Balance tube

[David Kemp]

My understanding of the Jowett Javelin Carb balance tube is to even mixture & vacumn between banks of cylinders. I notice on most modified or multi carb engines the same is done. My query is, isn't this contrary to the point of having twin carbs, that is each bank has unrestricted access to a carb? My way of thinking truly good flow would be each carb with it's own carb, of note is the VW flat four which does not have a balance tube.If a balance tube is necessary, would it not be better to have a large water cooled manifold like a Bradford?

[Forumadmin]

In essence you are correct when you have one carb per cylinder; but many twin carb systems support 2 or more cylinders. The reason the Jowett system requires a balance tube is the firing order does not put an even load on each carb. Note that the carbs are also "joined at the hip" in the airbox on a Javelin or an early Jup. This can cause imbalance due to refection pressure changes from the closing valves, similar to water hammer in water systems or in tuned exhaust systems (which the Jowett is via the balance pipe across the front).

Regarding water cooling (actually it is water heating) a water jacket would help prevent icing in cool damp conditions which can stop a Jav or Jup in its tracks.

[PJGD]

I don't have a really good explanation as to what the balance pipe on the Javelin/Jupiter does, but I suspect that it is necessary. When you have multiple carburetors on and engine, it is almost certain that they will be out of balance to some extent, thus a balance pipe should mitigate that issue to some extent. On the Javelin/Jupiter, it is even more likely that they will be out of balance because of the differential expansion of the aluminium crankcase at circa 90°C and the steel throttle rod out in the air flow at circa 20°C. In other words, if you set the balance right with the engine cold, it will be wrong when the engine is warm. That may not be a big effect, but it is an unhelpful fact.

Although the Javelin engine is almost certainly better off for the breathing and combustion development work that Harry Weslake performed for JCL during the engine development phase, it is most unfortunate that they ended up with a siamesed inlet port per cylinder head. One would like the induction strokes for each bank to be separated by a "quiet" period before the adjacent cylinder breathes, but in fact the firing order [1423] dictates that the adjacent cylinders of one bank follow each other, and then it switches to the other bank where those cylinders follow each other. Petrol, being heavier than air, has greater inertia and thus the first cylinder to breath on each bank runs a little bit lean, while the second cylinder runs a little bit rich. It may be that the balance pipe minimizes the downsides of this situation. Separate individual ports per cylinder would be much better along with a bank-to-bank cross-over manifold, possibly with separate carbs for cylinders 1 & 2 and 3 & 4. I believe that the VW engine that is referenced above has a cross-over manifold with individual ports.

Philip