Ideal inlet balance pipe ID?

What would be the best size ID for the balance pipe on the inlet manifolds on a commando? Did Norton get it right in the first place ? Also would it be best to add an extra spigot to each manifold for a vacuum gauge when balancing the carbs so that when the balance tube is refitted it remains the same? Cheers

No idea if they got it right sizewise but you balance the carb without the tube or block it off or you are not balancing the two carbs fully independently as its effectively a leak between the carbs.

kommando said:

No idea if they got it right sizewise but you balance the carb without the tube or block it off or you are not balancing the two carbs fully independently as its effectively a leak between the carbs.

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yes thats one of the things i was wondereing about if you disconnect the balance tube and use the spigots for your vacum guages you then balance the carbs and afterwards you remove the guages and re fit the balance tube once more are the carbs still perfectly balanced? cheers

Why would they not be still balanced, unless the tube has some directional bias ie the internal lining acting as a one way check valve, there should be no unbalancing effect.

baz said:

What would be the best size ID for the inlet manifolds on a commando? Did Norton get it right in the first place ? Also would it be best to add an extra spigot to each manifold for a vacuum gauge when balancing the carbs so that when the balance tube is refitted it remains the same? Cheers

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The flow of the balance tube will affect the fuel curve in the lower 1/4 throttle range quite a lot.

If you install fittings with an ID of ~3/8th inch and install a flow control valve in the line you can use it to help fine tune the throttle response in that range. It works rather well. Jim

kommando said:

Why would they not be still balanced, unless the tube has some directional bias ie the internal lining acting as a one way check valve, there should be no unbalancing effect.

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I did wonder if uneven wear between the slides may cause uneven balance once the pipe was re connected cheers

comnoz said:

baz said:

What would be the best size ID for the inlet manifolds on a commando? Did Norton get it right in the first place ? Also would it be best to add an extra spigot to each manifold for a vacuum gauge when balancing the carbs so that when the balance tube is refitted it remains the same? Cheers

Click to expand...

The flow of the balance tube will affect the fuel curve in the lower 1/4 throttle range quite a lot.

If you install fittings with an ID of ~3/8th inch and install a flow control valve in the line you can use it to help fine tune the throttle response in that range. It works rather well. Jim

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that's very interesting the reason I ask is I recently re fitted my amals with a rubber mount kit that had no balance pipe so I used the spigots that came with the vacuum gauge kit but the bore is rather small and and I wondered what effect a larger bore tube would have cheers

I have done this to 2 x sets of PWK flatslides from JS Motorsport.
It certainly helps with the idle set up.
I drilled out the small boss of the carb (not the manifolds) in front of the flatside body and used the brass choke tubes out of an old set of AMALs . Undersize drill from the tube diameter and press them in about 5 mm does the trick. Or drill to exact size and JB weld them in place.
Where I have them was originally drilled, but Jim (or supplier) had filled them in with Devcon or JB Weld.
Probably a 3mm pilot hole through to the carb throat.
I installed them originally to fit tune gauges.
Mr Comstock maybe able to shed light on where the correct placement might be best, manifold or carb body, and also hole size through the carb body (or manifold) if you dont use a control Vv like he does.
Regards Mike

.ok thanks for the info i have already installed the balance pipe spigots to the inlet manifolds close to the head,i will have to check the id of them because I think they are under 3mm but it does seem to idle better with the balance pipe fitted but maybe even better slightly larger? Cheers

i have measured the ID of my balance pipe spigot and it is 2.5 mm diameter would it be worth opening this up ? and if so how big? or leave it alone ? cheers

"If you install fittings with an ID of ~3/8th inch and install a flow control valve in the line you can use it to help fine tune the throttle response in that range. It works rather well. Jim"

I wonder why that would be ? I don't know exactly why and what the balance pipe does. But I do know that it tends to equalize the vacuum on the high vacuum side of both slides and that, presumably, equalizes the flow across both idle jets. I can only imagine that Jim's valve is a way of answering Baz' original question, to wit, what is the ideal diameter for the tubing? That is, Jim, starting with 3/8 tubing and then cutting the effective diameter down with the valve is showing that something less than three eighths is optimal. And, stock is what? 3/16 or 1/4? It could be that the real issue is sufficient flow through the balance pipe and that the real risk is too small in diameter but that can't be right since Jim gets a better result by choking the 3/8 tube with his valve. Perhaps that is because peak vacuum is not simultaneous between the two cylinders

I think it provides a tad more air or possibly residual fuel/air mist to the equation on whatever side is drawing a breath. The valve would allow precise control of the amount.

what happens if you don't use the balance tube at all and just use the ports for vacuum gauges and cap them afterwards? I have noticed that after reconnecting the balance tube after tuning with vacuum gauges I have to set the idle down on both sides a small amount. My theory is the balance tubes were used to give the Amals a bit more latitude, a technique used before vacuum gauges became readily available to the average rider.

Also, could you make a video of the 3/8 tube and flow control valve?

A balance tube can be used as a tuning device to help get the part throttle mixture curve correct. If the carbs are tuned for use with a crossover tube, then eliminating the crossover tube will make the mixture wrong.

When you are fitting carbs that were designed for a different engine, the adjustable balance tube can be a big help in getting the off idle mixture and throttle response just right.

I have not used a crossover since I have gone to fuel injection, so I don't have one to demonstrate. Jim

The balance tube weakens the signal on the side being drawn on intake at the moment. Larger orifices and hose diameter and shorter hose will increase this weakening of signal on the side being drawn on intake.

Deep southerners love V8's with dual exhausts and learned to connect them with a smaller tube for distinctly better idle and throttle stab response plus deeper softer notes. Works well no matter where dual pipes connected d/t stuff in the way but ideal is found by long wax/crayon mark, then run up to rpm ya want best effect then shut off and connect the pipe hot spots together. Allows both mufflers to share the flow by bleeding "pressure spikes" off that'd otherwise bounce back more than flow through a single practical size/wt muffler.

Only plenum type (Amal, Holley) carbs pairs benefit by balance tubes. Function is to divert one side intake valve closing against flow inertia - reversion pulse into the lower pressure carb rather than out carb month creating the 'standing reversion fog/cloud, which is over rich as passed the spray tube twice. Non plenum carbs (Webber) often require long trumpets stuck on mouths to help stifle reversion issues. Balancing carbs tend to soften low rpm response so trick is restricting the connection till the sluggishness resolves yet still getting anti-reversion benefit.

Further benefit can be gotten by plumbing inline plenum/canister sized to give best butt bump on snap opens. Two Smokers call them "boost bottles" and sized similar to a cylinder's displacement, usually somewhat less is best.

The tube is called a balance tube because it is an attempt to balance or equalize the vacuum seen by each carburetor. This vacuum might (see footnote) differ on each side due to unequal ring, bore, or valve guide wear. If the vacuum is unequal, then the fuel draft will be unequal, causing the engine to idle erratically, which in turn will create an imbalanced vacuum, creating a greater fuel draft imbalance. etc.

Conditions such as unequal wear mentioned above, cannot be rectified with an external crosstube, hence this tube is merely an attempt to rectify the imbalance caused by such wear. In short, it helps. In theory, if an engine is well built to specification on each cylinder, the balance tube should be unnecessary, although in practice, it may still help.

If the balance tube mainly compensates for the vacuum difference due to unequal wear, then the ideal tube diameter and length cannot be rigidly specified for any particular engine condition. Moreover, even with equal wear on each cylinder, should there be an ideal balance tube diameter, it will require adjustment as wear progresses.

Footnote: the main cause of vacuum imbalance between cylinders is usually due to the carbs themselves being whacked out by a novice or inept carb tuner. The obvious causes are leaky slides, unequal slide cut-aways, unequal idle jet sizes, and most likely ( as I have usually found when helping out someone who claimed his carbs were "hopelessly whacked out"), the engine was running on one carb due to a combination of faulty throttle stop and idle jet adjustment.

Slick

A balance tube is the moniker and concept but is there really any significant vacuum in the intake tract on the cylinder that is past it's intake stroke where the intake valve has been closed. As best as I can tell, if one wanted near complete balance the manifolds would need to be merged or connected with a big low restriction connection. Maybe it's for very low speed minimal throttle openings as Jim C. alluded to

To achieve this balance would cause a weakening of signal on the side with the greater vacuum which intuitively is the side undergoing the intake stroke and a strengthening of signal on the side which is not in the intake stroke.

Dances with Shrapnel said:

A balance tube is the moniker and concept but is there really any significant vacuum in the intake tract on the cylinder that is past it's intake stroke where the intake valve has been closed. As best as I can tell, if one wanted near complete balance the manifolds would need to be merged or connected with a big low restriction connection. Maybe it's for very low speed minimal throttle openings as Jim C. alluded to

To achieve this balance would cause a weakening of signal on the side with the greater vacuum which intuitively is the side undergoing the intake stroke and a strengthening of signal on the side which is not in the intake stroke.

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All true ...... which is why I said the balance tube was "an attempt"

Put another way, the balance tube is a bodge to make better what is less than perfect.

IMO, fretting over the diameter of the balance tube, is much ado about nothing.

Slick

Balance tubes are not a bodge in any way as Amal can be tuned find to start idle and leap w/o them. As logically stated above, too much equal vac balancing nullifies the piston draw in. They work on pressure differences behind the throttle slides so can't do much to compensate for loose sloppy slides. Its a mistake in flow physics to view balance connection from a vacuum perspective as its valve close reversion bounce back spikes being diverted.

Resonate wave tuning similar to 2-1 exhausts which can be improved by fiddling with size and length of tube volumes. As 2 smokes need to tame the whole volume of crankcases on intake piston closing, they need larger boost bottles to be felt, while 4 strokes only need volumes similar to manifold track. Use vac meter to get both carbs drawing the same then put balance hose back on with a T to meter both at once then diddle each carb the same to give best vac at idle then replace with normal tube. Can put tiny clamp on tube or restriction inside to recover best response lost from balancing vac too much.