Carb balance pipe

Status
Not open for further replies.
comnoz said:
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
Click to expand...

got it, thanks
 
Surely if the pipes at each end of the balance tune are resonating out of phase across the ends of the balance pipe, the balance pipe must also be resonating ? I have trouble imagining how flow occurs in the balance pipe under those conditions. Perhaps where the balance pipe is situated (at a node at certain revs ) is critical ? I've seen the cross shaped exhaust under the engine of the SFC Laverda 750. It seems to work.
 
needing said:
A Commando is effectively two single 4-stroke engines joined at the crank.
Both pistons rise and fall in unison in their cylinders.
As one piston is on power stoke downward the other is on intake stroke downward.
The inlet stroke is roughly only a quarter of the entire 4-stroke cycle.
When tuning, the manifold connector hose is removed and spigots blocked so each carby supplies only its own cylinder.

Connect an unrestricted vacuum gauge to a manifold spigot and an accurate readings at idle is unobtainable.
This is caused by the piston creating a vacuum in the manifold as it descends on intake.
As the inlet valve snaps shut the vacuum in the manifold reduces towards zero during the other 3 stokes of the cylinder.
These wild fluctuations in vacuum are in order of 400+mmHg.
This means the throttle slide is subjected to an intake pulse that sucks it forward in the carby body then releases it.
With well worn carby bodies and throttle slides, you can actually hear the slap between the two.

So, when tuning of each carby is complete, the connector tube must be replaced.
This is to allow vacuum from the piston on intake stroke to provide a forward holding force to the other carby's throttle slide (the one with its piston on power stroke and its inlet valve shut).
Failure to replace the connector tube will eventually result in throttle slide skirt breakage.
Also, I recommend using only Amal throttle slides marked MB5 as the MB3 cavity appears to have had tooling repairs that would make it more vulnerable to skirt breakage.

On the subject of tuning and carby balance, I have modified an old LH header to fit an oxygen sensor and a thermocouple. This allows Air:Fuel Ratio (AFR) and Exhaust Gas Temperature (EGT) gauging to be used as my (onroad dyno) tuning setup. The left side engine is tuned at each throttle opening with the right side settings duplicated. After tuning, the nice shiny LH header and the manifold connector tube are refitted.

I haven't figured out how to add photos to help explain all of the above. I hope that the words are enough until I do.
Click to expand...


If you connect a MAP sensor to an inlet port and watch the manifold pressure on a scope you will find that you need a 3/8th or 1/2 inch id tube between the intakes to maintain a vacuum in both ports at idle. Even then it will be pulsing wildly. Even a single carb manifold will not maintain a vacuum at idle. Jim
 
comnoz said:
If you connect a MAP sensor to an inlet port and watch the manifold pressure on a scope you will find that you need a 3/8th or 1/2 inch id tube between the intakes to maintain a vacuum in both ports at idle. Even then it will be pulsing wildly. Even a single carb manifold will not maintain a vacuum at idle. Jim
Click to expand...

Jim,

This might seem like a stupid question but why does the engine struggle to hold a vacuum? Is it something to do with the long firing distance between cylinders? Is this also why it's hard to get a smooth idle?

Dennis
 
dennisgb said:
comnoz said:
If you connect a MAP sensor to an inlet port and watch the manifold pressure on a scope you will find that you need a 3/8th or 1/2 inch id tube between the intakes to maintain a vacuum in both ports at idle. Even then it will be pulsing wildly. Even a single carb manifold will not maintain a vacuum at idle. Jim
Click to expand...

Jim,

This might seem like a stupid question but why does the engine struggle to hold a vacuum? Is it something to do with the long firing distance between cylinders? Is this also why it's hard to get a smooth idle?

Dennis
Click to expand...

The piston only draws for a half a revolution out of every two -so yes the time between intake strokes is long enough for the vacuum to drop completely away. Even at higher speeds there is no consistent vacuum in the port except under closed throttle deceleration.

Under load at higher speeds the pressure in the port will vary from a vacuum [below ambient pressure] during the intake stroke to a positive pressure after the intake valve closes and the still moving air piles up against the back of the valve.

The reason the Norton is a bit fussy at idle is due to the overlap of the quite racy stock cam. Take away the overlap and they will idle like a tractor. The overlap makes it hard to keep a consistent mixture in the intake. The blowback pollutes the intake charge with exhaust gas at low speed. Jim
 
This is a really useful discussion.
I have tried to balance my carbs using a home made manometer connected to the inlet manifold spigots after removing the balance tube.

Carb balance pipe


I guess this is why I found it a pointless exercise because the bike ran so badly at idle with the manometer connected?

David
 
pdl999 said:
This is a really useful discussion.
I have tried to balance my carbs using a home made manometer connected to the inlet manifold spigots after removing the balance tube.


I guess this is why I found it a pointless exercise because the bike ran so badly at idle with the manometer connected?

David
Click to expand...

Yeah you would have to have separate connections to make a manometer work very well. Then since the water has a lot of momentum you could get an average reading that may help with balance.
Of course a good ear and feel will do the same thing. Jim
 
Missed this thread before . . .

The balance pipe, by allowing the slide cutaway to be lower/smaller area, enables a stronger vacuum signal to pull on the fuel circuit at the slide at the moment that the slide is lifted. I know this from my experience with singles, where you have to jet it too rich at idle to work around the off idle stumble problem, or . . . . use a pumper carb. The pumper carbs really teach this lesson, greatly improving a single's low rpm jump; they deliver much more fuel than any off idle vacuum ever could.

With my JS PWKs, I too drilled them and then pressed hose barbs into them to allow a balance pipe. I did it before I even installed them so I can't actually say that it made a difference or not, though I believe that it is the better practice.
 
comnoz said:
...... 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.
Click to expand...
Comnoz:
Have you experimented a lot with the balance tubes?
Any idea on what the "too big" diameter of the pipe is?
What would the effects be with say a 1/2" ID balance pipe?
Bigger is Better, right?


Seriously, it never occurred to me that a balance pipe could be used in tuning and now I am curious. Is there an optimum size?
 
FWIW, the Commando Production Racers were delivered without a balance tube between the carbs. They had fabricated intake manifolds in place of the standard cast manifolds, with no provision for the balance tube. I never noticed a problem because of that, but it's not a bike that I used to putt around town with. It's not the sort of problem you'd notice much on the race track. Sounds to me like a good thing to keep on a street Commando.

Ken
 
Sounds right to me, no one needs a lean condition caused by a vacuum leak on a race bike.
A lot of the rubber used for tubing in that era would rot or easily tear.


Further, a balance pipe complicates jet/slide/needle changes, gets in the way, reattachment gets forgotten . . . . been there, done that.
 
I have used a few different types of carb synch tools and found the DIGI-SYNCH to be the best. But with all of them I've run into the same problem that started this thread....the idle changes when the balance tube is re-installed. So...would it work to make a balance tube (to be used only with the gauges) with a pair of "T" fittings giving me two access ports for the gauges and still have the crossover tube in play?
For most of us the area between idle and 1/4 throttle is where the motor spends most of it's life, therefore the idle and just off idle is of prime importance.
One thing I've found to be very useful is when installing or replacing the carbs is to get the slide height identical. I use an .040 wire gauge under the slide ON THE MOTOR SIDE and match the pair. This saves tons of time fishing around for starting and idle and cuts down the aggravation factor by a lot. This method seems more accurate than measuring on the air filter side.
 
Status
Not open for further replies.
Back
Top