the tube..

 

Twisty,

That makes some sense. However, I always assumed the opposite: that when you let more air in, that cylinder speeds up thereby increasing the vacuum. In practice I always end up using trial and error (just turn it a little and watch what it does). So either could be true and I wouldn't know the difference.

 

Andrew,

Glad to hear that balancing your father's RT went well. For others here is a picture of that bike.

348-009.JPG

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Hey, Nice picture! William, Glad to see you are back on the board! For others reading this post, You may remeber that I was looking for TBS help at the end of May. William heard my reply and incoprporated the TBS tutorial into a cross country trip. Thank you william!!! Not only did he teach me how but also a friendship was formed.

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Twisty,

That makes some sense. However, I always assumed the opposite: that when you let more air in, that cylinder speeds up thereby increasing the vacuum. In practice I always end up using trial and error (just turn it a little and watch what it does). So either could be true and I wouldn't know the difference.

Will, increasing engine speed (RPM) in & of itself will not raise engine vacuum.. There are some instances due to cam timing, road speed back driving the engine on over-run, or vacuum leaks that CLOSED throttle engine vacuum can increase slightly as engine RPM increases but as a basic rule intake manifold vacuum is a function of atmospheric pressure vs. throttle plate opening.. Intake manifold vacuum (that’s what you are measuring in a TBI balance) is the delta of atmospheric pressure OUTSIDE the manifold vs. atmospheric pressure INSIDE the manifold.. When the engine piston travels down the cylinder on the intake stroke it lowers the atmospheric pressure INSIDE the cylinder,, as it does that in turn lowers the atmospheric pressure inside the intake manifold due to the intake valve being open.. Engine vacuum is the comparison of atmospheric pressure outside the manifold to the negative pressure (vacuum) inside the manifold.. SO, that means the more air you let INTO the manifold the higher the inside pressure will be (less vacuum)..

 

Think of a glass of water with a straw in it.. The water in the glass has atmospheric pressure (weight of the air above it) pushing down on it’s surface.. At sea level that weight is appx. 14.7 pounds per square inch.. Seeing as the straw is open to atmospheric pressure at it’s top the water inside the straw ALSO has 14.7 pounds per square inch pushing on it so everything is equal (water in the straw is same height as the water level in the glass)..

NOW start sucking on the top of the straw!!! What that does is lowers the pressure ABOVE the water inside the straw.. The water in the glass still has that 14.7 pounds per square inch pushing on it’s surface but you have just lowered the atmospheric pressure inside the straw so the water starts flowing up the straw.. That is how people suck liquids out of a container, they aren’t actually sucking the liquid up but are really lowering the pressure above the liquid inside the straw & atmospheric pressure is doing all the work.. Same with your engine,, all the engine piston is doing is sucking the air out of the manifold & atmospheric pressure is pushing air back in through the BBS or open throttle plate..

 

Lets go back to that water glass/straw again.. If you drill a hole in the side of the straw above the water level then suck on the straw you won’t get any liquid to flow up the straw as atmospheric pressure will just push air in through that hole so you can’t suck the air out of the straw.. No matter how hard you suck or how many times a second you suck on that straw water will never raise out of that straw.. You are letting more air into the straw than you can suck out so never lower the pressure inside the straw.. It’s the same with those BBS (brass air by-pass screws) & TBI throttle plate.. The farther you open the BBS or farther you open the throttle plates the more air you let in so the negative pressure inside the manifold raises closer to the outside atmospheric pressure.. It helps if you think of vacuum as negative pressure..

 

Twisty

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You've convinced me, Twisty. I had it backwards. Not the first time I have learned something from this board. Thanks.

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Question......My LD says there is no TBS on the 1200RT's, and that it is all controlled by the computer, and 'stepper motors' thus there is no syncing of the TB's. What am I missing here?

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ShovelStrokeEd

Gordon,

I'll get back to you in a moment but there is an easy way to think of this whole vacuum thing.

 

There ain't no such thing as vacuum, there is only pressure. Atmospheric pressure varies all over the place from 760(approx) mm of mercury at sea level to 625 at Denver International, again, approximate to near zero at 300K feet up. All we are measuring is the differential pressure between two points. Pressure outside the manifold vs pressure inside the manifold. 100 mm difference in pressure is 100 mm difference in pressure at either location (Denver or Miami).

 

Now, Gordon, thought I forgot you didn't you.

 

The hex head series replaced the big brass screws with a pintle assembly whose position is controlled by stepper motors under control of the computer. Essentially, the 'puter turns the screws for you, maintaining idle synch and a smoother running motor in the lower RPM ranges. At higher speeds, the effect is more or less masked by the greater volume of air flowing past the throttle plates. High speed sync, at the cables, is still possible and, IMHO, necessary.

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What are the screws on the 1200's that are in the same place as the BBS? We adjusted those to dial in the idle. I was a little suspicious because they were chrome instead of brass.

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Question......My LD says there is no TBS on the 1200RT's, and that it is all controlled by the computer, and 'stepper motors' thus there is no syncing of the TB's. What am I missing here?
He's wrong. There is no idle sync adjustment, but there is still an above idle throttle body sync done by adjusting the right-hand cable.
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