Carb balance pipe

Hi.
Just replacing the isolastics on my old girl, and while I had it up on the ramp I thought I'd have the carbs down, to see if I could find the cause of a minor misfire from cold out the exhaust on one side as you tweek the throttle. I was casually casting my eye over things as you do, and I noticed that the manifold balance pipe was almost flat.
So, That got me wondering what effect would that have on the running? after all it must be there for a purpose right.
Terry

I'm not understanding what you mean that the balance pipe was almost flat. ??

But if there is a leak in the balance pipe/hose, the mixture will go lean and misfiring can result.

I didn't describe that very well, what I mean is the plastic has deformed over the years so that the normally round section of the pipe has become so oval (due to the fairly sharp bend it has to follow between the 2 manifolds) that it is just about flat. So no air (or vacuum) can pass between the 2 manifolds. I don't have an air leak, my misfire could be any number of the usual things that I'm sure I'll track down. Clearly if a balance pipe were leaking it would give you a weak mixture. But what exactly is it's purpose? what would be the outcome if you took it off and blanked the holes?

Terry

The balance tube flow will affect the transition from the idle circuit to the needle circuit. You could eliminate it if you are willing to do re-jetting and slide cutaway changes but it will be harder to tune. I would suggest keeping it in place. Jim

Hi Jim,

comnoz said:

The balance tube flow will affect the transition from the idle circuit to the needle circuit. You could eliminate it if you are willing to do re-jetting and slide cutaway changes but it will be harder to tune. I would suggest keeping it in place. Jim

Click to expand...

Can you expand on that, I wasn't planning on removing it, I'm Just very curious on what it's function is. Why didn't Nortons just jet it and fits slides to suit to start with.
Terry

Myford said:

Hi Jim,

comnoz said:

The balance tube flow will affect the transition from the idle circuit to the needle circuit. You could eliminate it if you are willing to do re-jetting and slide cutaway changes but it will be harder to tune. I would suggest keeping it in place. Jim

Click to expand...

Can you expand on that, I wasn't planning on removing it, I'm Just very curious on what it's function is. Why didn't Nortons just jet it and fits slides to suit to start with.
Terry

Click to expand...

A simple and incomplete answer would be if there is a balance tube then at low speed the engine rpm will be higher for the same slide position as there will be additional air supplied through the balance tube.
A lower slide position means less fuel will be delivered through the needle jet and more will be supplied through the idle circuit.
Eliminating the balance tube changes the fuel delivery curves at low speed. I have often installed a variable flow valve in the balance line to fine tune custom carb applications.

No balance tube at all makes the carb more sensitive to low speed changes and less forgiving to ambient changes. I always use a balance tube and tune the carb with the balance tube in place. Then I may vary the flow of the balance tube to get things "right on". Jim

You could eliminate it if you are willing to do re-jetting and slide cutaway changes

Click to expand...

and that is why Norton chose to have a balance pipe between the carbs, less hassle and expense

Jim,

Quote I always use a balance tube and tune the carb with the balance tube in place. Then I may vary the flow of the balance tube to get things "right on". Jim
I was going along that line of thought on my little ponder after I'd assumed it was there for areason - I usually set up the slow running jets by finding the sweet spot on revs, but I understand a lot of guys use vacuum gauges for this purpose by pulling off the pipe and plugging in a couple of gauges. You can see where I'm going with this - things are going to change when you put the balance pipe back on - yes?
Terry

Myford said:

Jim,

Quote - things are going to change when you put the balance pipe back on - yes?
Terry

Click to expand...

Yes. Think of the balance tube as another "jet". The ID of the tube and even the length of the tube will make minor changes. Sometimes these small changes will not be noticed right away in how the bike runs but they will contribute to how long the plugs can be run before they start crapping out because of deposits.

Tuning with a vacuum gauge is often not sensitive enough to be able to jet for good spark plug life or mixture that is well enough centered to keep the bike running well when the ambient conditions change a bit.
Reading spark plugs over a period of time and trial and error are about the only substitute for a good gas analyzer. Jim

Thanks Jim, fascinating,
one is never too old to learn from you guys.
Terry

comnoz said:

Myford said:

Jim,

Quote - things are going to change when you put the balance pipe back on - yes?
Terry

Click to expand...

Yes. Think of the balance tube as another "jet". The ID of the tube and even the length of the tube will make minor changes. Sometimes these small changes will not be noticed right away in how the bike runs but they will contribute to how long the plugs can be run before they start crapping out because of deposits.

Tuning with a vacuum gauge is often not sensitive enough to be able to jet for good spark plug life or mixture that is well enough centered to keep the bike running well when the ambient conditions change a bit.
Reading spark plugs over a period of time and trial and error are about the only substitute for a good gas analyzer. Jim

Click to expand...

Interesting. I would have thought that there would be standing waves in both inlets even at 3000 RPM and the balance pipe wouldn't deliver any flow. In my case I usually only ever use plug colour to set the main jets, and they are not really very important if you think about how long you actually use them even in racing. I've never tried to use a vacuum gauge because the system is pulsing and the vacuum reading is only a net effect and probably unresponsive - might be good for balancing the carbs, particularly on a four cylinder bike. I usually rely on pulling both slides right up with the twist grip and visually checking where the cutaways are. Almost all of my jetting is done on a race circuit looking for performance improvement. I don't think you can better that even with a dyno because of the weather effects.

I've always thought that balance pipes on exhaust systems were bullshit. Unless the mufflers are extremely restrictive and stifling the motor, I cannot see how the balance pipe would work when the pipes are resonating.

acotrel said:

I've always thought that balance pipes on exhaust systems were bullshit. Unless the mufflers are extremely restrictive and stifling the motor, I cannot see how the balance pipe would work when the pipes are resonating.

Click to expand...

I do believe they were to reduce decibels to satisfy the anal retentive US market.

Anyhow, I believe you have mistaken this thread as an exhaust thread.

To continue.
When i tuned my Amals, when I had them and I have had many, I would cap the orifice, pull the plug cap on the other side and bring each side to best running individually. This is of course after machanically equalizing slides, cable play and related item to attain a starting place. Then put the tube back on and dial them in as a pair.

I never thought or considered the ID or length of the tube being a factor only that without it a good idle would be very hard to attain. Seems a little too finite for the rather open and simple concept of the Amal concentric.

acotrel said:

I've always thought that balance pipes on exhaust systems were bullshit. Unless the mufflers are extremely restrictive and stifling the motor, I cannot see how the balance pipe would work when the pipes are resonating.

Click to expand...

No, they actually work to add power AND reduce noise - a definite win/win. Been used for that purpose since at least the 60's and probably before that. Dunstall showed that with the Commando motor in his tuneup book. For reasonably current car info demonstrating the improvement: http://www.carcraft.com/techarticles/ex ... tallation/

We typically got +15 HP on a Mopar 426 with same exhaust system before/after adding crossover - no other change.

Sorry - I know this is not an exhaust thread...

A loose or leaking manifold tube can be source of mystery bad running. The cross over carb tubes help low rpm mixture flow sharing and some play with size of this reasonant 'chamber' to get more low end response. Boost cans are further extension though mostly done on 2smokes. Ext. crossovers almost always add some low down grunt and take some bark out of note and almost never hurt top end but don't add to it. Resonate waves make extra heat in places along headers so connecting at the hottest spots gives best pay back but about any where on headers before muffler works as advertised.

comnoz said:

Myford said:

Jim,

Quote - things are going to change when you put the balance pipe back on - yes?
Terry

Click to expand...

Yes. Think of the balance tube as another "jet". The ID of the tube and even the length of the tube will make minor changes. Sometimes these small changes will not be noticed right away in how the bike runs but they will contribute to how long the plugs can be run before they start crapping out because of deposits.

Tuning with a vacuum gauge is often not sensitive enough to be able to jet for good spark plug life or mixture that is well enough centered to keep the bike running well when the ambient conditions change a bit.
Reading spark plugs over a period of time and trial and error are about the only substitute for a good gas analyzer. Jim

Click to expand...

Jim can you help me to understand HOW the tube has this effect? I understand it to be that it reduces the "stop/start" of air flow through the carb at low speed, but assumed the mixture on both sides was equal, so... ???
Doe these engines (because of cam timing) double carburet? On old piston port two stroke sled engines it was an issue at low speed, dampening the riders jacket with pre-mix.

I'm sure not all twin or multi carb set-ups have this balance tube, but besides the Norton my twin Stromberg TR6 and TR4 cars had one via the "T" in the crankcase breather system and so did my Corvair van. That one was an 18" or so pipe as the carbs were not close together. My Triumph T150 triple does not have a balance pipe and I think it could use one so it's triple Amal's could communicate. Do BMW boxer twins use them.

A carb manifold balance tube allows both carbs to "communicate" with each other, sensing the vac pulse (or "signal") between them and equalizing it, making idling and throttle response smoother. There is enough time at low rpm and small enough pressure differences it don't take much a tube cross section.

concours said:

comnoz said:

Myford said:

Jim,



Jim can you help me to understand HOW the tube has this effect? I understand it to be that it reduces the "stop/start" of air flow through the carb at low speed, but assumed the mixture on both sides was equal, so... ???
Doe these engines (because of cam timing) double carburet? On old piston port two strokes it was an issue at low speed, dampening the riders jacket with pre-mix.

Click to expand...

Click to expand...

Click to expand...

The balance tube on a Norton or most 4 stroke engines is only there for idle and off idle regions. It does not affect anything at higher RPM. It really does not have anything to do with pulsing or ram effects like the resonance bottle on some two strokes.

The main thing it does is it allows a higher RPM for the same slide position because each cylinder can draw a little extra air [mixture] from the opposite cylinder during it's intake stroke.

When you are tuning a single cylinder engine you will often find that the transition from the idle circuit to the needle circuit is the hardest to get right. On a single the transition happens at a low RPM as you first apply the throttle and unless it is just right a single will often cough or be rough right off idle.

When you have a twin with a balance tube then the engine will be able to run faster for the same throttle setting because of the extra air supplied by the balance tube. That means the transition from the idle circuit to the needle circuit is going to happen at a higher RPM where it is going to be less noticeable and easier to tune.

The transition is also going to be less noticeable at a higher RPM when the ambient conditions change enough to throw the mixture off a bit.

If you pinch the balance tube on an idling Norton engine you will find the RPM will drop. When you raise the slides to get the RPM back to where it was you also pull the needle farther from the jet and change the air pressure under the slide. Then to get the mixture correct again you will usually need to adjust the idle mixture screws or jet and change the slide cutaway and sometimes a different needle or jet will be needed.

Getting the transition from the idle to the needle just right is critical when the engine speed is only 1500 rpm. It is a lot easier if the transition does not happen until 2000 or higher. Jim

mike996 said:

acotrel said:

I've always thought that balance pipes on exhaust systems were bullshit. Unless the mufflers are extremely restrictive and stifling the motor, I cannot see how the balance pipe would work when the pipes are resonating.

Click to expand...

No, they actually work to add power AND reduce noise - a definite win/win. Been used for that purpose since at least the 60's and probably before that. Dunstall showed that with the Commando motor in his tuneup book. For reasonably current car info demonstrating the improvement: http://www.carcraft.com/techarticles/ex ... tallation/

We typically got +15 HP on a Mopar 426 with same exhaust system before/after adding crossover - no other change.

Sorry - I know this is not an exhaust thread...

Click to expand...

( http://www.bikesters.com/bikepics.htm DONT try this at Home . )

We get MORE with a Turbo . :wink: :lol: . :x

Just reading something from the dim dark ages quoteing Doug Hele on the cross over pipe on Triumphs . He seemed quite definate that it improved the power in the MIDRANGE . ( = early 60s )