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MG MGB Technical - Weber balance tube

According to my experience and (a little) understanding the weber DCOE carb, I think that the problem to get a smooth idle is because there is no balance tube between the two manifold runners.

Has anyone already tryed to tune the carb with a balance tube in place?

what are the results?

Any disadvantages?


Thanks
André

My B/GT is fitted with a 45 DCOE and a Warneford manifold with a fabricated balance tube.

I've never run or tuned my car without a balance tube so I cannot attest to any real differences based on not having one.

Because of the engine firing order of 1-3-4-2...the rear-most carb throat will tend to run a bit leaner at
higher revs because it gets two intake pulses in quick succession. In comparison, the forward carb throat has a chance to "catch up" between the longer times between the #1 & #2 cylinder intake
cycles. This leaning tends to become a little more aggravated at higher revs and with bigger cams.

A balance tube helps to even out these intake pulses.

Some Weber DCOE manifolds for MGB have a balance tube molded in place. Some (not all) Cannon
and PaceSetter manifolds have this feature as part of the casting - just inside the mouth of each runner near the carb mount flanges, however, I do not prefer either of these manifolds because their
straight runners tend to lean the #2 and #3 cylinders at higher revs (- but that's another subject).
Other manifolds have threaded bosses molded on the top of each runner for accepting a fabricated
tube. Warneford Design manifolds for MGB (among others) have this. I've also seen welded steel
tube intake manifolds that employ a balance tube.

Having said all that - I think there should be no real tuning problems at lower rpms (ie: idle) using a manifold that is without a balance tube, and that you should re-check (and adjust, if necessary) your
jet settings, mixture screws, choke sizes, gaskets seals, and ignition setup. I've found that DCOE's
prefer a little more advance in ignition timing. Turn the idle mixture screws in until the engine is about to die - then back them out about 1-2 turns. This is "baseline", but you will have to experiment what is
best for your particular engine setup. This takes a bit of patience (and a handful of different jets).
Daniel Wong

"Because of the engine firing order of 1-3-4-2...the rear-most carb throat will tend to run a bit leaner at
higher revs because it gets two intake pulses in quick succession."

I'm not sure I understand this Daniel. Couldn't 1-3-4-2 just as easily be written as 2-1-3-4? Meaning there is no difference on the load seen between the front and rear barrels?

I can confirm what Daniel says about the balance tube, it's not necessary for a smooth idle using a DCOE. I've got a smooth idle, it pulls strongly from idle up with no flat spots and no bog when I open the throttle.

The only problem I have (other than worse fuel efficiency than the SU's) is an annoying exhaust popping on underrun. I can't seem to eliminate it (any ideas?) other than using too rich an idle setting, which does cause a rough idle. I have read it's the nature of the DCOE45-152s, but I'm not sure I accept that.
Mike Polan

Mike-
The most common cause of exhaust pooping and/or backfiring in an MGB is the simplest to diagnose and fix: a leak in the exhaust system. As the pressure wave of a pulse of exhaust gases passes through the exhaust system, it leaves a partial vacuum behind it, sucking in fresh air through the leaky joint in the exhaust system. Unburnt fuel condenses in the exhaust system due to the induction of the cooler air and mixes with it. When a pulse of hot exhaust gases hit it- Bang! This problem can be aggravated by a too-lean or too-rich fuel/air mixture which results in the production of increased amounts in unburned fuel. To find out if this is the origin of your particular problem, mix up a thick solution of water and dishwashing detergent. Not the kind you put in the dishwasher, the other kind that your wife uses when she washes stuff in the sink. You know, the liquid stuff that she uses to cut grease with. With the system cold, squirt it on the joints of the exhaust system (don't forget the joint at the bottom of the header), then fire up the engine and look for bubbles. If you see bubbles, you've found the leak. If tightening up the clamps doesn't cure the problem, your friendly local auto parts store or muffler shop can supply you with some exhaust system putty to take up the gaps in the connections that result from poorly-matched exhaust system tubing diameters. Sometimes it's the little things..........
Steve S.

Hi Steve, yeah, that and an intake vacuum leak were my first thoughts, but I couldn't find any. I know of 2 other DCOE fitted B's with the same problem, and there was no popping with my SUs before I swapped them out for the webers.

I've narrowed it down to 3 theories that are harder to verify: 1) The vibration isolation mounts (carb to manifold)aren't very good. 2) Under hood heat is boiling off fuel. 3) There's a design problem with the progressive circuit on 152's. I'm open to worrying about a few more :).

There's some stuff at http://www.teglerizer.com/dcoe/dcoe_a_letter_from_an_end_user.htm that suggests it's theory #3 and the older 9 models work better, but I'm not sure I accept his point about the air bypass screw (I think it's there simply to balance the venturis at idle, not to mess with the a/f mixture).

Mine that makes me suspect #1 or #2 - it doesn't happen until the engine is good and warm, which implies some driving distance as well. Plenty of opportunity to froth up the fuel or heat the carb. I have read of someone that added a heat sheild and sorted out his car's popping so I want to give that a try next (it sure works for the SUs). I've considering heat wrapping my exhaust manifold but I've heard that can lead to a cracked head so I'm trying to decide how to bang out and mount an aluminium heat shield.

Andre, another though occurred to me - I'm using a stock cam and a smallish main venturi, you might have a lumpier idle if you've got a more radical cam or venturi. Generally though you can get a smooth idle with just about any idle jet (since you get to control the mixture with the idle screw). It's as soon as you crack open the throttle that bad choices in venturi, pump jet, idle or main jets that will give you fits. I'd be tempted to check everything else first on the idle problem - the weber guys say the same thing the SU guys do - 90% of carb problems are electrical.
Mike Polan

Mike:

The rear cylinder lean-out tends to happen more with higher reving, bigger cam engines. This is
probably not such a big deal on a street engine. Nothing to be overly concerned with.

This phenomena isn't a result so much because of loading, but more as a result of harmonics and
pressure waves (reversion) caused by a combination of the different timing of the clapping intake
valves between cylinders 1 & 2 and 3 & 4 , length and shape of the manifold runners, position of
the throttle plates - and the fact that the intake ports on these engines are siamesed (the biggest
single villain, here).

Those OEM dual S.U. manifolds don't have rather big balance tubes built on them, for nothing.

The 1-3-4-2 firing order subjects the rearmost carb throat to two quick gulps of air-fuel charge and
opening & closing claps of the intake valves; whereas the forward throat gets an extra 2 crankshaft
revolutions of "rest" between intake cycles. For illustration purposes only: it's as though the rear throat is momentarily feeding a 2-stroke engine, whereas the forward throat is feeding a 4-stroke engine (I'm for a lack of a better description for this, so please bear with me). Either throat on the
DCOE carb is capable of handling it's own separate responsibility of air-fuel delivery, but they are
doing rather different duty nontheless, and each throat has to handle a different set of intake
pulses and harmonics and reversion waves between every 4 revolutions of the crankshaft.

All this tends to make an engine more "peaky" (power spikes) at the mid and upper rev ranges.

Another side-effect of this includes the # 3 & 4 cylinders will tend to run a bit lean at higher revs.
It's very small and almost unperceptable at normal street revs, but if you hook up a gas analyzer
probe to each separate exhaust runner at higher revs...
Daniel Wong

Daniel, sorry, I must be thick, I just don't see it. Any time I picture it, I keep seeing that the 1342 cycle feeds 34 and then 21, with exactly the same rest and flow for the front and back barrels. I can't even see the siamese ports cause a difference front to rear - the firing is inside port then outside port on the front throat and the back throat - very symmetrical.

I'm aware there's a charge robbing effect due to the siamese ports, and if I understood it it's because the gas is already moving for the outside cylinders thanks to the inside cylinders getting it moving for them first (which is what I assumed the balance tube helped to dampen). But I've never heard of and can't fathom the front pair back pair problem you're describing.

Anyway, don't sweat it, I'll poke around some more and see if I can find anything that explains it for me a little slower :).
Mike Polan

Mike:

Discard the idea of the intake feeding cycle as being 3-4 and then 2-1.

Try to imagine the intake order more as being:

1...

...then, 3-4...

...then, 2.


-----------------

A drum player knows this beat as a "paradiddle roll".

One beat with the right hand...

...then, two quick beats with the left hand...

...then, one more beat with the right hand.


...now... try doing that with your hands a few hundred times
or so a minute so that it all sounds smooth, even, and seamless.

...now...imagine your engine having to do this several thousand times
a minute.
Daniel Wong

Daniel,

Here is what I get:

1

3-4

2-1

3-4

Etc.

If we go by your drum beat theory we get:

1

3-4

2

3-4

1

3-4

2

Etc.

Now if your engine is running this way, it's missing alternately on 1 and 2.

Sean
Sean Brown

Sean,


What's your opinion about a balance pipe?

Daniel,

Could you specify your size of venturis and jets?
I have a big valve head and a fairly radical cam (kent 718 sp 290°)
The car will have a check on the rollingroad, but a comparison of other setups is always interesting

Thanks
André

Ande -

My particular engine setup:

+.020 bore,
Isky T-32 cam (260/264*, .435/.430 lift)
flatop forged pistons by Arias, 2x taper floating pins, Total Seal gapless rings,
ported, polished & cc'd head (standard valves),
mechanical dizzy (modified by Nobu) w/ pertronix,
rods & pistons micro-balanced by Eldebrock,
crank modified, polished and micro-balanced by Hank The Crank,
Weber 45 DCOE-9, on a Warneford Design manifold w/ balance tube and Subaru PCV valve,
Special Tuning (Leyland factory) tube exhaust manifold, 2" pipes, Walker reverse-flow muffler.
K&N airfilter w/ cold air box.

Choke = 34mm
2nd venturi = 3.5
Main = 145
Idle = 50F9
Air = 165
Emulsion = F16
Cold start = 60F5
Intake valve = closed
Pump = 45

-----------

Sean & Mike:

1,3,4,2...1,3,4,2...1,3,4,2...

...or...

3,4,2,1,...3,4,2,1, ...3,4,2,1...

huh?...damn...now I dunno anymore!

...hey, I was just forwarding info that I was told by a couple of old-timer engineers at Eldebrock
(back in '75) as they were bench-spinning my engine and flowing my cylinder head on their dynos
and flow benches.

They were explaining the 3-4 cylinder-robbing phenomena and odd runner pulses and reversion
waves on siamesed heads (something I had not even thought about until they mentioned it) - and
they were attributing some of it to the 1,3,4,2 firing order. I figuired that the guys at Eldebrock must
know a thing or two about intake cycles and gas flows, and I was much younger (greener) back then,
so I just stood there like a dope and took their word at face value.
Daniel Wong

As Sean said, regardless of how you count, the firing interval between #3 and #4 will be identical to the firing interval between #2 and #1. From the firing position of #1 the engine spins 180 degrees for #3 to fire, #4 fires after the engine spins another 180 degrees. #2 fires 180 degrees later. When does #1 fire again? It has to be after the engine spins 180 degrees due to the 180 degree crankshaft throws. Two complete turns and the cycle repeats with no delay from one cycle to the next. FWIW, Clifton
Clifton Gordon

Well, this is a relief. I feel asleep last night trying to picture how to get an asymmetric firing pattern from a distributor with evenly spaced contacts. Maybe the Eldebrock guys meant to say cylinders 1 & 4?

I've put a vacuum gauge on a car running an aggressive cam with webers, and it's no wonder it's not usual to run a vacuum advance with them - it was pulsing like crazy at idle.
Mike Polan

I have never run without a balance pipe with either SU's or Webers, but my opinion of a balance pipe in general, is that it is only needed for proper idle quality. I know people who have blanked off the balance pipe on standard SU manifolds and drilled a hole in the blanking plate. They finally got down to a 5/16" hole before they found a problem. Additionally, it should be noted that Maniflow SU intakes use a very small (by comparison to stock pieces) balance pipe and function quite well as Steve S. can probably confirm.

This leads me to believe that a small balance pipe is adequate to balance the two passages for the purpose of taming the idle.

Now, it's important to think of something else too. The Weber carb does not work on quite the same principal as the SU. The SU's piston position is a function of the amount of air flowing through the carb and thus the difference in pressure above and below it. In steady flow this works pretty well, but when the flow pulses it can cause some problems. This is going to be a problem in both carbs, but I feel the SU will be less able to cope, due to the piston movement becoming out of sync with the flow movement.

In the Weber carb, the mixture ratio should also change and would be due to the pulsing air flowing both ways through the chokes at low RPM's. The reduced mixture quality is not likely to help the idle much, but I am sure that once RPM's rise sufficiently, these effects are reduced enough to be negligible on either count.

To answer the original question,

I can't see any disadvantage unless the balance tube joins the manifold runner in such a fashion as to disrupt the flow (which is the problem with the stock SU manifold). I would make a 1/2" balance tube and join it to the runners carefully with flow in mind.

Sean

Sean Brown

It is a well recognized phenomena that a pulsing air flow will pull much higher vacuum across a venturi (in pulses) and pull more fuel. i.e. a single cylinder 500 cc motorcycle engine will use a smaller - leaner jet than the same carb on a 2 cylinder 500 cc engine. With the smoother flow a richer jet is required to flow the same amount of fuel. Changing to a modified cam can cause the stock carb to run richer. ( at higher rpms the greater air flow can lean out the mixture) I am sure that there are dynamic peaks and valleys in the amount of reverberation or surging resulting from the siamese intake ports as the rpms increase or decrease. This variation in pulsation depending upon port size, compression ratio, cam timing, balance tubes, exhaust systems, rpms etc. all makes it hard to come up with a formula. Cut & try.
Barry
Barry Parkinson

This thread was discussed between 23/07/2003 and 25/07/2003

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