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itb balancing method question

General engine tech -- Drag Racing to Circle Track

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Caprimaniac
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Re: itb balancing method question

Post by Caprimaniac » Mon Mar 23, 2020 6:47 am

With carbs instructions say use idle stop at one row of carbs. Last time trying I had tu use both rows, and even then not good. Now- with a better linkage- should I make up a central idle stop? Or will the carbs be mor forgiving than the TB’s?
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Re: itb balancing method question

Post by Belgian1979 » Mon Mar 23, 2020 10:23 am

Hm, I've seen pulsating airflow cause the throttle blades to open and close with the pulses. Right now each axis is firmly held against its stop by the force of the spring acting on the central link which in fact counteracts any effect of airflow pulses.
If I would put the stops on the central linkage, the axes would be able to move with the flow and reverse airflow depending on how much play there is in the ball joints at the end of the steel linkages between the axis and the central link. Been there,done that and the jitter that this caused with the tps caused me to put the acceleration enrichment sensitivity so high that at low throttle openings it wouldn't do anything anymore.

Somehow at higher rpm I don't see the effect of pulses when the linkage is off the stops. I think this is due to the position of the airblades and the airflow's tendency to try and close the blades again thereby taking up any slack there might be in the linkage

I'm not so much convinced that steel wire would be a better solution but have no experience with them either.

As for balancing with 2 idle stops, when all air bypass screws are closed you normally make each cyl on each bank more or less equal to each other and then you adjust the throttle stops so each bank is the same as the other on the fluid manometer

I agree that play in the linkage will cause balancing problems. However, I'm not sure if a zero play system actually exists.

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Re: itb balancing method question

Post by naukkis79 » Mon Mar 23, 2020 1:13 pm

Caprimaniac wrote:
Mon Mar 23, 2020 6:47 am
With carbs instructions say use idle stop at one row of carbs. Last time trying I had tu use both rows, and even then not good. Now- with a better linkage- should I make up a central idle stop? Or will the carbs be mor forgiving than the TB’s?
Carbs are more forgiving than EFI, if EFI isn't calculating fuel per row.

But with two idle stops it's practically not possible to balance throttle blades, you have to use multiple idle stops only when your throttle linkage is so flexible that it cannot support throttles at all.

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Re: itb balancing method question

Post by naukkis79 » Mon Mar 23, 2020 1:27 pm

Belgian1979 wrote:
Mon Mar 23, 2020 10:23 am
Hm, I've seen pulsating airflow cause the throttle blades to open and close with the pulses. Right now each axis is firmly held against its stop by the force of the spring acting on the central link which in fact counteracts any effect of airflow pulses.
If I would put the stops on the central linkage, the axes would be able to move with the flow and reverse airflow depending on how much play there is in the ball joints at the end of the steel linkages between the axis and the central link. Been there,done that and the jitter that this caused with the tps caused me to put the acceleration enrichment sensitivity so high that at low throttle openings it wouldn't do anything anymore.

Somehow at higher rpm I don't see the effect of pulses when the linkage is off the stops. I think this is due to the position of the airblades and the airflow's tendency to try and close the blades again thereby taking up any slack there might be in the linkage

I'm not so much convinced that steel wire would be a better solution but have no experience with them either.

As for balancing with 2 idle stops, when all air bypass screws are closed you normally make each cyl on each bank more or less equal to each other and then you adjust the throttle stops so each bank is the same as the other on the fluid manometer

I agree that play in the linkage will cause balancing problems. However, I'm not sure if a zero play system actually exists.
Your linkage isn't rigid enough to support forces that throttle blades see. With higher rpm range they will set to whatever angle throttle position and linkage flex offers - at idle your setup won't work at all as it can't handle the forces.

With central linkage and two throttle cables attached to same side of linkage all forces will try to close blades. Flex is only to one direction and not affecting sync. With that star-central linkage closing forces from other throttle row can open other row blades - and do that totally uncontrollable way as that star linkage can flex to every possible direction.

And by syncing throttles target isn't try to sync just idle. Target is to get all throttle blades open simultaneously. How do you think to do that when at idle every blade is resting to it's own stop screw? Using more than one idle stop is just fundamentally wrong way to do ITB setups, and not to acknowledging that basic thing makes it absolutely impossible to do proper syncing.

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Re: itb balancing method question

Post by MadBill » Mon Mar 23, 2020 10:05 pm

Seems to me if you equalize flow at idle via the individual stops, then use the throttle pedal to hold a fast idle (assuming a reasonably stiff linkage), adjusting it as necessary to re-establish equal flow should be all that's needed. :-k
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Re: itb balancing method question

Post by modok » Mon Mar 23, 2020 11:10 pm

:lol: essentially, yes.

It's a little less crucial with carbs, as they meter fuel individually.

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Re: itb balancing method question

Post by Belgian1979 » Tue Mar 24, 2020 8:25 am

Couple of things though:
-there are only 2 idle stops and that is in the center on each bank. There is not a stop on each body.
- I'm not exactly seeing where you see all the flex coming from. Each tb has 2 bearings, so every banks axle is supported by 4 bearings in total.The connection between each 2 tb's on each bank is with an interlocking system, which is bolted on the axle (with set screws). The turning of the axle is accomplished by a lever that is bolted to each main tb of each bank which also has the idle stop screw (and end of travel stop) built into it. The linkage between the lever and the star pivot is by steel rods with spherical bearings. Those last ones are the ones that have a little give in them. Actually I cannot move the axles up/down, left to right nor can I make the blades move unless I put so much force into it that it overcomes the return springs in the system or it overcomes the thightening torque on the set screws of the interlocking connector.
- fuel is metered individually as well by way of EGO's per cylinder.

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Re: itb balancing method question

Post by Belgian1979 » Tue Mar 24, 2020 9:06 am

MadBill wrote:
Mon Mar 23, 2020 10:05 pm
Seems to me if you equalize flow at idle via the individual stops, then use the throttle pedal to hold a fast idle (assuming a reasonably stiff linkage), adjusting it as necessary to re-establish equal flow should be all that's needed. :-k
Thus the way I now did it, balancing them at 2000 rpm and fine tuning it at idle. They are within an 1/2 inch now.

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Re: itb balancing method question

Post by naukkis79 » Tue Mar 24, 2020 11:58 am

Belgian1979 wrote:
Tue Mar 24, 2020 8:25 am
- I'm not exactly seeing where you see all the flex coming from.
Pictures are bit blurry, but

it's that rolling star pivot what transfer throttle pedal movement to levers that go to throttle bodies. I haven't seen one yet that has been near enough rigid to actually handle needed forces. Push it lightly down in some lever attach point and it will rock. Any rock in that place affects directly balance between rows. And as that rock isn't in same direction as forces opening and closing throttle bodies it can't be preloaded. And if there's no way to preload throttle linkage whole sync process is pretty much pointless.

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Re: itb balancing method question

Post by Belgian1979 » Tue Mar 24, 2020 2:07 pm

The star is made from a steel plate, 5 mm thick. It rotates on an axle that is 12 mm in diameter. I assure you there is no flex there.

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Re: itb balancing method question

Post by naukkis79 » Wed Mar 25, 2020 9:24 am

Belgian1979 wrote:
Tue Mar 24, 2020 2:07 pm
The star is made from a steel plate, 5 mm thick. It rotates on an axle that is 12 mm in diameter. I assure you there is no flex there.
Did you try it? Keep balance pipe connected to different rows and push that star. To correctly balance throttles you need accuracy of something like 1/100mm in travel, systems where bearing tolerances can't be preloaded just aren't accuracy enough. To the point that tps sensor is able to detect that movement.

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