Ideal inlet balance pipe ID?

The balance tube, I believe, was put in place to take care of the minor to negligible differences left after a proper mechanical synchronization process, detailed by many on the forum. The balance tube will not compensate for unevenly worn slides, mismatched slides, worn of partially obstructed jetting circuits or air leaks.

I usually do the mechanical sync and check with a manometer, It is rare for me to change the settings using the manometer data.

somewhat related is something I experienced about 30 years ago with my MK3 going from Yerington Nevada to Reno often. The roads are long and straight and little traffic, offering ideal conditions for playing with jetting. This was early in my Nortoneering days so my knowledge was so so. I had rebuilt the Amals and used the phenolic spacer and Yamabond on the manifolds. I noticed that the plugs were turning white so I added more fuel via this jet and that jet. It still turned the plugs white. However, as I increased the fuel supply the bike developed more power. It always started in one or two kicks. I put in 107 needle jets. I raised the needles. I put in #3 slides. I upped the mains by 2 sizes. It just ran faster and better. It should be drowning in fuel but it only ran better. With no flat spots, a good and predictable idle, easy starting. Baffling but in my favor. What I finally discovered was that the phenolics were very poor at sealing, all the gasket sealer had been sucked away from the spacer allowing them to inhale air ...both at about the same rate. From then on I never trusted the phenolic spacers. It is curious how well this controlled air leak worked but I never investigated it further. It is probably something that could be used to one's benefit given a lot of investigation. I also wonder how many people are running their Nortons and not realizing how poorly the phenolics work as a gasket.

When I was a kid about 12, Dad explained to me about multiple carbs, (1 for each cyl.) while a boon to high rpm airflow, became a compromise/liability at low rpm. The reason being, the air had to come to a complete stop between intake (valve open) events, (idle or near idle speed) and it was disruptive to the vacuum signal in the venturi, and therefore fuel flow and more importantly the atomization of the fuel. An overly rich mixture would be required to compensate.

This was ONLY his theory, no flow bench, dyno or racetrack to test.

It may explain the intent of the balance tube.

YMMV

concours said:

When I was a kid about 12, Dad explained to me about multiple carbs, (1 for each cyl.) while a boon to high rpm airflow, became a compromise/liability at low rpm. The reason being, the air had to come to a complete stop between intake (valve open) events, (idle or near idle speed) and it was disruptive to the vacuum signal in the venturi, and therefore fuel flow and more importantly the atomization of the fuel. An overly rich mixture would be required to compensate.

This was ONLY his theory, no flow bench, dyno or racetrack to test.

It may explain the intent of the balance tube.

YMMV

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Actually, the air comes to a complete stop in the intake tract [near the valve] at any RPM with one carb per runner.

I always looked at the differences the crossover tube makes like this:

When you have a crossover tube, at low speed, each cylinder sees two throttle openings, so it runs faster with less slide [and needle] lift.

Therefore it changes the rpm at which the fuel delivery changes from mostly idle circuit to mostly needle circuit.

The closest thing to compare it to is the slide cutaway, but it's effect is centered a little higher in the throttle range.

Trying to keep Trixie all Norton but with Amal slides worn fluted and bore beaten wider, got the anodized slide with #3 cut out instead of 3.5 which solved idle stability and gave good off idle response with normal balance tube. Peel had vac/boost gauge Tee'd into balance tube which was interesting to watch and helpful in tuning. To assess how much slide leaking was messing with mixture I greased the slides, which distinctly improved behavior till the grease washed away over 100ish miles.

comnoz said:

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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Hi Jim, out of interest, what is a "flow control valve" ?

cliffa

cliffa said:

Hi Jim, out of interest, what is a "flow control valve" ?

cliffa

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I used a large needle type valve that I built into the manifold flange. Like a giant air adjusting jet.

Any type of valve could be used. Just a barrel valve [fuel petcock] in the hose would work fine.

To see if worth while changing fittings in close quarters to fit 3/8" hose to insert a valve, might consider my stop gap, slip on/off clamp device to test effects, then do it the right way eh. Laboratory supply have em.
https://www.humboldtmfg.com/product-images/std/H-8700.png

comnoz said:

I used a large needle type valve that I built into the manifold flange. Like a giant air adjusting jet.

Any type of valve could be used. Just a barrel valve [fuel petcock] in the hose would work fine.

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Thanks Jim, So using a 3/8 bore as your maximum, you can simply fine tune the flow between the manifolds with the valve until you get the best (cleanest) transition of lower to mid range RPM ?

cliffa said:

Thanks Jim, So using a 3/8 bore as your maximum, you can simply fine tune the flow between the manifolds with the valve until you get the best (cleanest) transition of lower to mid range RPM ?

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Yeah, that about covers it.

I wonder if the reason a balance tube helps low speed response, which is my experience, is that by transferring some of its signal to the other side (when the other side has its valve closed), gasoline is tricked into continuing to flow up into that "dead" other side, such that that other side's fuel circuit remains primed and fueled and ready for its own signal, and so, is better able to deliver gas instantly when its valve opens? Asked another way, does a balance pipe cause gasoline, on the way up from the bowl to the low vacuum side's pilot circuit, to retain its inertia, such that its flow is more continuous and less stop/start? And vice versa.

Just to be clear to all trying to envision this, the balance tubes are sited between the carb's slide and the intake valve, and not on the air cleaner side.

xbacksideslider said:

I wonder if the reason a balance tube helps low speed response, which is my experience, is that by transferring some of its signal to the other side (when the other side has its valve closed), gasoline is tricked into continuing to flow up into that "dead" other side, such that that other side's fuel circuit remains primed and fueled and ready for its own signal, and so, is better able to deliver gas instantly when its valve opens? Asked another way, does a balance pipe cause gasoline, on the way up from the bowl to the low vacuum side's pilot circuit, to retain its inertia, such that its flow is more continuous and less stop/start? And vice versa.

Just to be clear to all trying to envision this, the balance tubes are sited between the carb's slide and the intake valve, and not on the air cleaner side.

Click to expand...

Yes.

I am sure it helps keep the fuel flow more constant but I think that is only a small help since the restriction in the fitting is rather small.
There is no doubt that is why a single carb can have such good off idle throttle response.

My logic looking into this implies there is a pressure wave back into carb the valve closed on, so instead of pushing mixture back out this carb mouth and also the fuel column in spray tube back into bowel the balance tube diverts pressure spikes into other side about to open valve to draw in. It can not have much to do with low pressure/vac idea as benefit is most noticed on low rpm throttle snaps that essentially relieves any vac signal so accelerator pumps used to fill in hesitation. Also keep in mind its not really piston vacuum drawing fuel up spray tube but the venturi flow past the spray tube chimney. WOT has nil vac but plenty of air flow. Its mainly racers with over lapped cam lobe openings that tend to spend time on balance tubes. Over lapped aggressive hi rpm cams are infamour for poor low rpm response because they ain't got much/any vac to draw fuel so sharing very low to no vac side to side is not logical and would need large size connection, not a restriction, to move enough mix at such low pressure differences.